Quad 34 Solid state Preamp Background noise common fault ?

[blindboybenton]

Hi

I don't normally do hifi unless its valve. Had a returning customer give me a quad 34. Reported fault Background noise. Seems to be worse on the right channel
Without any siginal you can here what seems to be like a radio not tuned in and sort of noise drifting in and out.same with any input selected
He said that this is a common fault ?

Ive found a few faults. Leaking caps changed about 6 x 100uf caps

Does anybody no of this fault, Is is common ?

Just hoping for some inside info before i sit down a try to chase the fault

BBB

Happy new year to you all !!

[Enzo]

You have noise in a channel, and apparently white noise or similar. Everyone wants everything to be a "known issue" or "common problem". But life doesn;t give us a look up list of answers.

In my experience, such noise is USUALLY a bad semiconductor, in your case, probably an op amp. But you also have the transistor input stages as well as the signal switching which appears to be a bunch of 4066 CMOS chips. Those are semiconductors too. Likewise, in my experience, weak filter caps do not make hiss, they make hum.

Your amp is full of TL071 op amps. There is no reason to think one particular one would go noisy every time, so if one guy has IC8 go noisy, there is no particular reason to think the next guy with noise has the same one bad.


As to being common, sure, I can;t think of any amp circuit that cannot have a part go noisy and result in noise. Happens all the time, on any kind of amp or other signal path item.

The thing to do is simply troubleshoot the noise. it is on one channel, so most of the rest of the amp is innocent. Turn the controls up and down. ANy control that affects the noise itself is AFTER the source of the noise. Once narrowed down to a section, follow the signal path with a signal tracer or a scope and see where the noise comes from.

[blindboybenton]

Hi Enzo

And happy new year too you. I changed the caps as a matter of course as they were leaking over the board> Was hoping that its was a common fault and some inside info would help
Short cut i know , But i work alone and im busy, which is strange for after xmas.

BBB