Is the brown stuffs glue?

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Is the brown stuffs glue? It kind of looks like the liquid from electrolytic capacitors, except there is no electrolytic capacitors around it.

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Have you measured for a DC offset ?

Whether the glue is an issue or not is impossible to say without you faultfinding on it but glue has been an industry wide issue in older equipment (more so TV/Video than audio).

It wouldn't be the first suspect... you need to do some basic checks and see why the relay is tripped if that is the problem.
 
I have managed to isolate the problem to one channel, but I have not been able to measure the DC, the spot is hard to reach without taking everything apart.

For some reason, the amp starts instantly now. I have not been able to make it not start at the moment, so I can't make any measurement until the problem comes up again.
 
The amp is a about 20 years old now, the problem only shows up often recently (only rarely in the past). Would a bad joint worsen only after all these years? My guess is it's an age related problem, I thought it was electrolytic capacitor drying up, but not so sure now.
 
"Would a bad joint worsen only after all these years? "
Possibly. This is one of those things that is impossible to predict. It is unlikely to get better by itself.

"My guess is it's an age related problem, I thought it was electrolytic capacitor drying up"
Why did you think it was an electro? Something you have read?

I am afraid guesses and speculation are unlikely to help you diagnose the problem. Measurements and logic are. Help comes from freezer spray and a heat gun (provoking track breaks etc), and tapping with a chop-stick.

You said earlier that you could not reach the channel output. This would be the same as the rear panel amp output (until the disconnect relay opens!)

Be prepared to take some things apart, attach (solder) some wires to difficult to reach points, connect your meters / 'scope and wait it out.

Very hard without a schematic to advise further.
 
When the offending channel is disconnected, the relay turns on. I think it's safe to say that channel has DC on its output. You can't measure DC at the binding post, when there is appreciable DC the relay would instantly disconnect.

It work well for the past 20 years, and then the problem came on quite often lately. Age related problem is not much of a wild guess isn't it? Typical electrolytic capacitor is rated at 2000 hours at 85˚C. 1 hour a day for 20 years would put it at close to 8000 hours. Often I even run it upwards of 10 hours a day. Even if I don't run it at all, 20 years is still a long time for electrolytic capacitor. Sure there are many amps that has older electrolytic capacitors running in them, but if they happen to fail now I wouldn't call it premature at all.

Schematic:

ScreenShot2013-06-28at123529AM1_zpsc74e6cb6.png
 
Then you seem to think you know what you are doing, so why are you asking??

It would surprise me if your amp is operating at +85C. Or ANYWHERE near that. This is important as the life expectancy extends by many times as the temperature is lowered. It is the heat that dries them out.

If you want to go for the current fashion of wholesale swapping of electros without knowing their condition, then go for it.

As for me, I would open it up apply some test connections and make some measurements.
 
I must sound like I know what I am doing :D, but not really.

As I said, I am not so sure about the replacement of electrolytic capacitor. Unless there are obvious signs of damage, I am inclined to think that it is not the problem, although I can not rule it out. I don't mind replacing them, if it works, but I wouldn't want to spend money on new caps for a dead amp that I can't fix.

And I can only see what is wrong when it happens again, for now it seems to work properly.
 
OK.

It could be anything, most likely a bad solder joint, connection or PCB track.

That is why I hinted at using freezer spray when it is switched on and working and monitoring points. Disconnect any loudspeaker load and use a resistor.

But it is not possible to (at least I can't!) go into precise detail in the general case.

I have fixed hundreds of amps over the last 40 (yuk!) years and there was no common factor:

Leaking caps, diodes, transistors.
Bad solder joints, connectors dirty or filmed.
Surface gunge on the PCB

The least you can do at low risk is:

unplug / clean / reseat any connections
Use magnifier and bright light and resolder any suspect joints (cracked, dull etc)

I'm sorry it is not so simple.
 
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It may not be convenient to look for DC on the output when binding posts are disconnected. But there are the 0.22 ohm ballast resistors in the emitter leads of the output transistors. Those are usually accessible, and are connected directly to the output buss on the amplifier side of any speaker relay. SO the leads of those 0.22 ohm resistors are a convenient place to check.
 
Unfortunately the ballast resistor is also inaccessible (the 5 channel of amp boards are tightly stacked together), the most accessible part is the input wire on the relay board, but there are so much wires on it.

If it happen again, I'll take it out, clean it and reflow the solder joins.

Thanks guys for all your advices. :)
 
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