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#1
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Variac as a Troubleshooting Aid
For intermittent problems possibly caused by a microscopic fracture in a pc
clad, try lowering the AC input voltage with a variac. Lower the AC until the circuit just starts working, then clad fractures and often cold solder connections will manifest themselves. Cordially, west |
#2
Posted to rec.audio.tubes
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Variac as a Troubleshooting Aid
west wrote: For intermittent problems possibly caused by a microscopic fracture in a pc clad, try lowering the AC input voltage with a variac. Lower the AC until the circuit just starts working, then clad fractures and often cold solder connections will manifest themselves. Is that so ? Could you explain what you mean by a 'pc clad' btw ? Graham |
#3
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Variac as a Troubleshooting Aid
"Eeyore" wrote in message ... west wrote: For intermittent problems possibly caused by a microscopic fracture in a pc clad, try lowering the AC input voltage with a variac. Lower the AC until the circuit just starts working, then clad fractures and often cold solder connections will manifest themselves. Is that so ? Could you explain what you mean by a 'pc clad' btw ? Graham Traces. west |
#4
Posted to rec.audio.tubes
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Variac as a Troubleshooting Aid
On Apr 24, 2:31 pm, "west" wrote:
For intermittent problems possibly caused by a microscopic fracture in a pc clad, try lowering the AC input voltage with a variac. Lower the AC until the circuit just starts working, then clad fractures and often cold solder connections will manifest themselves. Cordially, west West: With all straightforwardness here, please look up some of my rants on Variacs. You are looking for thermal effects based on low voltage, with the (mostly mistaken) belief that if a circuit runs cool it will be less likely to "open up" than if it runs hot. Right. The rant below was written against vintage equipment of all sorts, and in the case you cite, it is even more critical to have meaningful metering. If there actually is an intermittent in the circuit, traces or connections, that will manifest as significant movement in the meter. Furthermore, the general current draw of any device is easily calculated, so any variations above or below should be viewed with great suspicion with diagnosis not being valid until the cause is found and verified. __________________________________________ 1. A Variac *can* be useful. But only if used properly and with the proper metering. Otherwise it will serve merely to slow down whatever awful discovery (or success) you would have found by plugging it in and "letting her rip". 2. In any case, DON'T justplugitinandletitrip....... I am continuously surprised by the almost mythical status variable autotransformers have achieved in the hobby to somehow diagnose the condition of vintage equipment, reform caps, and do all sorts of other magical things..... without any reference to the required peripherals for the variac to achieve these ends. First, unless you have a current measuring device on the variac that indicates in meaningful increments exactly how much current the Item Under Test (hereinafter IUT) is drawing as current is applied, then you have no way of knowing whether you are heading to operation or destruction as you apply current. Sure, you can use a dim-bulb device to approximate current, but being within the 20-30% (at best) that can be gleaned from this process is way too innacurate for trust. Example: You have a typical 7-tube PP-type 25wpc amp on the bench. Nameplate is usually around 100 watts, actual usually around 60-70 watts. As you have no meter, you bring it up on the variac, all seems well... it may even play. BUT, you are drawing, unbeknownst to you 85 watts. You are a happy camper. The Unit plays. You bring it down(up)stairs and say: Hey, Hon, the Unit plays... let's listen to some chamber music on it. About 20 minutes into the disc, there is this funny smell, and a small (or large) *POP*.... maybe even some smoke. That 15-25-or-so extra watts was going somewhere. Typically heat. So something had to give. Second, there are "dim-bulb" devices that insert standard wattage lightbulbs in series with the IUT. These will indicate VERY ROUGHLY whether there are any spectacular shorts in an item, but again the margin of error is pretty immense for the long haul. They are minimally useful when used with a variac, but only because they are better than nothing. Cutting to the chase, before applying current to ANY item under any conditions, one should have a direct method in place to measure current drawn. There are several methods, the most "elegant" of which is to have meaningful meters built into the variac (together with an Isolation Transformer (to be saved for another rant). The other method is to measure across a resistor (in series) of a specific value. The drop across the resistor will indicate current thereby utilizing a common VOM instead of hard-to-find and calibrate AC ammeters, or the rare units with it all built in. In this way, one can REALLY determine what a IUT is doing, and REALLY get a decent idea where to begin troubleshooting (if necessary). Without meaningful information, one is entirely flying blind. Meaningful information includes accurate measurements of Voltage and Amperage, using meaningful increments. In the realm of equipment mostly discussed here, increments of 0.01 or at worst 0.02 A on a difference-scale of 0-1A must be clearly discernable, with ranges up to 5A being useful. Peter Wieck Wyncote, PA |
#5
Posted to rec.audio.tubes
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Variac as a Troubleshooting Aid
On Apr 24, 2:31 pm, "west" wrote:
For intermittent problems possibly caused by a microscopic fracture in a pc clad, try lowering the AC input voltage with a variac. Lower the AC until the circuit just starts working, then clad fractures and often cold solder connections will manifest themselves. Cordially, west One mo _____________________________________________ Isolation transformers are critical for any workbench, and after good hand-tools the first thing any electronic hobb7 person should get... perhaps even before said tools. Variacs, on the other hand are either very handy diagnostic tools or worse-than-useless anchors fit only to let the magic smoke out of an otherwise salvageable item. I am not really sure if there is an in-between other than their original function as light dimmers. Without both current and voltage metering, they are worse-than- useless. With the proper metering, they can be very handy tools. Myths: A variac will help reform caps (with specific reference to tube/ valve equipment). Fact: Only on those specific items with solid-state rectifiers that pass B+ current at any voltage, and only if done over more time than one cares to consider. In the case of a tube rectifier, most of them do not start to pass DC until the filament voltage reaches between 65% and 75% of rated voltage. If the B + on the particular item is say.... 350V, that means that the first voltage the caps-to-be-reformed would see would be something between 227V and 263V... not exactly a soft-start. Myth: A variac will allow one to apply voltage gently to a item, so as to discover problems before they become fatal. Fact: No, not at all. True, a variac does allow an item to see the minimum amount of current to trigger its functions... an item with bad filter caps may begin to hum at 80V rather than 120V but if the caps are not so bad as to show audible hum, the variac ceases to be useful. And a variac will *not* tell you whether a repaired item is OK or not. However, if the proper meters are put on the Isovariac, then some real diagnosis can happen: An item which should, by calculation draw say... 65 watts or so (0.55A), and draws 90 watts is dissipating 25 watts of heat somewhere... perhaps the output transformer(s). Comes down to a current meter with fine enough increments to give meaningful information. Such a tool is useful right down the line from initial diagnosis to testing the completed results. __________________________________ Peter Wieck Wyncote, PA |
#6
Posted to rec.audio.tubes
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Variac as a Troubleshooting Aid
"Peter Weeke the ****** " Isolation transformers are critical for any workbench, and after good hand-tools the first thing any electronic hobb7 person should get... perhaps even before said tools. ** Totally false. Isolation transformers need only be used when working on a live chassis item. Otherwise, you do NOT need one and using one INCREASES the electrocution hazard for a technician. See: http://sound.westhost.com/articles/electrocution.htm a.. Do not assume that isolation or stepdown transformers are safe - they are not. Isolation transformers will cheerfully kill you if you become connected across the output - an isolation transformer has no active (live) and neutral. Many stepdown transformers do not provide isolation. Isolation transformers can lead to a complacent attitude, and one lead can connect to chassis if there is an insulation failure - with no effect on the equipment or transformer. The other lead is now a killer! Isolation transformers will prevent an electrical safety switch from operating if a fault develops on the isolated side! ....... Phil |
#7
Posted to rec.audio.tubes
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Variac as a Troubleshooting Aid
On Apr 25, 10:27 pm, "Phil Allison" wrote:
"Peter Weeke the ****** " Isolation transformers are critical for any workbench, and after good hand-tools the first thing any electronic hobb7 person should get... perhaps even before said tools. ** Totally false. Isolation transformers need only be used when working on a live chassis item. Otherwise, you do NOT need one and using one INCREASES the electrocution hazard for a technician. See:http://sound.westhost.com/articles/electrocution.htm a.. Do not assume that isolation or stepdown transformers are safe - they are not. Isolation transformers will cheerfully kill you if you become connected across the output - an isolation transformer has no active (live) and neutral. Many stepdown transformers do not provide isolation. Isolation transformers can lead to a complacent attitude, and one lead can connect to chassis if there is an insulation failure - with no effect on the equipment or transformer. The other lead is now a killer! Isolation transformers will prevent an electrical safety switch from operating if a fault develops on the isolated side! ...... Phil Phil: Other than being a not-half-bad tech, you have a strange view of the world which arrests your understanding at the most basic level. Put another way, you are almost entirely unencumbered by the thought process. Accordingly, you give advice out of your ass rather than out of your brain. "Hot Chassis" items can happen for any number of reasons (as recent posts here illustrate), from shorted bypass caps and similar failures to errors on the part of a technician, assembler, kit-builder... or even someone as perfect as you. An isolation transformer is a simple precaution. It does NOT protect anyone against all error, and it is *only* useful on a bench during the diagnostic process. Certainly when an item leaves the bench it also leaves that very special environment and needs to join polite society... Futhermore, I do not think I mentioned anything about step-down/up transformers either. Mostly they are auto-transformers anyway. What I do mean is a laboratory-grade isolation transformer designed for exactly the purpose. Not silly little Radio-Shack 32VAC transformers mounted back-to-back or other expedients. The one I keep is: http://www.oaktreeent.com/web_photos...20_collage.jpg Repeat: You are entirely unencumbered by the thought process. When you get your meds adjusted and learn to think clearly rather than like a gerbil on PCP, please do come back and revisit this. Peter Wieck Wyncote, PA |
#8
Posted to rec.audio.tubes
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Variac as a Troubleshooting Aid
Peter Wieck wrote: Repeat: You are entirely unencumbered by the thought process. When you get your meds adjusted and learn to think clearly rather than like a gerbil on PCP, please do come back and revisit this. What Phil is quite correctly pointing out is that the use of an isolation transformer means that you will get no protection from an 'earth leakage current' breaker feeding it. Your very life might depend on it. Do not use isolation transformers 'willy nilly'. They have their dangers. Graham |
#9
Posted to rec.audio.tubes
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Variac as a Troubleshooting Aid
Phil Allison wrote
Isolation transformers are critical for any workbench, and after good hand-tools the first thing any electronic hobb7 person should get... perhaps even before said tools. ** Totally false. Isolation transformers need only be used when working on a live chassis item. Otherwise, you do NOT need one and using one INCREASES the electrocution hazard for a technician. See: http://sound.westhost.com/articles/electrocution.htm a.. Do not assume that isolation or stepdown transformers are safe - they are not. Isolation transformers will cheerfully kill you if you become connected across the output - an isolation transformer has no active (live) and neutral. Many stepdown transformers do not provide isolation. Isolation transformers can lead to a complacent attitude, and one lead can connect to chassis if there is an insulation failure - with no effect on the equipment or transformer. The other lead is now a killer! Isolation transformers will prevent an electrical safety switch from operating if a fault develops on the isolated side! Seems to me you are both right, or wrong, depending on how you work. As long as you are both aware of the dangers, one way or the other, which I am sure you both are, then you are both right. You have both done lots of work on dangerous equipment. You are both still alive. Why should either of you insist that your way is better? cheers, Ian |
#10
Posted to rec.audio.tubes
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Variac as a Troubleshooting Aid
On Apr 26, 8:10 am, Eeyore
wrote: Peter Wieck wrote: Repeat: You are entirely unencumbered by the thought process. When you get your meds adjusted and learn to think clearly rather than like a gerbil on PCP, please do come back and revisit this. What Phil is quite correctly pointing out is that the use of an isolation transformer means that you will get no protection from an 'earth leakage current' breaker feeding it. Your very life might depend on it. Do not use isolation transformers 'willy nilly'. They have their dangers. Graham I have no clue as to what sorts of GFIC devices you use, and how they are configured, but in this country (USA), based on NEC requirements the Device measures any deviations between input and output and will trip on any leakage to ground. Let's look at the nature of a GFIC device: It is designed to trip when the current goes anywhere other than from hot to neutral. Can we agree on that? If that is the case, and you insert yourself into a circuit *between the hot and the neutral*, whether there is a GFIC device or not, you will get zapped, as it cannot differentiate between you and a 1/4 watt 10,000 ohm resistor put in the same location.. or a 25 what incandescent lamp, for that matter. Assume no isolation transformer. Assume a properly functioning GFIC Device. Assume that you insert yourself into a ground path. Assume that the GFIC Device trips properly. *You* are still subjected to a momentary surge at whatever voltage is happening at that moment. What an Isolation Transformer does is isolate you from said ground paths. It does not obviate any protection from any additional outboard devices, and it does not take the place of them either. Nor does it do any better (or worse) than any other device if you insist on inserting yourself into the *designed* current path. Lastly, a properly designed isolation system does have a ground-path through it to prevent test probes and/or attached devices to test- equipment from setting up a potential hot-chassis situation outside the Isolated Circuit. In this case, the GFIC device will trip as in any other similar case. If you do not believe me, set it up for yourself and try it. Mine trips just as it should under that scenario when I go through the monthly tests. NO piece of equipment rates "willy-nilly" use. All have their limitations and dangers, and lack of wit and understanding will overcome even the most carefully designed safety systems. At the same time just because problem or fault *is* possible, that does not mean one should discard all precautions as meaningless. Airbags can kill under certain conditions. Seatbelts can trap one in a vehicle under certain conditions, Anti-Lock brakes and/or ESP can cause problems over no such systems under certain conditions. You appear to fall into that logical trap that requires believing that the opposite of Black is only and necessarily White. When, in fact, the opposite of Black is anything that is simply *Not Black*. Just ask the question: Is it black or not? Anything that is not black rates the NOT. Whether it is the polar opposite of White, or merely Puce, it is still NOT BLACK. Peter Wieck Wyncote, PA |
#11
Posted to rec.audio.tubes
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Variac as a Troubleshooting Aid
On Apr 26, 8:36 am, "Ian Iveson"
wrote: Phil Allison wrote Isolation transformers are critical for any workbench, and after good hand-tools the first thing any electronic hobb7 person should get... perhaps even before said tools. ** Totally false. Isolation transformers need only be used when working on a live chassis item. Otherwise, you do NOT need one and using one INCREASES the electrocution hazard for a technician. See:http://sound.westhost.com/articles/electrocution.htm a.. Do not assume that isolation or stepdown transformers are safe - they are not. Isolation transformers will cheerfully kill you if you become connected across the output - an isolation transformer has no active (live) and neutral. Many stepdown transformers do not provide isolation. Isolation transformers can lead to a complacent attitude, and one lead can connect to chassis if there is an insulation failure - with no effect on the equipment or transformer. The other lead is now a killer! Isolation transformers will prevent an electrical safety switch from operating if a fault develops on the isolated side! Seems to me you are both right, or wrong, depending on how you work. As long as you are both aware of the dangers, one way or the other, which I am sure you both are, then you are both right. You have both done lots of work on dangerous equipment. You are both still alive. Why should either of you insist that your way is better? cheers, Ian- Hide quoted text - - Show quoted text - Depends on how one defines "life"... Phil may be the way he is due to too many shocks? He certainly does have an electric personality... In any case, I have come to the conclusion that electricity must behave differently in the Antipodes and all the strictures and standards of the NEC, NEMA UL and ANSI are relatively meaningless in that environment. Peter Wieck Wyncote, PA |
#12
Posted to rec.audio.tubes
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Variac as a Troubleshooting Aid
Peter Wieck wrote: Eeyore wrote: Peter Wieck wrote: Repeat: You are entirely unencumbered by the thought process. When you get your meds adjusted and learn to think clearly rather than like a gerbil on PCP, please do come back and revisit this. What Phil is quite correctly pointing out is that the use of an isolation transformer means that you will get no protection from an 'earth leakage current' breaker feeding it. Your very life might depend on it. Do not use isolation transformers 'willy nilly'. They have their dangers. I have no clue as to what sorts of GFIC devices you use, and how they are configured, but in this country (USA), based on NEC requirements the Device measures any deviations between input and output and will trip on any leakage to ground. It works bt detecting any line-neutral imbalance. Let's look at the nature of a GFIC device: It is designed to trip when the current goes anywhere other than from hot to neutral. Can we agree on that? Yes. If that is the case, and you insert yourself into a circuit *between the hot and the neutral*, whether there is a GFIC device or not, you will get zapped, as it cannot differentiate between you and a 1/4 watt 10,000 ohm resistor put in the same location.. or a 25 what incandescent lamp, for that matter. Which is exactly what happens when you use an isoaltion transformer if you're touching the live and ground say and the neutral is also grounded. You become a perfectly valid load and get electrocuted whilst the breaker sees no problem with that. Remove the isolating transformer and the breaker will trip. Graham |
#13
Posted to rec.audio.tubes
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Variac as a Troubleshooting Aid
On 25 Apr 2007 18:09:34 -0700, Peter Wieck wrote:
On Apr 24, 2:31 pm, "west" wrote: For intermittent problems possibly caused by a microscopic fracture in a pc clad, try lowering the AC input voltage with a variac. Lower the AC until the circuit just starts working, then clad fractures and often cold solder connections will manifest themselves. Cordially, west One mo _____________________________________________ Isolation transformers are critical for any workbench, and after good hand-tools the first thing any electronic hobb7 person should get... perhaps even before said tools. Variacs, on the other hand are either very handy diagnostic tools or worse-than-useless anchors fit only to let the magic smoke out of an otherwise salvageable item. I am not really sure if there is an in-between other than their original function as light dimmers. Without both current and voltage metering, they are worse-than- useless. With the proper metering, they can be very handy tools. Myths: A variac will help reform caps (with specific reference to tube/ valve equipment). Fact: Only on those specific items with solid-state rectifiers that pass B+ current at any voltage, and only if done over more time than one cares to consider. In the case of a tube rectifier, most of them do not start to pass DC until the filament voltage reaches between 65% and 75% of rated voltage. If the B + on the particular item is say.... 350V, that means that the first voltage the caps-to-be-reformed would see would be something between 227V and 263V... not exactly a soft-start. Myth: A variac will allow one to apply voltage gently to a item, so as to discover problems before they become fatal. Fact: No, not at all. True, a variac does allow an item to see the minimum amount of current to trigger its functions... an item with bad filter caps may begin to hum at 80V rather than 120V but if the caps are not so bad as to show audible hum, the variac ceases to be useful. And a variac will *not* tell you whether a repaired item is OK or not. However, if the proper meters are put on the Isovariac, then some real diagnosis can happen: An item which should, by calculation draw say... 65 watts or so (0.55A), and draws 90 watts is dissipating 25 watts of heat somewhere... perhaps the output transformer(s). Comes down to a current meter with fine enough increments to give meaningful information. Such a tool is useful right down the line from initial diagnosis to testing the completed results. __________________________________ Peter Wieck Wyncote, PA Why not just plug it into a watt meter? J.P. |
#14
Posted to rec.audio.tubes
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Variac as a Troubleshooting Aid
On Apr 26, 1:39 pm, Eeyore
wrote: Peter Wieck wrote: Eeyore wrote: Peter Wieck wrote: Repeat: You are entirely unencumbered by the thought process. When you get your meds adjusted and learn to think clearly rather than like a gerbil on PCP, please do come back and revisit this. What Phil is quite correctly pointing out is that the use of an isolation transformer means that you will get no protection from an 'earth leakage current' breaker feeding it. Your very life might depend on it. Do not use isolation transformers 'willy nilly'. They have their dangers. I have no clue as to what sorts of GFIC devices you use, and how they are configured, but in this country (USA), based on NEC requirements the Device measures any deviations between input and output and will trip on any leakage to ground. It works bt detecting any line-neutral imbalance. Let's look at the nature of a GFIC device: It is designed to trip when the current goes anywhere other than from hot to neutral. Can we agree on that? Yes. If that is the case, and you insert yourself into a circuit *between the hot and the neutral*, whether there is a GFIC device or not, you will get zapped, as it cannot differentiate between you and a 1/4 watt 10,000 ohm resistor put in the same location.. or a 25 what incandescent lamp, for that matter. Which is exactly what happens when you use an isoaltion transformer if you're touching the live and ground say and the neutral is also grounded. You become a perfectly valid load and get electrocuted whilst the breaker sees no problem with that. Remove the isolating transformer and the breaker will trip. Graham- Hide quoted text - - Show quoted text - Clearly you do not understand the nature of lab-grade isolation transformers. ALL power through them is 'above ground', the only valid path is hot-to-neutral. There is no ground path from the isolated hot side. Whether the neutral is grounded or not. There is still no path except between hot and neutral... same as there would (should) be anyway. Don't take my word for it. Set it up and test it for yourself. You can ground the test piece to a good direct ground. And then measure a VOM from the hot side to it. No current will flow. And you can short the neutral to the ground and do the same. It still won't go through the ground. Or, measure current from neutral to ground. Nothing. Connect your GFIC device. Use a leaky test-probe from a signal generator or scope not isolated to the test piece... *POP* goes the breaker. As it should. Lots-O-Speculation here, little actual experience testing from appearances. It ain't nohow black magic. This is an unique condition, and isolated (pun intended) to test-bench conditions, to be used by someone who clearly understands the limitations of the device with great specificity to what it does NOT do. Now, of course, consider your scenario under hospital conditions where all sorts of wet stuff, machines, needles and monitors are operated through isolation transformers to protect the patient (and the machinery) from stray currents that could be anything from life- threatening to causing the machines to malfunction. If your scenario obtained, there would be no benefit to their use... which, of course, is not the case. http://www.reo.co.uk/files/kbase/Saf...ansformers.pdf http://www.mhennessy.f9.co.uk/isolating/index.htm http://www.sussex.ac.uk/Units/safety/sgn/sgn08.pdf http://www.reo.co.uk/news_article/71/0 http://jobfunctions.bnet.com/whitepa...px?docid=58852 http://www.freelists.org/archives/pc.../msg00012.html http://www.trft.org/TRFTPix/CHRSSafety.pdf http://www.wireless-workshop.com/safety.htm And on and on... Peter Wieck Wyncote, PA |
#15
Posted to rec.audio.tubes
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Variac as a Troubleshooting Aid
On Apr 26, 1:47 pm, J.P. wrote:
On 25 Apr 2007 18:09:34 -0700, Peter Wieck wrote: On Apr 24, 2:31 pm, "west" wrote: For intermittent problems possibly caused by a microscopic fracture in a pc clad, try lowering the AC input voltage with a variac. Lower the AC until the circuit just starts working, then clad fractures and often cold solder connections will manifest themselves. Cordially, west One mo _____________________________________________ Isolation transformers are critical for any workbench, and after good hand-tools the first thing any electronic hobb7 person should get... perhaps even before said tools. Variacs, on the other hand are either very handy diagnostic tools or worse-than-useless anchors fit only to let the magic smoke out of an otherwise salvageable item. I am not really sure if there is an in-between other than their original function as light dimmers. Without both current and voltage metering, they are worse-than- useless. With the proper metering, they can be very handy tools. Myths: A variac will help reform caps (with specific reference to tube/ valve equipment). Fact: Only on those specific items with solid-state rectifiers that pass B+ current at any voltage, and only if done over more time than one cares to consider. In the case of a tube rectifier, most of them do not start to pass DC until the filament voltage reaches between 65% and 75% of rated voltage. If the B + on the particular item is say.... 350V, that means that the first voltage the caps-to-be-reformed would see would be something between 227V and 263V... not exactly a soft-start. Myth: A variac will allow one to apply voltage gently to a item, so as to discover problems before they become fatal. Fact: No, not at all. True, a variac does allow an item to see the minimum amount of current to trigger its functions... an item with bad filter caps may begin to hum at 80V rather than 120V but if the caps are not so bad as to show audible hum, the variac ceases to be useful. And a variac will *not* tell you whether a repaired item is OK or not. However, if the proper meters are put on the Isovariac, then some real diagnosis can happen: An item which should, by calculation draw say... 65 watts or so (0.55A), and draws 90 watts is dissipating 25 watts of heat somewhere... perhaps the output transformer(s). Comes down to a current meter with fine enough increments to give meaningful information. Such a tool is useful right down the line from initial diagnosis to testing the completed results. __________________________________ Peter Wieck Wyncote, PA Why not just plug it into a watt meter? J.P.- Hide quoted text - - Show quoted text - For a couple of reasons. a) Most watt-meters are not very fine-reading. For them to be effective, they must be able to divide into meaningful 1-2 watt increments. b) There is no isolation or voltage control. For starters. Peter Wieck Wyncote, PA |
#16
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Variac as a Troubleshooting Aid
"Peter Weeke the CRIMINAL ****WIT ****** "
Isolation transformers are critical for any workbench, and after good hand-tools the first thing any electronic hobb7 person should get... perhaps even before said tools. ** Totally false. Isolation transformers need only be used when working on a live chassis item. Otherwise, you do NOT need one and using one INCREASES the electrocution hazard for a technician. See: http://sound.westhost.com/articles/electrocution.htm a.. Do not assume that isolation or stepdown transformers are safe - they are not. Isolation transformers will cheerfully kill you if you become connected across the output - an isolation transformer has no active (live) and neutral. Many stepdown transformers do not provide isolation. Isolation transformers can lead to a complacent attitude, and one lead can connect to chassis if there is an insulation failure - with no effect on the equipment or transformer. The other lead is now a killer! Isolation transformers will prevent an electrical safety switch from operating if a fault develops on the isolated side! ( snip this totally ASD ****ed CRIMINAL IDIOT's brain dead abuse ) See: http://sound.westhost.com/articles/electrocution.htm "Hot Chassis" items can happen for any number of reasons ** In which case the item is faulty and will trip a breaker or ELCB ....... INSTANTLY !! Only a INTENTIONAL hot chassis items need an Iso Tranny for use with scopes etc. An isolation transformer is a simple precaution. ** WRONG !!!!!!!!!!!!!!!!!!!!! It is a lethal POS that kill technicians. God , how I hope YOU are the next one See: http://sound.westhost.com/articles/electrocution.htm ....... Phil |
#17
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Variac as a Troubleshooting Aid
Bret Ludwig wrote: Brit practice is very different In what way exactly ? and will confuse people badly. I fail to see any reason for confusion. Please elaborate. Graham |
#18
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Variac as a Troubleshooting Aid
"Bret Ludwig ****ing Mental Retard. " "Peter Weeke the CRIMINAL ****WIT ****** " Phil, you are a ****. ** I am simply trying to prevent innocent folk from being electrocuted by the INSANE advice dished out by that arrogant, autistic **** called Weeke. No way exists to do that WITHOUT discrediting the incorrigible, asshole as much as possible. YOU are just as mentally defective as Weeke. So drop ****ing dead. ........ Phil |
#19
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Variac as a Troubleshooting Aid
Bret Ludwig wrote:
On Apr 26, 5:49 pm, "Phil Allison" wrote: "Peter Weeke the CRIMINAL ****WIT ****** " Phil, you are a ****. Wieck is basically sane, he just hasn't worked in service shops and electronics facilities. If you were to learn to communicate better you wouldn't be such a **** then, would you?? Yeah he would.... P -- "Yes, it is a good thing you are handy, as you clearly suck at being smart." - Herb |
#20
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Variac as a Troubleshooting Aid
On Apr 27, 7:37 pm, Bret Ludwig wrote:
On Apr 26, 5:49 pm, "Phil Allison" wrote: "Peter Weeke the CRIMINAL ****WIT ****** " Phil, you are a ****. Wieck is basically sane, he just hasn't worked in service shops and electronics facilities. If you were to learn to communicate better you wouldn't be such a **** then, would you?? Bret: I have observed four (4) electronics "service shops" in my time, two are still extant. ALL of them use(d) isolation transformers. ALL of them *when they became commonplace in the 1970s* used dedicated GFIC Breakers on the workbenches. And, yeah, I have a very good ground at my workbench. Peter Wieck Wyncote, PA |
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Variac as a Troubleshooting Aid
"Windsor Fox ****wit TROLL Idiot " ** Drop dead, ****. ........ Phil |
#22
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Variac as a Troubleshooting Aid
"Ian Iveson" wrote in message . uk... Phil Allison wrote Isolation transformers are critical for any workbench, and after good hand-tools the first thing any electronic hobb7 person should get... perhaps even before said tools. ** Totally false. Isolation transformers need only be used when working on a live chassis item. Otherwise, you do NOT need one and using one INCREASES the electrocution hazard for a technician. See: http://sound.westhost.com/articles/electrocution.htm a.. Do not assume that isolation or stepdown transformers are safe - they are not. Isolation transformers will cheerfully kill you if you become connected across the output - an isolation transformer has no active (live) and neutral. Many stepdown transformers do not provide isolation. Isolation transformers can lead to a complacent attitude, and one lead can connect to chassis if there is an insulation failure - with no effect on the equipment or transformer. The other lead is now a killer! Isolation transformers will prevent an electrical safety switch from operating if a fault develops on the isolated side! Seems to me you are both right, or wrong, depending on how you work. As long as you are both aware of the dangers, one way or the other, which I am sure you both are, then you are both right. You have both done lots of work on dangerous equipment. You are both still alive. Why should either of you insist that your way is better? cheers, Ian The vexing question, Iverson, is ... why are YOU still alive? west |
#23
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Variac as a Troubleshooting Aid
On Apr 27, 7:34 pm, Bret Ludwig wrote:
On Apr 26, 3:49 pm, Peter Wieck wrote: On Apr 26, 1:47 pm, J.P. wrote: On 25 Apr 2007 18:09:34 -0700, Peter Wieck wrote: On Apr 24, 2:31 pm, "west" wrote: For intermittent problems possibly caused by a microscopic fracture in a pc clad, try lowering the AC input voltage with a variac. Lower the AC until the circuit just starts working, then clad fractures and often cold solder connections will manifest themselves. Cordially, west One mo _____________________________________________ Isolation transformers are critical for any workbench, and after good hand-tools the first thing any electronic hobb7 person should get... perhaps even before said tools. Variacs, on the other hand are either very handy diagnostic tools or worse-than-useless anchors fit only to let the magic smoke out of an otherwise salvageable item. I am not really sure if there is an in-between other than their original function as light dimmers. Without both current and voltage metering, they are worse-than- useless. With the proper metering, they can be very handy tools. Myths: A variac will help reform caps (with specific reference to tube/ valve equipment). Fact: Only on those specific items with solid-state rectifiers that pass B+ current at any voltage, and only if done over more time than one cares to consider. In the case of a tube rectifier, most of them do not start to pass DC until the filament voltage reaches between 65% and 75% of rated voltage. If the B + on the particular item is say.... 350V, that means that the first voltage the caps-to-be-reformed would see would be something between 227V and 263V... not exactly a soft-start. Myth: A variac will allow one to apply voltage gently to a item, so as to discover problems before they become fatal. Fact: No, not at all. True, a variac does allow an item to see the minimum amount of current to trigger its functions... an item with bad filter caps may begin to hum at 80V rather than 120V but if the caps are not so bad as to show audible hum, the variac ceases to be useful. And a variac will *not* tell you whether a repaired item is OK or not. However, if the proper meters are put on the Isovariac, then some real diagnosis can happen: An item which should, by calculation draw say... 65 watts or so (0.55A), and draws 90 watts is dissipating 25 watts of heat somewhere... perhaps the output transformer(s). Comes down to a current meter with fine enough increments to give meaningful information. Such a tool is useful right down the line from initial diagnosis to testing the completed results. __________________________________ Peter Wieck Wyncote, PA Why not just plug it into a watt meter? J.P.- Hide quoted text - - Show quoted text - For a couple of reasons. a) Most watt-meters are not very fine-reading. For them to be effective, they must be able to divide into meaningful 1-2 watt increments. b) There is no isolation or voltage control. The main purpose of a straight variac (for tube audio) IMO is to serve as a variable source for outboard DC, high and low voltage power supplies you build yourself. You will use it more in its pure AC supply function for solid state audio and, should you still wish to, TV repair. I particularly liked the old Lab-Volt power panels that had a variac, a HV variable AC outlet, a LV variable AC outlet from a transformer, and a variable HV and/or LV DC output from transformers and rectifier- filters. And some metering. Today you would use an isoformer on occasion, simply because the old standby of drilling and pounding your own ground rod in the basement is illegal and would trip the guffer. In the old days that's what shops and hobbyists in basements did. They had one hot wire to the bench and their own good ground. They would tie their own good ground to the main building ground with the web copper strap cars used for the ground cable sometimes. Today, smart people have a high amperage four wire range or dryer outlet installed so they know whare ground, neutral, and each of the two hots are, though most people will only use one of them.- Hide quoted text - - Show quoted text - Any hobbyist that runs an electronics shop in a basement more-or-less deserves what he/she gets. There are far too many unknown grounds (including the concrete slab) for that to be a fit place to have a proper shop. Not that it cannot be done safely, but the amount of planning and care required is considerable. Given the general amount of ignorance displayed in this particular thread as examples there are damned few individuals here who have that capacity. Starting with Mr. Allison who clearly has no clue about this subject... at least as it applies in the good old USA. Peter Wieck Wyncote. PA |
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Variac as a Troubleshooting Aid
On Apr 26, 4:47 pm, Peter Wieck wrote:
On Apr 26, 1:39 pm, Eeyore wrote: Peter Wieck wrote: Eeyore wrote: Peter Wieck wrote: Repeat: You are entirely unencumbered by the thought process. When you get your meds adjusted and learn to think clearly rather than like a gerbil on PCP, please do come back and revisit this. What Phil is quite correctly pointing out is that the use of an isolation transformer means that you will get no protection from an 'earth leakage current' breaker feeding it. Your very life might depend on it. Do not use isolation transformers 'willy nilly'. They have their dangers. I have no clue as to what sorts of GFIC devices you use, and how they are configured, but in this country (USA), based on NEC requirements the Device measures any deviations between input and output and will trip on any leakage to ground. It works bt detecting any line-neutral imbalance. Let's look at the nature of a GFIC device: It is designed to trip when the current goes anywhere other than from hot to neutral. Can we agree on that? Yes. If that is the case, and you insert yourself into a circuit *between the hot and the neutral*, whether there is a GFIC device or not, you will get zapped, as it cannot differentiate between you and a 1/4 watt 10,000 ohm resistor put in the same location.. or a 25 what incandescent lamp, for that matter. Which is exactly what happens when you use an isoaltion transformer if you're touching the live and ground say and the neutral is also grounded. You become a perfectly valid load and get electrocuted whilst the breaker sees no problem with that. Remove the isolating transformer and the breaker will trip. Graham- Hide quoted text - - Show quoted text - Clearly you do not understand the nature of lab-grade isolation transformers. ALL power through them is 'above ground', the only valid path is hot-to-neutral. There is no ground path from the isolated hot side. Whether the neutral is grounded or not. There is still no path except between hot and neutral... same as there would (should) be anyway. Don't take my word for it. Set it up and test it for yourself. You can ground the test piece to a good direct ground. And then measure a VOM from the hot side to it. No current will flow. And you can short the neutral to the ground and do the same. It still won't go through the ground. Or, measure current from neutral to ground. Nothing. Connect your GFIC device. Use a leaky test-probe from a signal generator or scope not isolated to the test piece... *POP* goes the breaker. As it should. Lots-O-Speculation here, little actual experience testing from appearances. It ain't nohow black magic. This is an unique condition, and isolated (pun intended) to test-bench conditions, to be used by someone who clearly understands the limitations of the device with great specificity to what it does NOT do. Now, of course, consider your scenario under hospital conditions where all sorts of wet stuff, machines, needles and monitors are operated through isolation transformers to protect the patient (and the machinery) from stray currents that could be anything from life- threatening to causing the machines to malfunction. If your scenario obtained, there would be no benefit to their use... which, of course, is not the case. http://www.reo.co.uk/files/kbase/Saf...ansformers.pdf http://www.mhennessy.f9.co.uk/isolating/index.htm http://www.sussex.ac.uk/Units/safety/sgn/sgn08.pdf http://www.reo.co.uk/news_article/71/0 http://jobfunctions.bnet.com/whitepa...px?docid=58852 http://www.freelists.org/archives/pc.../msg00012.html http://www.trft.org/TRFTPix/CHRSSafety.pdf http://www.wireless-workshop.com/safety.htm I always see problems in if the people are not aware of what a regular isolation does and is connected. I was reverifying how they are connected. For servicing equipment, I make use of a transformer that has the secondary completely isolated, which is not the case with approved devices. By the way, I use variacs, isolation transformers, and light bulbs, as needed to control power. According to rules, isolation transformers have one side of the secondary tied to ground. The ground goes from input to output. The secondary is ground referenced and can be hazardous working on equipment. An isolations transformer is designed to minimize common mode leakage current from primary to secondary. It also effectively filters common mode noise. Drawn like this............... http://zekfrivolous.com/iso/isozoom.JPG. Like all these and the medical approved...................... http://www.tripplite.com/products/co...eneral_purpose The medical units have the better plugs but are otherwise the same. Operating rooms have a 100 ua limit but even that can be hazardous with open tissue. I found some info from the original stuff from Mark Waller. http://www.smartpowersystems.com/New...%20article.pdf greg |
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Variac as a Troubleshooting Aid
I use a lab-grade variable A/C supply with a current meter for basic
troubleshooting and other tasks. It can reform electrolytic caps. This requires that you have SS rectification or replace tube rectifiers with a SS equivalent. I use it to look for dangerous conditions, e.g. excessive current draw, before applying full line voltage to the device under test. Can't tell you how many transistors, etc. this has saved. It can determine transformer step-up / step-down ratios, or figure out taps on unknown iron. Often audio amplifiers powered through a regulated A/C supply sound worse. Sometimes dramatically so. One wonders why . . . . Jon |
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Variac as a Troubleshooting Aid
On May 2, 2:52 pm, Jon Yaeger wrote:
I use a lab-grade variable A/C supply with a current meter for basic troubleshooting and other tasks. It can reform electrolytic caps. This requires that you have SS rectification or replace tube rectifiers with a SS equivalent. I use it to look for dangerous conditions, e.g. excessive current draw, before applying full line voltage to the device under test. Can't tell you how many transistors, etc. this has saved. It can determine transformer step-up / step-down ratios, or figure out taps on unknown iron. Often audio amplifiers powered through a regulated A/C supply sound worse. Sometimes dramatically so. One wonders why . . . . Jon Chopped DC from minimal filtration... "regulated" supplies tend to be designed against the lowest anticipated load. So when the amp is pushing, the power-supply isn't up to it. There are always exceptions as a well-designed power-supply of almost any type will put out clean DC as needed. "Design" includes proper isolation (noise and other artifcacts) as well as simple capacity. For related reasons, running an amp from inverter power (unless a true (pure) sine-wave type) can cause many otherwise good amps to sound pretty wretched. And yes, a lab-grade iso-variac can be quite versatile. But as you note, not without proper metering. Peter Wieck Wyncote, PA |
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