AA-1800 amp failing

[rrcrain]

One of my two amps sounds like it's starting to fail. With nothing else turned on, it's starting to produce "thumps" and the protection circuit is tripping. Is there any hope this might be caused by the original caps starting to fail or is it more likely something worse is failing?

[Pete B]

One of my two amps sounds like it's starting to fail. With nothing else turned on, it's starting to produce "thumps" and the protection circuit is tripping. Is there any hope this might be caused by the original caps starting to fail or is it more likely something worse is failing?

The first thing to do is to replace all the electrolytic caps. You can probably

leave the main computer grade caps for now. You could bypass them with

small 1000 uF caps just to test if the ESR has gone up, however this would not

find leakage issues or internal arcing. Look for any components that

run hot, even resistors if they look like they are cooking or the PC board is

discolored underneath replace them. When/if you recap it look for cold solder

joints. Any solder joints to components that run hot are more likely to fail.

Are there numerous connectors in the amp? Hit them with Deoxit if so.

If there are supply line fuses I would drop them way down to say 1A while

you are working on it. If not you could use the old series light bulb trick to

limit current while debugging. You really don't want to wipe out a bunch of

expensive output transistors. A very standard thing to do is tap around with

a plastic rod such as a ball point pen case, see if you can make it pop, narrow

it down to the exact component and it will either be a solder joint or obviously

the component itself. It could be a semiconductor failing, really anything.

Connections and electrolytic caps are the least reliable parts.

I have seen hairline cracks in PC boards that opened up lands, another thing

to look for. Is it temperature sensitive? Cold can might help you narrow it

down or a heat gun not too close and on a low setting.

[Pete B]

http://www.diyaudio.com/forums/solid-state...-amplifier.html

There are a lot of techs on DIYaudio who might have experience with this amp.

[rrcrain]

http://www.diyaudio.com/forums/solid-state...-amplifier.html

There are a lot of techs on DIYaudio who might have experience with this amp.

I found the time to pull the amp out of it's rack mount yesterday and discovered several things.

First and most annoying is my wife and I can't find the manual or schematic. I agreed to give her another week to hunt for it before ordering a copy. I'll not pass judgement on why I think it's not where I remember it being left.

Offset voltage, as expected was extremely unstable in the left channel and bias was around 70mv, double what it should be.

I liberal application of contact cleaner freed up the pots on the iinput and mostly stabalized the offset. The same treatment to the offset and bias pots, as of last night, appeared to resolve the remainder of the problems. I'll know with a bit more certainty tonight when I power it up again cold after everything has evaporated.

I think one offset pot is on it's last legs for various reasons. If I'm right, it's probably contributing to the problem. (left channel of course)

None of this addresses 30+year old caps so that will be my next goal after I find my old manual or order a copy.