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#1
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
I have a 1987 MFA Magus A2 preamp that a few weeks ago started making
a buzzing/humming sound from both channels on any source setting. It is strongest in the right channel but audible from both. I have swapped out the line stage tube (6DJ8) to no effect. I have verified that it is the Magus because if the amps are on (MFA M75 monoblocks) and the preamp off there is no noise from the speakers. I have also tried removing the phono stage tubes to eliminate them as the problem but again to no effect. The noise seems to start 20 or 30 seconds after the preamp relay clicks on and after the amps warm up - in other words, it doesn't start as soon as the amps warm up but maybe 20 seconds later. Anyone have any ideas regarding what the problem could be and what I should do to fix it? Local technicians have little or no knowledge about tube gear so taking it to a shop isn't an option. It seems to be slowly getting louder over time so I need to fix it. The noise level does increase as the volume or gains are turned up if that helps. Any ideas? |
#2
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
On Nov 3, 11:39 pm, wrote:
I have a 1987 MFA Magus A2 preamp that a few weeks ago started making a buzzing/humming sound from both channels on any source setting. It is strongest in the right channel but audible from both. I have swapped out the line stage tube (6DJ8) to no effect. I have verified that it is the Magus because if the amps are on (MFA M75 monoblocks) and the preamp off there is no noise from the speakers. I have also tried removing the phono stage tubes to eliminate them as the problem but again to no effect. The noise seems to start 20 or 30 seconds after the preamp relay clicks on and after the amps warm up - in other words, it doesn't start as soon as the amps warm up but maybe 20 seconds later. Anyone have any ideas regarding what the problem could be and what I should do to fix it? Local technicians have little or no knowledge about tube gear so taking it to a shop isn't an option. It seems to be slowly getting louder over time so I need to fix it. The noise level does increase as the volume or gains are turned up if that helps. Any ideas? Filter caps. Peter Wieck Wyncote, PA |
#3
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
wrote in message ups.com... I have a 1987 MFA Magus A2 preamp that a few weeks ago started making a buzzing/humming sound from both channels on any source setting. It is strongest in the right channel but audible from both. I have swapped out the line stage tube (6DJ8) to no effect. I have verified that it is the Magus because if the amps are on (MFA M75 monoblocks) and the preamp off there is no noise from the speakers. I have also tried removing the phono stage tubes to eliminate them as the problem but again to no effect. The noise seems to start 20 or 30 seconds after the preamp relay clicks on and after the amps warm up - in other words, it doesn't start as soon as the amps warm up but maybe 20 seconds later. Anyone have any ideas regarding what the problem could be and what I should do to fix it? Local technicians have little or no knowledge about tube gear so taking it to a shop isn't an option. It seems to be slowly getting louder over time so I need to fix it. The noise level does increase as the volume or gains are turned up if that helps. Any ideas? I have to think that you may have a problem in the power supply. The culprit could be a filter cap breaking down, because the noise comes on after the relay is engaged, thus drawing the max current draw from the ps. These caps can appear ok with less than a full load. At least this is a good place to start since the ps is common to both channels. If you do not have a scope you can put your dvm in the AC mode while measuring the ps output and see if the voltage goes up significantly after the relay kicks in. It shouldn't, but if it does it can indicate excessive ac on your dc. If the cap is good, check the diodes and most of all, BE CAREFUL OF LETHAL VOLTAGES. Good Luck, west |
#4
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
On Nov 4, 4:53 pm, Bret Ludwig wrote:
Very likely but he needs a scope to see what's on those lines.- No he doesn't. Read the description... 20 seconds after warm-up. The classic symptom of bad filters. Then the vintage - the amp is 20 years old. Of course, it could be bad traces and so-forth, but it would have to be a strange combination of things for the amp not to be silent, and it *not* be bad filters. West... too many words and mostly wrong. Bad diodes will not make those symptoms unless they fail-short, something diodes simply do not do under these conditions. And if they fail-open, then the amp is silent. Even if one or two fail (and consider on a bridge how that would have to happen for the amp _not_ to be silent), the amp would not hum, just have low B+. So, please learn the basics before you give bad and/or dangerous advice. Peter Wieck Wyncote, PA |
#5
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
Peter Wieck wrote in
I have a 1987 MFA Magus A2 preamp that a few weeks ago started making a buzzing/humming sound from both channels on any source setting. It is strongest in the right channel but audible from both. I have swapped out the line stage tube (6DJ8) to no effect. I have verified that it is the Magus because if the amps are on (MFA M75 monoblocks) and the preamp off there is no noise from the speakers. I have also tried removing the phono stage tubes to eliminate them as the problem but again to no effect. The noise seems to start 20 or 30 seconds after the preamp relay clicks on and after the amps warm up - in other words, it doesn't start as soon as the amps warm up but maybe 20 seconds later. Anyone have any ideas regarding what the problem could be and what I should do to fix it? Local technicians have little or no knowledge about tube gear so taking it to a shop isn't an option. It seems to be slowly getting louder over time so I need to fix it. The noise level does increase as the volume or gains are turned up if that helps. Any ideas? Filter caps. Why would it be stronger in one channel, Peter? I was thinking either filter or cathode resistor bypass cap but figured one would be on both channels, and the other on just one. Would it be wise for the OP to renew all the electrolytic caps he can find? Same values and voltage please, OP, high temperature, low ESR. Ian |
#6
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
"Peter Wieck" wrote in message
ps.com... On Nov 3, 11:39 pm, wrote: I have a 1987 MFA Magus A2 preamp that a few weeks ago started making a buzzing/humming sound from both channels on any source setting. It is strongest in the right channel but audible from both. I have swapped out the line stage tube (6DJ8) to no effect. I have verified that it is the Magus because if the amps are on (MFA M75 monoblocks) and the preamp off there is no noise from the speakers. I have also tried removing the phono stage tubes to eliminate them as the problem but again to no effect. The noise seems to start 20 or 30 seconds after the preamp relay clicks on and after the amps warm up - in other words, it doesn't start as soon as the amps warm up but maybe 20 seconds later. Anyone have any ideas regarding what the problem could be and what I should do to fix it? Local technicians have little or no knowledge about tube gear so taking it to a shop isn't an option. It seems to be slowly getting louder over time so I need to fix it. The noise level does increase as the volume or gains are turned up if that helps. Any ideas? Filter caps. Peter Wieck Wyncote, PA I am not about to engage in a ****ing contest with you. My post was meant to teach and be friendly since it appeared that the OP had little or no experience in electronics. OTOH, your post, Mr. Wiecked, was again designed not to edify, but to poison the well in order to promote yourself. Nice going ###hole. How many people are you going to chase away from this ng because of your insecurities? I'm a professional biomedical engineer and taught many college level electronic courses while you are a pathetic net stalker. unsigned out of utter contempt. |
#7
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
On Nov 5, 12:04 am, "West" wrote:
"Peter Wieck" wrote in message ps.com... On Nov 3, 11:39 pm, wrote: I have a 1987 MFA Magus A2 preamp that a few weeks ago started making a buzzing/humming sound from both channels on any source setting. It is strongest in the right channel but audible from both. I have swapped out the line stage tube (6DJ8) to no effect. I have verified that it is the Magus because if the amps are on (MFA M75 monoblocks) and the preamp off there is no noise from the speakers. I have also tried removing the phono stage tubes to eliminate them as the problem but again to no effect. The noise seems to start 20 or 30 seconds after the preamp relay clicks on and after the amps warm up - in other words, it doesn't start as soon as the amps warm up but maybe 20 seconds later. Anyone have any ideas regarding what the problem could be and what I should do to fix it? Local technicians have little or no knowledge about tube gear so taking it to a shop isn't an option. It seems to be slowly getting louder over time so I need to fix it. The noise level does increase as the volume or gains are turned up if that helps. Any ideas? Filter caps. Peter Wieck Wyncote, PA I am not about to engage in a ****ing contest with you. My post was meant to teach and be friendly since it appeared that the OP had little or no experience in electronics. OTOH, your post, Mr. Wiecked, was again designed not to edify, but to poison the well in order to promote yourself. Nice going ###hole. How many people are you going to chase away from this ng because of your insecurities? I'm a professional biomedical engineer and taught many college level electronic courses while you are a pathetic net stalker. unsigned out of utter contempt.- Hide quoted text - - Show quoted text - West, you are the quintessential pillock. You make a post with bad information in it with the sole purpose being to look as if you know something. And in the context of someone who is not obviously familiar with reasons for the problem described, bad information is _very_ dangerous. Despite your conscience-soothing warning. You and Andre do share that characteristic of being invincibly ignorant much of the time. Peter Wieck Wyncote, PA |
#8
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
On Nov 4, 10:40 pm, "Ian Iveson"
wrote: Peter Wieck wrote in I have a 1987 MFA Magus A2 preamp that a few weeks ago started making a buzzing/humming sound from both channels on any source setting. It is strongest in the right channel but audible from both. I have swapped out the line stage tube (6DJ8) to no effect. I have verified that it is the Magus because if the amps are on (MFA M75 monoblocks) and the preamp off there is no noise from the speakers. I have also tried removing the phono stage tubes to eliminate them as the problem but again to no effect. The noise seems to start 20 or 30 seconds after the preamp relay clicks on and after the amps warm up - in other words, it doesn't start as soon as the amps warm up but maybe 20 seconds later. Anyone have any ideas regarding what the problem could be and what I should do to fix it? Local technicians have little or no knowledge about tube gear so taking it to a shop isn't an option. It seems to be slowly getting louder over time so I need to fix it. The noise level does increase as the volume or gains are turned up if that helps. Any ideas? Filter caps. Why would it be stronger in one channel, Peter? I was thinking either filter or cathode resistor bypass cap but figured one would be on both channels, and the other on just one. Would it be wise for the OP to renew all the electrolytic caps he can find? Same values and voltage please, OP, high temperature, low ESR. Ian: Depends on the configuration and how the supply is laid out. It would be an unusual situation that the PS would account for the imbalance by itself. But I suspect the imbalance has nothing to do with the common PS caps but other effects (possibly of age) including (perhaps) weak or unmatched tubes, an imprecise balance control or similar... or your suggestion of bad caps after the PS. But in general when one electrolytic goes, they all should be shotgunned, you are correct in that suggestion. However, and here is the serious isssue - the one that West & Bret missed entirely: If the OP is insufficiently familiar with the workings of tube equipment such that the diagnosis would not come "automagically" based on prior experience - and there is certainly no shame in that - then it is most likely that the OP should take the amp to someone who is familiar with the species. And most certainly NOT go prodding and poking around inside testing diodes or messing about with a scope to check waveforms. None of this is rocket science by any means, but first-time-through one should have an Elmer/Mentor on-site and not over the net. Were that my amp to diagnose, I would start with the filters, check the tubes, shotgun the rest of the electrolytics -each step checking the results - and then start worrying about what a scope might show. Chances are it would be unnecessary by that point anyway. My albeit limited experince with instrument amps vs. audio amps is that they are remarkably simple beasts, but generally more rugged than audio stuff. So, they typically respond well to a simple approach. Peter Wieck Wyncote, PA |
#9
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
Ian:
Depends on the configuration and how the supply is laid out. It would be an unusual situation that the PS would account for the imbalance by itself. But I suspect the imbalance has nothing to do with the common PS caps but other effects (possibly of age) including (perhaps) weak or unmatched tubes, an imprecise balance control or similar... or your suggestion of bad caps after the PS. But in general when one electrolytic goes, they all should be shotgunned, you are correct in that suggestion. However, and here is the serious isssue - the one that West & Bret missed entirely: If the OP is insufficiently familiar with the workings of tube equipment such that the diagnosis would not come "automagically" based on prior experience - and there is certainly no shame in that - then it is most likely that the OP should take the amp to someone who is familiar with the species. And most certainly NOT go prodding and poking around inside testing diodes or messing about with a scope to check waveforms. None of this is rocket science by any means, but first-time-through one should have an Elmer/Mentor on-site and not over the net. Were that my amp to diagnose, I would start with the filters, check the tubes, shotgun the rest of the electrolytics -each step checking the results - and then start worrying about what a scope might show. Chances are it would be unnecessary by that point anyway. My albeit limited experince with instrument amps vs. audio amps is that they are remarkably simple beasts, but generally more rugged than audio stuff. So, they typically respond well to a simple approach. Peter Wieck Wyncote, PA- Hide quoted text - Thanks for the suggestions. Let me provide a little more info on the unit and about myself. I bought this preamp used from a local stereo dealer (now out of business) in 1991. I had to replace some caps on the circuit board that were part of the power supply circuit because the relay kept failing (staying "on" all the time). This was back when I first bought it. I have since replaced the power resistors in the separate power supply myself. I have also replaced the selector switch with a sealed Elna (sp?) model due to failure of the original part from oxidation. This unit has a separate gain control for each channel as well as a common volume control. I replaced these with sealed Alps Blue Velvet pots for increased reliability. The unit has a separate power supply with an umbilical cord to the unit. Tube complements are a 6DJ8 in the line stage and 2 12ax7s and a 12at7 in the phono stage. The 6dj8 was recently swapped out and the noise did not change. I am not a trained electronics professional but have done some circuit board repair work (replacing caps,resistors) on comuter CRTs and printers as well as audio gear. I own and can use a multitester but have no clue about using an oscilloscope or other electronics test gear. I have not done much troubleshooting in the last several years but can probably still do simple stuff like check parts. BTW I know about the voltages in the power supply a friend told me that it's stepped up several times above house current and I got "bit" once by it, LOL, shocked me from my thumb to my elbow once. I should also mention that Scott Frankland himself ( the "F" in the company MFA) worked on the unit two years ago and he upgraded the rectifier and checked solder connections. It was expensive for him to work on it and the unit was out of my hands for a month so I would prefer not to send it out again. Do you guys still feel the filter caps are the likely culprits? |
#10
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
On Nov 5, 11:36 am, wrote:
Do you guys still feel the filter caps are the likely culprits? I do. For any number of reasons, the first of which is that you may not have replaced all the filter caps, you may not have used caps of the correct voltage so they held a while and then failed again, other caps in the PS may have failed. The magic clue word above is "some" when you described the caps you replaced. As you know about voltages, and you likely know that caps can store a heavy charge for a considerable period of time, you are likely able to do some basic service yourself - just be careful that you use equal-or- greater voltage and equal-or-greater capacity and *MIND POLARITY". Good luck with it. One more clue as you imply without direct statement that this amp has PC boards. Check the traces for hairline cracks. Heat/cool cycles can cause eventual failures such that as the amp warms up the traces open. A bright light underneath, or a good lens is a place to start. If you do come across a bad trace, be sure to bridge it with a few strands of wire, not just fresh solder. Peter Wieck Wyncote, PA |
#11
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
Could you describe for our readership the errors I made and let them be the
judge (hope to hear from some, pro & con). I know it's difficult for you to disagree objectively and present your contrasting opinions. Instead, its the old and tired ad hominem approach. Do you really thing that the majority of rodents buy your propaganda? Discuss the facts my man, if you can. That's what this NG needs and deserves. Cordially, west "Peter Wieck" wrote in message oups.com... On Nov 5, 12:04 am, "West" wrote: "Peter Wieck" wrote in message ps.com... On Nov 3, 11:39 pm, wrote: I have a 1987 MFA Magus A2 preamp that a few weeks ago started making a buzzing/humming sound from both channels on any source setting. It is strongest in the right channel but audible from both. I have swapped out the line stage tube (6DJ8) to no effect. I have verified that it is the Magus because if the amps are on (MFA M75 monoblocks) and the preamp off there is no noise from the speakers. I have also tried removing the phono stage tubes to eliminate them as the problem but again to no effect. The noise seems to start 20 or 30 seconds after the preamp relay clicks on and after the amps warm up - in other words, it doesn't start as soon as the amps warm up but maybe 20 seconds later. Anyone have any ideas regarding what the problem could be and what I should do to fix it? Local technicians have little or no knowledge about tube gear so taking it to a shop isn't an option. It seems to be slowly getting louder over time so I need to fix it. The noise level does increase as the volume or gains are turned up if that helps. Any ideas? Filter caps. Peter Wieck Wyncote, PA I am not about to engage in a ****ing contest with you. My post was meant to teach and be friendly since it appeared that the OP had little or no experience in electronics. OTOH, your post, Mr. Wiecked, was again designed not to edify, but to poison the well in order to promote yourself. Nice going ###hole. How many people are you going to chase away from this ng because of your insecurities? I'm a professional biomedical engineer and taught many college level electronic courses while you are a pathetic net stalker. unsigned out of utter contempt.- Hide quoted text - - Show quoted text - West, you are the quintessential pillock. You make a post with bad information in it with the sole purpose being to look as if you know something. And in the context of someone who is not obviously familiar with reasons for the problem described, bad information is _very_ dangerous. Despite your conscience-soothing warning. You and Andre do share that characteristic of being invincibly ignorant much of the time. Peter Wieck Wyncote, PA |
#12
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
No one suggested using a scope except for illustration. You are somewhat
clever in trying desperately to dig for insignificant and irrelevant issues that you can exploit for your agenda. Measuring a DC power supply with a DVM in the AC mode is standard procedure for a service tech to quickly observe ac ripple on the power supply. Later, if necessary and desired, you can whip out your scope. I know that you are not a professional (paid for this type of work) but a hobbyist. I also understand that the bs is about to flow with all sorts of accounts on how you made money doing this sort of work. Remember: mindless shotgunning of components can complicate a problem. I pray that you don't again accuse me of using too many words. (verbose post to Ian, scope issue, etc.) Cordially, west "Peter Wieck" wrote in message oups.com... On Nov 4, 10:40 pm, "Ian Iveson" wrote: Peter Wieck wrote in I have a 1987 MFA Magus A2 preamp that a few weeks ago started making a buzzing/humming sound from both channels on any source setting. It is strongest in the right channel but audible from both. I have swapped out the line stage tube (6DJ8) to no effect. I have verified that it is the Magus because if the amps are on (MFA M75 monoblocks) and the preamp off there is no noise from the speakers. I have also tried removing the phono stage tubes to eliminate them as the problem but again to no effect. The noise seems to start 20 or 30 seconds after the preamp relay clicks on and after the amps warm up - in other words, it doesn't start as soon as the amps warm up but maybe 20 seconds later. Anyone have any ideas regarding what the problem could be and what I should do to fix it? Local technicians have little or no knowledge about tube gear so taking it to a shop isn't an option. It seems to be slowly getting louder over time so I need to fix it. The noise level does increase as the volume or gains are turned up if that helps. Any ideas? Filter caps. Why would it be stronger in one channel, Peter? I was thinking either filter or cathode resistor bypass cap but figured one would be on both channels, and the other on just one. Would it be wise for the OP to renew all the electrolytic caps he can find? Same values and voltage please, OP, high temperature, low ESR. Ian: Depends on the configuration and how the supply is laid out. It would be an unusual situation that the PS would account for the imbalance by itself. But I suspect the imbalance has nothing to do with the common PS caps but other effects (possibly of age) including (perhaps) weak or unmatched tubes, an imprecise balance control or similar... or your suggestion of bad caps after the PS. But in general when one electrolytic goes, they all should be shotgunned, you are correct in that suggestion. However, and here is the serious isssue - the one that West & Bret missed entirely: If the OP is insufficiently familiar with the workings of tube equipment such that the diagnosis would not come "automagically" based on prior experience - and there is certainly no shame in that - then it is most likely that the OP should take the amp to someone who is familiar with the species. And most certainly NOT go prodding and poking around inside testing diodes or messing about with a scope to check waveforms. None of this is rocket science by any means, but first-time-through one should have an Elmer/Mentor on-site and not over the net. Were that my amp to diagnose, I would start with the filters, check the tubes, shotgun the rest of the electrolytics -each step checking the results - and then start worrying about what a scope might show. Chances are it would be unnecessary by that point anyway. My albeit limited experince with instrument amps vs. audio amps is that they are remarkably simple beasts, but generally more rugged than audio stuff. So, they typically respond well to a simple approach. Peter Wieck Wyncote, PA |
#13
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
On Nov 6, 1:22 am, "West" wrote:
Could you describe for our readership the errors I made and let them be the judge (hope to hear from some, pro & con). I know it's difficult for you to disagree objectively and present your contrasting opinions. Instead, its the old and tired ad hominem approach. Do you really thing that the majority of rodents buy your propaganda? Discuss the facts my man, if you can. That's what this NG needs and deserves. Cordially, west Asking Worthless Wiecky to specify the exact cause of any particular generic bilious attack will result only in another generic bilious attack. Worthless is a big pile of bile. Bile is a homogenous substance, undifferentiated, undistinguished; it is what makes poor Worthless simultaneously so dull and so offensive. A bilious attack from Worthless is like a Wiki, compendious, meaningless, free and -- dare we say it? -- worthless. Andre Jute "Blessed is the man who, having nothing to say, abstains from giving us wordly evidence of the fact."-- George Elliot "Peter Wieck" wrote in message oups.com... On Nov 5, 12:04 am, "West" wrote: "Peter Wieck" wrote in message ups.com... On Nov 3, 11:39 pm, wrote: I have a 1987 MFA Magus A2 preamp that a few weeks ago started making a buzzing/humming sound from both channels on any source setting. It is strongest in the right channel but audible from both. I have swapped out the line stage tube (6DJ8) to no effect. I have verified that it is the Magus because if the amps are on (MFA M75 monoblocks) and the preamp off there is no noise from the speakers. I have also tried removing the phono stage tubes to eliminate them as the problem but again to no effect. The noise seems to start 20 or 30 seconds after the preamp relay clicks on and after the amps warm up - in other words, it doesn't start as soon as the amps warm up but maybe 20 seconds later. Anyone have any ideas regarding what the problem could be and what I should do to fix it? Local technicians have little or no knowledge about tube gear so taking it to a shop isn't an option. It seems to be slowly getting louder over time so I need to fix it. The noise level does increase as the volume or gains are turned up if that helps. Any ideas? Filter caps. Peter Wieck Wyncote, PA I am not about to engage in a ****ing contest with you. My post was meant to teach and be friendly since it appeared that the OP had little or no experience in electronics. OTOH, your post, Mr. Wiecked, was again designed not to edify, but to poison the well in order to promote yourself. Nice going ###hole. How many people are you going to chase away from this ng because of your insecurities? I'm a professional biomedical engineer and taught many college level electronic courses while you are a pathetic net stalker. unsigned out of utter contempt.- Hide quoted text - - Show quoted text - West, you are the quintessential pillock. You make a post with bad information in it with the sole purpose being to look as if you know something. And in the context of someone who is not obviously familiar with reasons for the problem described, bad information is _very_ dangerous. Despite your conscience-soothing warning. You and Andre do share that characteristic of being invincibly ignorant much of the time. Peter Wieck Wyncote, PA |
#14
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
If I must. And as you asked:
On Nov 4, 3:50 pm, "West" wrote: I have to think that you may have a problem in the power supply. The culprit could be a filter cap breaking down, because the noise comes on after the relay is engaged, thus drawing the max current draw from the ps. These caps can appear ok with less than a full load. No they cannot. First error. Once an electrolytic starts to fail (open or short), the indications are blatant, obvious and such that an experienced hand with as limited instrumentation as a decent VOM can get a pretty good idea which way they are failing - cold and unloaded. NO! A VOM is not the final say by any means but serves (only) to confirm what is already suspected. And with an ESR meter, the failings will also be blatantly obvious - cold and unloaded. At least this is a good place to start since the ps is common to both channels. If you do not have a scope you can put your dvm in the AC mode while measuring the ps output and see if the voltage goes up significantly after the relay kicks in. It shouldn't, but if it does it can indicate excessive ac on your dc. And, you have this person measuring on the output side of the power- supply. Better he puts his meter in the Amp mode and measure the current draw of the unit (assuming that he knows the proper quiescent state draw). If the filters have failed "short" the draw of the unit will start to increase rapidly as the B+ kicks in. That is why an ammeter in the testing stages can be most useful in indicating overall condition. I would not be wanting to advise an individual of unkown skills to be putting the probes of his VOM onto a PC board of unknown clearances, perhaps poorly marked or unmarked where he might be screwing around with 500+V on two traces 1/8" apart or less. Now, as to AC-over-DC - only if the diodes have failed short. And that will not happen. Chopped DC will still indicate DC, even as the unit hums like a bumblebee. If one has the basic ability to discern audibly between 60hz and 120hz, then one will also be able to discern between chopped DC (typically 120hz) and AC (60hz). Back in the days of tube rectifiers, AC passing the tube was kinda- sorta possible under some conditions and so AC-over-DC was a reasonable concern. And one of the several reasons chokes were used (and also drafted as speaker field coils) as they pass DC quite nicely, thank you, but do not pass AC. But the device in question uses silicon diodes. Not hardly the same thing. If the cap is good And how would one tell that? In-circuit, perhaps with an ESR meter. Better out-of-circuit with a proper cap tester, one that tests under full operating voltage. Otherwise, only the purest speculation, and contrary to the symptoms already listed. , check the diodes Here is where you not only derail, but tumble to the bottom of the ravine and wind up steaming wreck. You are postulating that the diodes fail-short. They won't. Perhaps in the day of selenium or copper-oxide stacks, or the very earliest metal-cased silicon diodes. But this amp is a mere 20 years old. Fail-open will cause the amp to be silent. Not Humming. and most of all, BE CAREFUL OF LETHAL VOLTAGES. About the only solid advice you are giving. Good Luck, No comment. Peter Wieck Wyncote, PA |
#15
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
On Nov 5, 8:40 pm, "West" wrote:
No one suggested using a scope except for illustration. You are somewhat clever in trying desperately to dig for insignificant and irrelevant issues that you can exploit for your agenda. Measuring a DC power supply with a DVM in the AC mode is standard procedure for a service tech to quickly observe ac ripple on the power supply. YIKES - please explain to the assembled multitude how AC will pass a bridge rectifier such as to be measurable on an AC meter even with no filtration at all. Chopped DC ain't nohow AC. Peter Wieck Wyncote, PA |
#17
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
"Peter Wieck" wrote in message
ps.com... On Nov 5, 8:40 pm, "West" wrote: No one suggested using a scope except for illustration. You are somewhat clever in trying desperately to dig for insignificant and irrelevant issues that you can exploit for your agenda. Measuring a DC power supply with a DVM in the AC mode is standard procedure for a service tech to quickly observe ac ripple on the power supply. YIKES - please explain to the assembled multitude how AC will pass a bridge rectifier such as to be measurable on an AC meter even with no filtration at all. Chopped DC ain't nohow AC. Peter Wieck Wyncote, PA Don't you understand? I'll try again. If there is ac on the dc rail because of leaky filter cap, bad diode, etc., it can be roughly measured with a dvm on the ac mode. If you have a bank of various dc supplies as in a system network or any commercial equipment you can easily go down the line with your dvm measuring the various dc levels and switching over to ac on your meter to measure an ac component, if present. This will give you the level of dc and if ripple is present, the ac level. Naturally all these simple test can be performed with a scope, switch on the input to DC, then AC. I don't know how many times and ways I can explain it to you. Perhaps someone else can do a better job but this is how I taught it to my field engineering students for several years with no problem. I believe you owe me and the OP a heartfelt apology. We will not hold our breath, of course because I do not think Peter the Great has such capabilities west PS I bottomed posted this time because your question had no insults and may actually be a legit question. |
#18
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
"Andre Jute" wrote in message oups.com... On Nov 6, 1:22 am, "West" wrote: Could you describe for our readership the errors I made and let them be the judge (hope to hear from some, pro & con). I know it's difficult for you to disagree objectively and present your contrasting opinions. Instead, its the old and tired ad hominem approach. Do you really thing that the majority of rodents buy your propaganda? Discuss the facts my man, if you can. That's what this NG needs and deserves. Cordially, west Asking Worthless Wiecky to specify the exact cause of any particular generic bilious attack will result only in another generic bilious attack. Worthless is a big pile of bile. Bile is a homogenous substance, undifferentiated, undistinguished; it is what makes poor Worthless simultaneously so dull and so offensive. A bilious attack from Worthless is like a Wiki, compendious, meaningless, free and -- dare we say it? -- worthless. Andre Jute "Blessed is the man who, having nothing to say, abstains from giving us wordly evidence of the fact."-- George Elliot "Peter Wieck" wrote in message oups.com... On Nov 5, 12:04 am, "West" wrote: "Peter Wieck" wrote in message ups.com... On Nov 3, 11:39 pm, wrote: I have a 1987 MFA Magus A2 preamp that a few weeks ago started making a buzzing/humming sound from both channels on any source setting. It is strongest in the right channel but audible from both. I have swapped out the line stage tube (6DJ8) to no effect. I have verified that it is the Magus because if the amps are on (MFA M75 monoblocks) and the preamp off there is no noise from the speakers. I have also tried removing the phono stage tubes to eliminate them as the problem but again to no effect. The noise seems to start 20 or 30 seconds after the preamp relay clicks on and after the amps warm up - in other words, it doesn't start as soon as the amps warm up but maybe 20 seconds later. Anyone have any ideas regarding what the problem could be and what I should do to fix it? Local technicians have little or no knowledge about tube gear so taking it to a shop isn't an option. It seems to be slowly getting louder over time so I need to fix it. The noise level does increase as the volume or gains are turned up if that helps. Any ideas? Filter caps. Peter Wieck Wyncote, PA I am not about to engage in a ****ing contest with you. My post was meant to teach and be friendly since it appeared that the OP had little or no experience in electronics. OTOH, your post, Mr. Wiecked, was again designed not to edify, but to poison the well in order to promote yourself. Nice going ###hole. How many people are you going to chase away from this ng because of your insecurities? I'm a professional biomedical engineer and taught many college level electronic courses while you are a pathetic net stalker. unsigned out of utter contempt.- Hide quoted text - - Show quoted text - West, you are the quintessential pillock. You make a post with bad information in it with the sole purpose being to look as if you know something. And in the context of someone who is not obviously familiar with reasons for the problem described, bad information is _very_ dangerous. Despite your conscience-soothing warning. You and Andre do share that characteristic of being invincibly ignorant much of the time. Peter Wieck Wyncote, PA Thanks for the tough but true input. Wiecky is indeed a netstalker. This NG is his very life. He usually is the first one to post an answer. If I happen to walk by my computer and get an urge to answer a new post, by the time I finish writing it, Worthless has beaten me to the punch. Once I posted a question and in less than 10 seconds Wiecky was there with an insulting (to put it mildly) answer. There is something unnatural for someone to hang out seemingly 24/7 on any NG ready to prounce with his keyboard. This is not a hobby for him and I know he doesn't get any enjoyment out of it. He gets his substance from it. When he thought I was a newbie, he accused me of wanting to take over this group. Can you imagine that? A most glaring example of projection, I'd say. He then emailed me with many threats and insults to try and scare me off this group, then strangely, in the same email he told me that he "runs a diagnostic clinic." I though he was some kind of doctor, an unstable one to wit. west |
#19
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
On Nov 6, 2:14 am, "West" wrote:
"Peter Wieck" wrote in message ps.com... On Nov 5, 8:40 pm, "West" wrote: No one suggested using a scope except for illustration. You are somewhat clever in trying desperately to dig for insignificant and irrelevant issues that you can exploit for your agenda. Measuring a DC power supply with a DVM in the AC mode is standard procedure for a service tech to quickly observe ac ripple on the power supply. YIKES - please explain to the assembled multitude how AC will pass a bridge rectifier such as to be measurable on an AC meter even with no filtration at all. Chopped DC ain't nohow AC. Peter Wieck Wyncote, PA Don't you understand? I'll try again. If there is ac on the dc rail because of leaky filter cap, bad diode, etc., it can be roughly measured with a dvm on the ac mode. If you have a bank of various dc supplies as in a system network or any commercial equipment you can easily go down the line with your dvm measuring the various dc levels and switching over to ac on your meter to measure an ac component, if present. This will give you the level of dc and if ripple is present, the ac level. Naturally all these simple test can be performed with a scope, switch on the input to DC, then AC. I don't know how many times and ways I can explain it to you. Perhaps someone else can do a better job but this is how I taught it to my field engineering students for several years with no problem. I believe you owe me and the OP a heartfelt apology. We will not hold our breath, of course because I do not think Peter the Great has such capabilities west PS I bottomed posted this time because your question had no insults and may actually be a legit question.- Hide quoted text - - Show quoted text - West: Please answer the question as-posed: How does an AC meter measure across a bridge rectifier? Even if no filtration at all? Hint: Before I posted back to you, I tried it on the bench last night. Results: Single diode, 120V in, 84VDC out, ~4.3V of AC ripple (half-wave) Dual Diode, 120V in, 84VDC out, ~2V of AC rippple Bridge, 120V in, 169VDC out, 100mv of AC ripple. No caps involved, Fluke "True RMS" DVM Try it yourself. We are discussing Vacuum Tube equipment with solid- state rectifiers at (relatively) high voltage. Not Solid-State equipment typically operating at a B+ of 80V or less, and where half- wave rectifiers might be common. HINT: When bridge diodes fail in this situation, the B+ drops big- time. The amp goes silent. Just for round figures, half-wave rectifier = Vin x 0.7 Bridge Rectifier = Vin x 1.4 Point: I do not take money to service anything. This is a hobby. Nor do I typically take items to service without the owner being present (to learn what I did) unless I am dealing with someone who is elderly, too young, or otherwise unable (or unwise). Peter Wieck Wyncote, PA |
#20
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MFA Magus preamp hum/buzzing sound
Jon:
Please note the interpolations: On Nov 5, 11:26 pm, Gray Glass wrote: Peter (& West), I have seen rectifiers fail shorted. In my experience, that is how they most commonly fail in lower voltage DC applications. I don't see too many in mains rectification circuits, but it DOES happen from time-to-time. OK... you have seen something that I have not seen in 35 years - a mains rectifier failed-short. Assume that is the case, however. In a bridge situation, that would require at least two of the rectifiers to fail in this way to give a path for more than a very few volts of AC to reach the secondary to be measured. And, of course, that would immediately cut the B+ in half. The unit would go silent under those conditions. As far as electrolytics go, here is my rule of thumb: 1. An electrolytic that tests bad is bad 2. An electrolytic that tests good may be bad ESR meters are pretty useful, but they usually tests caps at a fraction of their working voltage, and that is a practical limitation in our hobby. If the cap has begun to fail, the ESR meter will show this as compared to a properly functioning cap, but I agree that the only legitimate way to test a cap is under full operating voltage out-of-circuit. When testing capacitors out of circuit, I use an adjustable high voltage DC power supply. I connect a 1000 ohm, 1 or 2W resistor between the minus lead of the cap and ground. I then measure the voltage across the resistor with a VOM set to the voltage scale, i.e. voltage measured across the resistor corresponds to current drawn through the capacitor. Poor Man's Ammeter. A similar device using a 1ohm, 5 watt resistor in series with one mains-lead may be used to test current draw with a VOM. Measure across the resistor to read current draw of the device under test. I gradually increase the voltage and keep an eye on the meter. This is a useful way to spot defective non-elecrolytics pretty fast. Electrolytics are a little harder because they will draw current with voltage changes until they stabilize. It is also a good way to reform electrolytics, as you can keep an eye on the current as you change slowly increase voltage. Everyone knows that an old electrolytic that hasn't been used in awhile can experience catastrophic failure with immediate application of high voltage, so before I condemn an electro, I reform it first. A really useful bench tool is a variable A/C supply with built-in ammeter and voltmeter. You can ramp up voltages and look for excess current draw, usually before toasting the defective part. I keep (two actually I feel a bit piggy over it) Heath IP-5220s for this purpose. http://www.oaktreevintage.com/web_ph...P-5220_web.jpg Voltage ramping (or reforming) is best done with any tube rectifiers removed and replaced with SS equivalents, such as those sold under the brand name "Copper Cap." Otherwise, no B+ flows until you reach the turn-on voltage of the rectifier. Common sense, right? Jon: You know better! Common Sense isn't. But unless there are prevailing reasons to attempt a reform, I tend not to trust this expedient as a permanent fix. Yes, it is possible, but the result is still an old, questionable cap that is now pretending to be good for an unknown period. On the other hand, I very nearly always "form" (& test) new electrolytics before installing them. And equipment that has sat about for 2-20 years between uses does get the approach you describe. Another hint: A 40 year old electrolytic NOS is just as suspect as a 40 year old electrolytic of unknown condition installed. A scope is very helpful to show amount of A/C ripple. It certainly is. Most AC ripple will come from Tube Rectifiers, however, not from silicon bridges. Electros that leak current sometimes leak electrolyte, and over time almost all get warm or hot. So one quick (and low-tech) way to gauge a failure is to check the can temperature. B+ dropping resistors located under some caps will warm them up, too, so you have to watch for that too. And remember - discharge electros before & after testing them. Why? You don't like char-marks and divots on tools? The feel of bits of molten metal on your skin? Finding yourself across the room in a heap with no memory of how you got there? Actually, a fully-charged large-value electro can be useful in "healing" open field-coil and transformer windings. Not something to do often, but if the alternative is scrap, worth a try. Peter Wieck Wyncote, PA |
#21
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MFA Magus preamp hum/buzzing sound
On Nov 6, 2:35 am, "West" wrote:
He then emailed me with many threats and insults to try and scare me off this group, then strangely, in the same email he told me that he "runs a diagnostic clinic." I though he was some kind of doctor, an unstable one to wit. Westiepoo.... care to publish the e-mails in full? All of them? Peter Wieck Wyncote, PA |
#22
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
"Peter Wieck" wrote in message ups.com... On Nov 6, 2:14 am, "West" wrote: "Peter Wieck" wrote in message ps.com... On Nov 5, 8:40 pm, "West" wrote: No one suggested using a scope except for illustration. You are somewhat clever in trying desperately to dig for insignificant and irrelevant issues that you can exploit for your agenda. Measuring a DC power supply with a DVM in the AC mode is standard procedure for a service tech to quickly observe ac ripple on the power supply. YIKES - please explain to the assembled multitude how AC will pass a bridge rectifier such as to be measurable on an AC meter even with no filtration at all. Chopped DC ain't nohow AC. Peter Wieck Wyncote, PA Don't you understand? I'll try again. If there is ac on the dc rail because of leaky filter cap, bad diode, etc., it can be roughly measured with a dvm on the ac mode. If you have a bank of various dc supplies as in a system network or any commercial equipment you can easily go down the line with your dvm measuring the various dc levels and switching over to ac on your meter to measure an ac component, if present. This will give you the level of dc and if ripple is present, the ac level. Naturally all these simple test can be performed with a scope, switch on the input to DC, then AC. I don't know how many times and ways I can explain it to you. Perhaps someone else can do a better job but this is how I taught it to my field engineering students for several years with no problem. I believe you owe me and the OP a heartfelt apology. We will not hold our breath, of course because I do not think Peter the Great has such capabilities west PS I bottomed posted this time because your question had no insults and may actually be a legit question.- Hide quoted text - - Show quoted text - West: Please answer the question as-posed: How does an AC meter measure across a bridge rectifier? Even if no filtration at all? Hint: Before I posted back to you, I tried it on the bench last night. Results: Single diode, 120V in, 84VDC out, ~4.3V of AC ripple (half-wave) Dual Diode, 120V in, 84VDC out, ~2V of AC rippple Bridge, 120V in, 169VDC out, 100mv of AC ripple. No caps involved, Fluke "True RMS" DVM Try it yourself. We are discussing Vacuum Tube equipment with solid- state rectifiers at (relatively) high voltage. Not Solid-State equipment typically operating at a B+ of 80V or less, and where half- wave rectifiers might be common. HINT: When bridge diodes fail in this situation, the B+ drops big- time. The amp goes silent. Just for round figures, half-wave rectifier = Vin x 0.7 Bridge Rectifier = Vin x 1.4 Point: I do not take money to service anything. This is a hobby. Nor do I typically take items to service without the owner being present (to learn what I did) unless I am dealing with someone who is elderly, too young, or otherwise unable (or unwise). Peter Wieck Wyncote, PA You are the one who suggested measuring AC across a diode. I always referred to measuring AC on the power supply's OUTPUT for ripple, etc. Now quit bothering me. west |
#23
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
"Peter Wieck" wrote in message ps.com... On Nov 6, 2:35 am, "West" wrote: He then emailed me with many threats and insults to try and scare me off this group, then strangely, in the same email he told me that he "runs a diagnostic clinic." I though he was some kind of doctor, an unstable one to wit. Westiepoo.... care to publish the e-mails in full? All of them? Peter Wieck Wyncote, PA You can do that if you wish because I never sent you an email. west |
#24
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
On Nov 6, 10:59 am, "West" wrote:
You are the one who suggested measuring AC across a diode. I always referred to measuring AC on the power supply's OUTPUT for ripple, etc. Now quit bothering me. When? Where? Peter Wieck Wyncote, PA |
#25
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MFA Magus preamp hum/buzzing sound
On Nov 6, 11:01 am, "West" wrote:
You can do that if you wish because I never sent you an email. west Oh, really? And if I post a few, that makes you what? Mistaken, lapse of memory or a liar? Do you remember the analogy of the fox and the scorpion? Peter Wieck Wyncote, PA |
#26
Posted to rec.audio.tubes
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MFA Magus preamp hum/buzzing sound
On Nov 6, 7:35 am, "West" wrote:
"Andre Jute" wrote: Asking Worthless Wiecky to specify the exact cause of any particular generic bilious attack will result only in another generic bilious attack. Worthless is a big pile of bile. Bile is a homogenous substance, undifferentiated, undistinguished; it is what makes poor Worthless simultaneously so dull and so offensive. A bilious attack from Worthless is like a Wiki, compendious, meaningless, free and -- dare we say it? -- worthless. Andre Jute "Blessed is the man who, having nothing to say, abstains from giving us wordly evidence of the fact."-- George Elliot Thanks for the tough but true input. Wiecky is indeed a netstalker. This NG is his very life. He usually is the first one to post an answer. If I happen to walk by my computer and get an urge to answer a new post, by the time I finish writing it, Worthless has beaten me to the punch. Once I posted a question and in less than 10 seconds Wiecky was there with an insulting (to put it mildly) answer. There is something unnatural for someone to hang out seemingly 24/7 on any NG ready to prounce with his keyboard. This is not a hobby for him and I know he doesn't get any enjoyment out of it. He gets his substance from it. When he thought I was a newbie, he accused me of wanting to take over this group. Can you imagine that? A most glaring example of projection, I'd say. He then emailed me with many threats and insults to try and scare me off this group, then strangely, in the same email he told me that he "runs a diagnostic clinic." I though he was some kind of doctor, an unstable one to wit. west Truest thing you ever said about Peter Wieck: "This NG is his very life." That's sad, actually. Our society should really offer even a janitor more suitable employment for his mind, perhaps a workingman's club in every neighbourhood. But it is also sad for us because Worthless Wiecky is a parasite who if not controlled and cut down could strangle RAT with his constant flow of bile.. Andre Jute No real corpses were harmed in the assembly of my golem Worthless Wieckless. I made him by stuffing a cow's bladder with pig offal. -- Creepy Mike LeFevre, Magnequest Transformers, Philadelphia |
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