Preamplifier Troubleshooting

[audioferret]

Greetings,

I am trying to determine what is wrong with my preamplifier or Amp. I have a circa 1978 Toshiba C15B Preamp and a M15B 45watt amp that I picked up and have been using for the last two months. Until now, no problems.

Recently, the right channel (and sometimes the 2nd R+L Pre-Out) have stopped working. Sometimes, they are fine, but then they will inexplicably cut out. Sometimes, they won't work even from the point I turn the preamp on. I have tried running my CD input directly to the amp and I have had no problems (no gain control either). I have noticed that this problem seems to be linked with the level of heat.

I am looking for a good guide on troubleshooting the Preamp. I suspect that the capacitors are reaching the failure point and am considering replacing them. What should I do?

[Neutron Bob]

Don't let this problem get the best of you- remember the key is that you have a stereo preamplifier here. There's a working signal path to compare against, in the other channel. Looks like a wee bit of exploration with the oscilloscope may point you to the trouble!

Bob

[gmphadte]

One of the problems could be damaged solder points due to long use. Inspect the solder points, there may be round cracks in them. Resolder if so

Gajanan Phadte

[wintermute]

If the RCA's on the back of the preamp are mounted on the circuit board check that the joints haven't cracked... I've found faults before where the slightest vibration can make the difference between working and not. Try wiggling the leads when the problem occurs to see if it comes good and cuts out depending on the flexing.

You can also try flexing the board to look check for dry joints (a plastic pen pressed on the board at various points works well) If the problem rectifies when you flex it, you likely have a dry joint, finding it can be a bit harder but a magnifying glass will probably help

As bob said you have one working channel for reference, if you don't have a scope the next best thing is a multimeter, check voltages at various points in the circuit and compare noting any major differences..... differences of maybe up to 10% or so could just be due to component tolerances.

Tony.

[anatech]

Hi audioferret,
If your preamp uses mechanical switches, operate each one slowly. Listen for intermittent operation. Rotate controls slowly and listen for crackling.

If you find a noisy control, clean it with a proper cleaner De-Ox-Id from GC, or DeOxid from Caig both work well. No tuner cleaner, no Radio Shack stuff. Use real, proper cleaners. Use as little as possible to get the job done in each control or switch affected.

You can usually find intermittents by tapping lightly on the PCB with a non-conductive object - not a rubber mallet! Resolder carefully using solder for electronics only, and a real soldering iron, not a soldering gun. Remember to unplug the unit first, I'm not trying to be smart as I've seen this happen.

-Chris

[anatech]

Check for muting relays also. Sometime the contacts become oxidized.

-Chris

[mastertech]

sounds like a bad connection or a faulty chip in the preamplifier
make sure/check ps dc supplies are rock solid down to the section
you suspect to be faulty

cheers