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Old 15th May 2015, 11:12 AM   #1
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Default Troubleshooting stereo amplifier

Hey,
I have a vintage 70s home stereo amplifier with built in preamplifier and FM tuner/receiver.

Problem with the amp is: left channel is weaker than right channel, moreover there's loss of bass too. Problems concern only the left channel, since right sound pretty much fine to me.

I'm an absolute beginner as an electrician, but with the help of several discussion topics and online guides to stereo repairs I have managed to revive some of the left channel, by recapping a few on preamplifier board. Now left channel have perhaps half of the power present on right channel. Balance control among other controls responds well.

I have used DMM and a extremely simple oscilloscope put together from old headphones to track down the possible culprits.

Now I'm pondering on whether I should just recap the whole left channel on the preamp and amp boards totally without any more testing, since it takes time as I'm no professional on this. I have detected that the voltages on preamp board (audio) exit points for left and right channel to amp board have about the difference you hear on the speakers, so I should focus on the preamp board alone? Makes me curious also, if the culprit could still be on the amp board and low voltage reverts back to preamp board exit points, is this possible?

Are there any other components apart from caps that may lead to loss of power on audio channel?

I know from the several guides that I should always pop out component from a pcb to accomplish a reliable test, but is it possible that on the board component resistance is the same on the both channels circuits, but component could be still broken?

Tips are appreciated
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Old 15th May 2015, 11:18 AM   #2
Mooly is offline Mooly  United Kingdom
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You need to do more conclusive tesing than replacing parts in hope. The problem could be physical, a problem control, switch or pot.

There are simple tricks like simply paralleling the inputs to the two power amps. That tells you in an instant which half is faulty, front end or power amp.

If DC voltages are different between L and R boards then that points to a problem that should be easy to find.

It would help to know the make and model and to see the circuit diagrams.
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Old 15th May 2015, 03:36 PM   #3
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Quote:
Originally Posted by Mooly View Post

If DC voltages are different between L and R boards then that points to a problem that should be easy to find.

It would help to know the make and model and to see the circuit diagrams.
Here are the schematics available for download:
Arena

I believe amp works on AC, if I understood correctly.

Oscillating the preamp board I noticed there's almost no bass at all on left channel bass pot test point, while the right sounds very rich in bass. I believe the pot is working correctly determining this by increasing bass and turning balance between channels there's more bass to both channels.
I placed an order for new set of PP film capacitors for preamp board. Whether these are the culprits I understood PP will increase the sound quality anyway.

I think there are no other means to check further the components else than removing them from board? Solder looks ok to me as far as I can tell.

Oscillating preamp shows major loss of power straight after cap C38. I replaced the 10uF and no difference. Testing resistor R64 resistance to exit points from C38 on board both (R64 & R65) check out fine. Any hints where to look next?
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Old 15th May 2015, 07:32 PM   #4
Mooly is offline Mooly  United Kingdom
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I'll try and take a look tomorrow. Lack of bass (assuming its nothing physical) points to a dried out electrolytic cap in the signal path or the feedback returns. You can just tag a replacement across any suspect ones as a quick test, no need to remove the old one.
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Old 16th May 2015, 06:55 AM   #5
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Originally Posted by Mooly View Post
I'll try and take a look tomorrow.
Thanks, really appreacite it.

I think would be so much easier to find totally dead component than hunting down the cause for uneven output.

According to schematics for left channel, I've replaced caps: C26, C34, C38 and oscillation gave me a reason to replace C24. This was a faulty cap, but others were okay. All of them are 10uF 35V. C24 had only 1.25uF capacity left, while the others were near 10uF as they should be. This restored some of the output for left channel.

I tried to tag across film cap C28 and it certainly gave more bass to left channel. Almost that could be the culprit for the loss in bass on left channel.

DMM measurement gives me equal output for both channels right before R48 and R49. However output after R48 is just around 5mV, and right channel after R49 there's around 15mV present power on. Here one would assume there's something to do with film caps or bass control pot?

During the tag across film cap C28 gave around 4-5mV more to the left channel after R48, but channels are not equal even if C28 is culprit.

Power off and tested resistance for R48 and R49 onboard and there are pretty much same readings, no major difference. Bass control works fine as I posted before. This would make me believe bass pot is working correctly.

Is it possible a resistor could be the one to blame? I just read about resistors, that they are not very prone to fail....
Further on the circuit there's equal voltage on both channels again just before caps C37 and C38, but after C38 there's major loss of voltage and output.

I have been pondering on whether it could be a hair crack in a solder joint or something... I just don't know what to do next other than replace the rest of the left channel caps, there aren't many left...

I believe I can pick up the new components late next week, when they arrive. Here I don't have a good supply for new components, so everything must be ordered.

Thanks for any further help.
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Old 16th May 2015, 07:12 AM   #6
Mooly is offline Mooly  United Kingdom
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It sounds as though you may have a combination of more than one problem part, probably caps. You have found one that was faulty... and its not fixed it... so that's a pretty good sign there will be more.

Its not a very clear diagram to follow.

You could try isolating power amp input and linking it to the other channels feed. That would prove if there was a problem in the power stages.

I suspect you will need to recap this completely, the speaker coupling cap could have lost value too.
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Old 16th May 2015, 08:17 AM   #7
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Originally Posted by Mooly View Post
Thanks for the quick reply and tips.
Its not a very clear diagram to follow.

I suspect you will need to recap this completely, the speaker coupling cap could have lost value too.
Yeah, for a beginner like myself now try to pinpoint from that diagram the components to the board. I'm lucky I got this far with this with so little experience. I haven't tried reading on any diagrams before this.

Initially I started repairing on the amp, when 2-3 weeks ago amp started humming and crackle, so the first thing I replaced were the big caps 2x output stage 2000uF and 2x power stage 2000uF. This restored the overall power to the amp and removed all the hum and crackle. Now would sound great, if both channels operated at same level.

Before this I have driven the amp with 2 speakers both connected on right channels, since the amp supports 2 set of speakers. So maybe that contributed to the fact now it's not working correctly, who knows...
Now that I managed to restore the overall power I'm determined to fix the left channel.

The idea of linking right channel to left power output stage is a great idea. However I'm planning to recap left output stage. There's thin copper wire from the preamp exit point going to output stage and would be a bitch to get it back on the solder joint once if I'd break it loose with my current tools. Anyway I will use this tip and find a solution, if recapping left output stage brings no change.

I have a question: do you think the caps before bass pot (R48) could be in perfect working order since the voltage levels equals on both channels until that point? Would narrow down the search.
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Old 16th May 2015, 08:30 AM   #8
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Running two sets of speakers won't have done any harm.

I've not got the circuit in front of me just now but the millivolt differences you measure are probably just caused by tiny leakage currents and slight normal variations in the DC conditons between the channels. If you measured say 6 volts on one transistor collector and 2 volts on the other channel, then is the time to start investigating.

99% certain your problem will be cap related from what you have described. All the electrolytics really need replacement (but its nice to find the problem parts/s by proper methods first).
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Old 16th May 2015, 09:07 AM   #9
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Running two sets of speakers won't have done any harm.

99% certain your problem will be cap related from what you have described. All the electrolytics really need replacement (but its nice to find the problem parts/s by proper methods first).
Thanks a lot for your expertise and quick replies, helped me a lot.
I'm also confident it's cap related issue from all of I read from guides, but I'm no expert so..

I'm waiting for the new components, so then I will recap the rest of the board and get better idea what to do after that, if it's still no help.
Also gonna replace the caps for the right channel, there are just maybe less than 10 caps in total on output stage and less of them left to replace on preamp board.

I'll get back to post my findings after recapping.
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Old 16th May 2015, 10:46 AM   #10
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That sounds like a plan. Just take it slowly and make sure you orientate them correctly.
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