Old stereo amplifier, right channel gives weak signal and keeps disappearing

[giltas]

Hey folks, I've been a bit into electronics lately (built a few guitar effect pedals) but not much other than that. Recently I've purchased a turntable which I connected to an old amplifier (I think it's from the 90's- possibly 80's). The turntable is connected to the amp via phono connections. When I connected it for the first time it didn't play a sound at all. After playing a bit with the connectors (spinning them around, in and out, etc.) the amp started working, however one channel (right one) didn't play at all. I opened the amp and when I touched the phono preamp curcuit (the back side of the transistors and capacitors) with my finger, the speakers produced a low frequency 'boom' sound and then the music started playing for a while and stopped. This repeated itself every time I touched the circuit. shaking the amp or hitting it (not to harshly of course) also makes it work sporadically.
I finally unscrewed the entire thing and sprayed everything with a tuner cleaner, and polished off the corrosion from the connectors. After that the amp started playing well (for a couple days) but now it spontaneously stops producing sound from the right speaker (or both of them). When it does work, the right channel is still much weaker then the left one so I have to rotate the balance pot almost all the way to the right.
I guess that this is mostly a dry joints problem but my knowledge in electronics is very poor. Will supply pictures if needed.
Thanks!

 

[duke37]

You have two channels which should be similar.
Get a piece of wire with a crock clip on each end and connect across similar points. I would start at the volume controls. If the output is different, then there is a fault after the volume control. You can also connect the two inputs together to check the input plugs.
It is a matter of logic.

 

[KrisBlueNZ]

Your description of touching a point in the circuit and getting a boom and "waking up" the circuit sounds like something I've seen a few times, due to a faulty transistor downstream of the point you touched. It's also possible that shaking or banging it could also cause a faulty transistor to start and stop working. But a dry joint is a possibility too.

You may be able to find a dry joint by brushing the track side of the board with a stiff brush such as a toothbrush, or gently wiggling components from above.

Yet another common problem that can cause these symptoms is dirty switches, and possibly even dirty potentiometers. You can test them individually by gently changing their positions, then for switches, bringing them back to their original position.

Some photos would help to give us an idea of the kind of technology used. Sometimes this can suggest other likely or possible types of failures.

 

[giltas]

Thanks guys. I'll open the amp again tomorrow so I can take pictures and throw a look in and try to fix it.

Kris- I don't think it's related to transistors as there are no transistors in the phono amp channel (there are diodes though). However, when I play with the volume pot it can trigger the channel to "wake up" so it's possible that it is a dirty pot. I did try to clean in but perhaps the spray didn't go deep enough.

oh yeah and I found the service manual here: http://www.eserviceinfo.com/download.php?fileid=44110

 

[Jimmybravo]

i think you have a faulty volume control,due to the age of that amplifier,volume controls carbon inside usually runs out,sorry but im not good in english i hope you understand my reply, goodluck bro

 

[Jimmybravo]

also check the variable for balance if any..