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JanErik 13th September 2010 07:39 AM

Amplifier troubleshooting, one channel weak
I am trying to fix my trash-found soviet Odisseij; one channel is very weak (or the other very strong?), something like 1/3 of the amplitude without load. The output signal looks good on the scope, both half waves are present and it is not distorted.
So far I have measured that both channels are equal after the input capacitors. The output stage is running a bias current as it should.

I have started to trace the PCB and measured semiconductors along the way; all power devices seem OK. Where should I start looking, what determines the gain of a typical class AB amp?

There are a few electrolytics of course, but can they cause this kind of problem?

nigelwright7557 13th September 2010 01:52 PM

I would guess the feedback resistors into the LTP will set the gain.

sregor 13th September 2010 03:58 PM

Check the gain - most audio amps have gain of 20 to 40. The feed back circuit determines the gain - normal circuit is a resistor going from output to base of the inverting input (one side of LTP) from there it usually goes through another resistor to ground via a small electrolytic. If this cap is open, gain drops to 1. Hope this helps. Good luck.

JanErik 14th September 2010 06:58 AM

OK, so that would be C3 in the P3A, 60-80W Power Amplifier ?

For the curious, here are some pics of the relic:

Weighs around 15 kg, PSU caps hold +-50 V and that is regulated down to +-35 V.

Mooly 14th September 2010 08:05 AM

Yes C3

Can you scope the signal on the volume control as that should tell whether the problem is in the pre or power stages. You say you measured "after input caps"... not quite sure where you mean... there could be many caps in the audio path.

Can you identify the audio inputs to the power amp boards ?
Sometimes problems can be physical... cracked print around the front panel controls.
There also appear to be attenuators on the front panel... faulty switches maybe ?

I remember an amp made by "Mashpriborintorg" ???? many many years ago called the "Odyssey" from around 1978

JanErik 14th September 2010 08:49 AM

The input caps I refer to are on the power amp board, preamp completely disconnected when testing. The DIN jack on the back below the outputs goes directly to the power amp and are obviously intended for use with an external preamp.

Mooly 14th September 2010 06:40 PM

That confirms the fault is in the power amp section then. Very difficult to advise without a circuit diagram or actually seeing the thing. As others have stated, the gain is set via the feedback network... but it's not unknown for power amps to have the tone controls etc withing there feedback networks.

Did you manage to locate the cap that was mentioned ?

firechief 14th September 2010 06:55 PM

Check the back of the small vertical board in Pix #3. Looks like something got hot.
Good Luck

JanErik 15th September 2010 07:04 AM

The vertical board is some sort of clipping indicator (drives the two LEDs on the front), disconnected now too when testing and does not make any difference when plugged in. But it only looks like it has been hot in this pic, some shadow.
And the tone controls are all in the preamp (behind the front panel), all disconnected now.

Guess I'll get back to it in the weekend.

JanErik 8th October 2010 05:41 PM

I have now traced the circuit this far:

Guess most of the interesting part - the feedback - is still missing, but the prime suspect is there, C14. There are no more electrolytics than there four.

What do you think about the circuit in general? Seems to be a fair number of diodes as voltage droppers and more current sources/sinks that I have usually seen.

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