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#1
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How to test how good my PSU electrolytics are in an old amp?
Hi,
I got hold of a 20yr old dead Adcom hifi power amp recently together with the service manual. The fuses on each +ve line have blown, the fuses on the -ve lines were ok. I have not tried the amp out - I have taken out these supply line fuses (between the psu and the main electronics), therefore isolating the power supply to the two amp boards, and checked the psu with a voltmeter and oscilloscope. The psu seems fine.... + and - 49.4V out, and all lines giving out about 10mV ripple. One rail had 20mV ripple. As this is quite an old amp, and I understand the electrolytics go first (after pots, swtiches and connectors), what tests can I do on the caps to see what kind of life I can get out of them. I can replace them, but if it is not necessary I don't want to do it. Thanks, guys. |
#2
Posted to sci.electronics.basics,rec.audio.tech
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How to test how good my PSU electrolytics are in an old amp?
Measure effective serial resistance with an ESR meter. But if they are
giving no symptoms, there's probably no reason to test them. You could try paralleling another good capacitor with each of them temporarily to see if there's a real drop in the ripple. Age alone is not what kills them; it's a kind of electrolytic deterioration that can be prevented by using them regularly. In fact, they may be better after a month of use than they are now. "royalmp2001" wrote in message ups.com... Hi, I got hold of a 20yr old dead Adcom hifi power amp recently together with the service manual. The fuses on each +ve line have blown, the fuses on the -ve lines were ok. I have not tried the amp out - I have taken out these supply line fuses (between the psu and the main electronics), therefore isolating the power supply to the two amp boards, and checked the psu with a voltmeter and oscilloscope. The psu seems fine.... + and - 49.4V out, and all lines giving out about 10mV ripple. One rail had 20mV ripple. As this is quite an old amp, and I understand the electrolytics go first (after pots, swtiches and connectors), what tests can I do on the caps to see what kind of life I can get out of them. I can replace them, but if it is not necessary I don't want to do it. Thanks, guys. |
#3
Posted to sci.electronics.basics,rec.audio.tech
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How to test how good my PSU electrolytics are in an old amp?
On Sat, 6 May 2006 17:51:04 -0400, "mc"
Gave us: Age alone is not what kills them; it's a kind of electrolytic deterioration that can be prevented by using them regularly. In fact, they may be better after a month of use than they are now. They "dry out". Age is the main factor for determining how close to "dried out" they might be. The only exceptions are hermetically sealed military versions of EL caps. Most all commercial EL caps are vented and eventually will "dry out". The easiest way to tell is to compare it with another of the same form factor, by weight. No instruments required other than an accurate scale. |
#4
Posted to sci.electronics.basics,rec.audio.tech
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How to test how good my PSU electrolytics are in an old amp?
royalmp2001 wrote: Hi, I got hold of a 20yr old dead Adcom hifi power amp recently together with the service manual. The fuses on each +ve line have blown, the fuses on the -ve lines were ok. I have not tried the amp out - I have taken out these supply line fuses (between the psu and the main electronics), therefore isolating the power supply to the two amp boards, and checked the psu with a voltmeter and oscilloscope. The psu seems fine.... + and - 49.4V out, and all lines giving out about 10mV ripple. One rail had 20mV ripple. As this is quite an old amp, and I understand the electrolytics go first (after pots, swtiches and connectors), what tests can I do on the caps to see what kind of life I can get out of them. I can replace them, but if it is not necessary I don't want to do it. Thanks, guys. Hi, Royal. The first check should be in the power supply itself. You seem to have already begun this. Try no load, then add some power resistors to approximate the load of the amplifier. Look at the loaded ripple, and see if it's anywhere near what it should be. For a 1 amp load, you should get 1 volt of ripple for an 8300uF cap. If your cap is 8300uF and you have 2 amps, you should have 2V of ripple. Double the load, double the ripple. If your capacitance is doubled, your ripple should halve. Work it out on paper beforehand, and compare your calcs with your observations. While you're at full load, run it for a few minutes, and feel the body of the cap. It shouldn't be getting more than slightly warm. If any of them are getting toasty, leakage current is probably well on its way to cooking the cap. One good way to check older large electrolytic power supply caps if you've got time is to disconnect one end, and then charge them up to rated voltage with a fairly good sized series resistor. For instance, if your caps are rated for 63WV, try charging them up to 60V through a 10K 1/2W resistor. If the cap can't make it up to anywhere near voltage, or the voltage starts ramping up, and then either stabilizes or goes down on its own, you're SOL -- the cap is bad. Once the cap voltage stabilizes at close to your applied voltage, read the voltage across your 10K resistor with your DVM. If it reads more than a couple of volts (a couple of tenths of mA of leakage), that indicates bad. You're saying that the PSU seems to work fine when the amplifier is disconnected, and you'd like some information on troubleshooting the PSU caps. I'm not sure I follow you. You might want to apply yourself to the amp itself, which seems to be where the problem is. It sounds like the power supply lines on the amp board are shorted out (likely a cap on the amp board?). Now you can get a start on tracking down short circuits if you've got a floating 13.8V regulated bench power supply, a DVM with a 200mV range, and some power resistors in your junkbox. If you set up something like this (view in fixed font or M$ Notepad): | | 100 ohm 3 watt | + ___ | o-----|___|-o---------------o | | , | | ,´ | | ´ | .-. | | 13.8V 1 ohm | | /+\ | | | (DVM) | '-' \-/ | | | | | \. | - | \ | o-----------o---------------o (created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de) you'll be able to make up a crude short finder, which shouldn't damage even semiconductors in the circuit. Use jumpers to clip the 2 lines parallel to the 1 ohm resistor on the probes of your DMM, as close as you can get to the tips of the probes. Now place your probes across the shorted power lines on the amp board, and move them around to try to get the minimum mV reading on the DMM. This will work down to a couple of milliohms, which might get you pretty close to where the short is, or at least narrow it down by quite a bit. The response is obviously non-linear, but it becomes most sensitive (as well as useful) at less than ten milliohms. (Note: you're not reading milliohms here, you're reading the voltage across the probes in parallel with a 1 ohm resistor. However, a reading of 0.1mV greater than the minimum reading you can get with the probes touching each other will correspond to roughly 1.4 milliohms extra resistance.) If you've got a more expensive DMM with a 20mV range, you can click down to less than two tenths of a milliohm per count at the low end, which should walk you right to the short. The advantage of this little trick is that you'll never apply more than 140mV or so to the board, so there's essentially no chance of smoking anything, even a JFET input. If you narrow it down to one area of the board, and you think a cap might be suspect, use a solder sucker to open one leg of the cap, and see if the short is still there. There are easier ways to do this, and some rather expensive instruments exist which can do some cap testing in-circuit (Sencore), but for hobbyist repair work this should be sufficient. Happy hunting Chris |
#5
Posted to sci.electronics.basics,rec.audio.tech
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How to test how good my PSU electrolytics are in an old amp?
Thanks Chris for such an excellent detailed and knowledgeable reply.
You sound as if you are in the repair/service business.... |
#6
Posted to sci.electronics.basics,rec.audio.tech
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How to test how good my PSU electrolytics are in an old amp?
On 6 May 2006 15:41:21 -0700, "royalmp2001"
Gave us: Thanks Chris for such an excellent detailed and knowledgeable reply. You sound as if you are in the repair/service business.... The best thing to do if you want another twenty plus years out of the equipment is to gather all the values up, and purchase all new 105 C units, and then replace them all. At that point, you have restored the circuit to original specs, and have increased the life of the unit ten fold over what it could be expected to last in its current state. I have a 25 year old laser disc player that most assuredly has EL cap problems. Other than that, it will be fine when I finally get around to servicing it. I have two ten dollar (thrift store) LD players, and one $30 Plays both side, does it all model (also thrift store), so I am not in a rush. |
#7
Posted to sci.electronics.basics,rec.audio.tech
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How to test how good my PSU electrolytics are in an old amp?
royalmp2001 wrote: Hi, I got hold of a 20yr old dead Adcom hifi power amp recently together with the service manual. The fuses on each +ve line have blown, the fuses on the -ve lines were ok. I have not tried the amp out - I have taken out these supply line fuses (between the psu and the main electronics), therefore isolating the power supply to the two amp boards, and checked the psu with a voltmeter and oscilloscope. The psu seems fine.... + and - 49.4V out, and all lines giving out about 10mV ripple. One rail had 20mV ripple. As this is quite an old amp, and I understand the electrolytics go first (after pots, swtiches and connectors), what tests can I do on the caps to see what kind of life I can get out of them. I can replace them, but if it is not necessary I don't want to do it. Thanks, guys. Why are you worried about the caps, when the first thing you should have been worried about if the fuses were blown are the rectifier diodes and amplifier output devices? I would check ALL the output devices for shorts with a DVM. If they all check out ok, then power the amp up. If you had a variac, you could slowly spool it up. Check for output offset and any abnormal heating of any device. In my opinion, fix what is wrong and get the amp at least working before you stray off into any modifications. Bob Bob ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
#8
Posted to sci.electronics.basics,rec.audio.tech
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How to test how good my PSU electrolytics are in an old amp?
royalmp2001 wrote:
Hi, I got hold of a 20yr old dead Adcom hifi power amp recently together with the service manual. The fuses on each +ve line have blown, the fuses on the -ve lines were ok. I have not tried the amp out - I have taken out these supply line fuses (between the psu and the main electronics), therefore isolating the power supply to the two amp boards, and checked the psu with a voltmeter and oscilloscope. The psu seems fine.... + and - 49.4V out, and all lines giving out about 10mV ripple. One rail had 20mV ripple. As this is quite an old amp, and I understand the electrolytics go first (after pots, swtiches and connectors), what tests can I do on the caps to see what kind of life I can get out of them. I can replace them, but if it is not necessary I don't want to do it. Thanks, guys. Electrolytics are probably not your issue, although they do age. It sounds like you have some sort of short on the B+ line. With the fuses out of circuit, compare the resistance to ground of load side of the +/- rails. My guess is that you're gonna find a shorted output device.... jak |
#9
Posted to sci.electronics.basics,rec.audio.tech
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How to test how good my PSU electrolytics are in an old amp?
Bob Urz wrote:
royalmp2001 wrote: Hi, I got hold of a 20yr old dead Adcom hifi power amp recently together with the service manual. The fuses on each +ve line have blown, the fuses on the -ve lines were ok. I have not tried the amp out - I have taken out these supply line fuses (between the psu and the main electronics), therefore isolating the power supply to the two amp boards, and checked the psu with a voltmeter and oscilloscope. The psu seems fine.... + and - 49.4V out, and all lines giving out about 10mV ripple. One rail had 20mV ripple. As this is quite an old amp, and I understand the electrolytics go first (after pots, swtiches and connectors), what tests can I do on the caps to see what kind of life I can get out of them. I can replace them, but if it is not necessary I don't want to do it. Thanks, guys. Why are you worried about the caps, when the first thing you should have been worried about if the fuses were blown are the rectifier diodes and amplifier output devices? I would check ALL the output devices for shorts with a DVM. If they all check out ok, then power the amp up. If you had a variac, you could slowly spool it up. Check for output offset and any abnormal heating of any device. In my opinion, fix what is wrong and get the amp at least working before you stray off into any modifications. Bob Oops, Bob...I gave him (some of) much the same advice before I read this post.... jak ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
#10
Posted to sci.electronics.basics,rec.audio.tech
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How to test how good my PSU electrolytics are in an old amp?
Thanks Jak,
This design is a dual monoblock config...separate psu transformers, etc per channel. Resistance of +ve line to ground was about 68K on either channel, and was 600K from -ve to ground on one channel and virtually open circuit on the other channel. I think that seems normal. I'll checkout the output devices also. |
#11
Posted to sci.electronics.basics,rec.audio.tech
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How to test how good my PSU electrolytics are in an old amp?
royalmp2001 wrote:
Thanks Jak, This design is a dual monoblock config...separate psu transformers, etc per channel. Resistance of +ve line to ground was about 68K on either channel, and was 600K from -ve to ground on one channel and virtually open circuit on the other channel. I think that seems normal. I'll checkout the output devices also. Nope...you've got a problem. Assuming identical left and right channels, one would have to assume identical readings from channel to channel. jak |
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