Amplifier troubleshooting, one channel weak

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JanErik

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Joined 2010
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?
 
S

sregor

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Joined 2006
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.
 
Mooly

Mooly

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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
 
Mooly

Mooly

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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 ?
 
J

JanErik

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Joined 2010
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.
 
J

JanErik

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Joined 2010
I have now traced the circuit this far: http://users.abo.fi/jskata/odisseij.pdf

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.
 
Mooly

Mooly

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Comparing is probably the best way forward here. DC conditions should be virtually identical between channels... and if so almost rules out any semiconductor problems. It's not easy tracing out the circuit visually, and without a full and complete 100% accurate circuit it's very difficult to offer anything other than general advice.
 
L

LAJ

Member
Joined 2008
n/a
Do you have any DC voltage on the output? If not, does it sound like the highs are maybe OK but the lows are gone? Then it is a coupling capacitor. Make a "Y" and feed both channels a signal and check corresponding points between the 2 channels with a scope. How old is the amp? If its 10+ years old, most likely it needs new capacitors. And don't forget, you may have more than one bad. Its not difficult to troubleshoot an amp without a schematic as long as one channel is working.
 
J

JanErik

Member
Joined 2010
astankov said:
There is the full circuit:

Hotfile.com: One click file hosting: ??????? ?-010.pdf

Power amplifier , Preamplifier and Power Supply.

I hope you understand russian. If you have other questions just ask.

Best Regards. Andrey.

EDIT: I just saw that there is a trimmer for the gain on one channel and probably there is the problem.
Click to expand...

Thank you very much! This is very hard-to-find info, did not even believe this was any longer in existance except for in some Ukranian dumpster! Do you think it is OK if I put it on my own webspace so others can download it easier without this one-minute wait and captcha?

I know a bit Russian but not many technical terms...
I have seen there is a tiny trimmer in the middle of the board but thought it had something to do with the clipping indicator. Did try to fiddle with it with no noticable change. Not yet traced it in my diagram. The other two trimmers set quiescent current and output offset.
 
J

JanErik

Member
Joined 2010
Found some pics and data now as I got the model number clear;
Ñòåðåîôîíè÷åñêèé óñèëèòåëü ''Îäèññåé Ó-010 ñòåðåî''.

Incredible that it is only specced to 50 W @ 8 ohm, still weighs 16 kg. If I understand correctly, it is 0.05% THD.

The same schematic is also found here: http://mexan1k.info/info/images/6/62/Odissey_u010s_pasport_19.jpg

Some manuals:
http://http://mexan1k.info/info/index.php/Одиссей_У-010_стерео_(паспорт)http://mexan1k.info/info/index.php/%D0%9E%D0%B4%D0%B8%D1%81%D1%81%D0%B5%D0%B9_%D0%A3-010_%D1%81%D1%82%D0%B5%D1%80%D0%B5%D0%BE_(%D0%BF%D0%B0%D1%81%D0%BF%D0%BE%D1%80%D1%82)
Sorry the cyrillic letters don't work on my computer but the links seem to work.
 
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L

LAJ

Member
Joined 2008
n/a
Most stuff here in the States is Chinese now and I can tell you Russian is easier to read than Chinese. I doubt the trimmer is the problem. Be careful that its not the bias pot. You don't want to blow up your output stage. And I think the reason its inverting is so if the output gets coupled back to the input, it doesn't self destruct.
 
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