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high DCv on the backs of electrolytics in Adcom amp
high DCv on the backs of electrolytics in Adcom amp
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Old 8th November 2019, 03:57 PM   #1
studiophone is offline studiophone  United States
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Default high DCv on the backs of electrolytics in Adcom amp

I recently repaired an Adcom 5802 amp. It had a blown output transistor. I replaced it and all the electrolytic caps and Zener diodes as well as any cap that I could replace with a Wima and matched input mosfets… 555 timer …relay … tested all the other transistors… tons of stuff.

This is my first time doing something like this. I’m learning a lot. I made a lamp limiter and hooked it up last night. 100w was not enough for the relay so I went to 200w. That worked. No smoke, no noise and the bulb is a faint glow after the initial power up. I set the offset to near 0 on each channel and have not messed with the bias yet, it’s still original. I let it warm up for 20 min. and so far so good.

In probing around with a meter (DC) I’m getting some high voltages on the metal tops of some of the electrolytic caps. Namely the 47uf (+100v) on the input board and the 100uf on the main amp board. (one is +170v on one channel and +70v on the other, strange that they are different…) I’m not sure if I should be looking at the back of these caps but my question is does this sound right to you? The voltage is over the rating on the cap.

My next step is to see if there is high voltage on the input XLR jack and if not try it with some signal into speakers but I need some advice from those who have seen this stuff. This could just be trepidation after all the work…

Second question: is it OK to do an audio test with the lamp limiter? (200w bulb I also have a 300w)
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Old 8th November 2019, 04:05 PM   #2
rayma is offline rayma  United States
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Take out the bulb, it's only for the initial power-up, in case of severe problems.
You'll have to rebias the amp now also.
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Old 8th November 2019, 05:49 PM   #3
studiophone is offline studiophone  United States
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ok good to know about the lamp limiter. thanks. as far as the bias I forgot to mention of course I'll set that before I hook it up, I ran out of time last night. You think that has anything to do with the big voltage on the body of some of the caps?
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Old 8th November 2019, 06:00 PM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by studiophone View Post
You think that has anything to do with the big voltage on the body of some of the caps?
The capacitor's can is not at a defined potential, so avoid measuring or touching it. The power supply
has +90V and -90V for the output stage, so the maximum you could measure is between those two
points, or 180VDC. Use only the proper chassis ground for a 0V reference for your meter.

Last edited by rayma; 8th November 2019 at 06:07 PM.
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Old 9th November 2019, 06:32 PM   #5
PRR is offline PRR  United States
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The electrolytic cap's can is typically (not always) at the same potential as the "-" leg of the capacitor. That's the simplest way to make them.

If the amp covers are on, this is "safe".

If the caps are exposed, some makers order the caps with paper or plastic covering all exposed surfaces.

90V is not safe to touch but rarely kills. There are some tube amps with caps stacked-up for higher total voltage rating. The can of the "upper" cap can sit at +295V (Ampeg VT40). This really stings!! And can kill. And was semi-exposed in use, right-there when re-tubing. These caps had heavy paper shells and insulated mounting plates. Later techs who do not understand sometimes 'refresh' with ordinary caps with shell exposed, ZAP.

While your hands are in there: the case and mounting screws of many power transistors are at circuit voltage and often high (like 90V). Occasionally they are mounted on heatsinks with no insulator and the whole heatsink is "hot"! This is a convenient place to rest your hand and it has bitten me a couple of time.
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