high DCv on the backs of electrolytics in Adcom amp

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studiophone

studiophone

Member
2018-03-05 3:17 am
Milwaukee
www.studiophon.com
  • #1
    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)
     
    R

    rayma

    Member
    2011-04-29 8:37 pm
  • #4
    studiophone said:
    You think that has anything to do with the big voltage on the body of some of the caps?
    Click to expand...

    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:
    PRR

    PRR

    Member
    Paid Member
    2003-06-12 7:04 pm
    Maine USA
  • #5
    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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