Amplifier repair

Thread Starter

lubnaan90

Joined Jan 13, 2010
198
Hello All ,

Just bought a Classic Soviet built Hi-Fi amplifier (stereo) today, just for the sake of my hobby of collecting classic power amplifiers.
A very powerful one indeed, Now the problem is That, output sound from the Left channel is very low (I mean very Low) when compared to the right channel output.

The amplifier when opened up , has three Big circuits , First is the Input circuit , second is the filter circuit (it has the treble & Bass & loudness control) , & the third one finally is the amplifier.

As per my inspection of all the three circuits, the problem lies some where in the Filter Circuit, since i checked the Input circuit & the amplifier Circuit They are working fine.

So, if any one can help me out to solve this Problem, i will deeply appreciate it.
 

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beenthere

Joined Apr 20, 2004
15,819
If you have a meter, one classic method is to apply a waveform from a signal generator to both channels inputs, and look for differences in the left/right circuitry. An oscilloscope with dual traces is even better for this.

You can also check for DC voltage differences at selected points. Or just examine for discolored components. If it is in the tone control, it's probably a Baxandall circuit. You could also have a bad level pot.
 

Thread Starter

lubnaan90

Joined Jan 13, 2010
198
In the picture attached, which is the Filter/tone control board, it has two detachable circuits, one for the left channel & the other for the right channel.

I tried switching the circuits from their own slots, Now in place of the right channel detachable circuit, rests the detachable circuit which was for the left channel & Vice versa.

The observation now is that, right channel has very low sound output & left channel is working fine.
Note: Earlier the problem was in the left channel & now the same problem in the right channel after exchanging the detachable circuits & the left channel now is working fine, so its obvious that problem lies in the one of the detachable circuits.
 

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studiot

Joined Nov 9, 2007
4,998
You have quite a few electrolytic capacitor cans on those boards.

75% chance it is one of these. Look for an emitter bypass one gone open circuit.

25% chance a transistor has failed.
 

Thread Starter

lubnaan90

Joined Jan 13, 2010
198
You have quite a few electrolytic capacitor cans on those boards.

75% chance it is one of these. Look for an emitter bypass one gone open circuit.

25% chance a transistor has failed.
Thank you for your reply ,

What about the Resistors & the Ceramic capacitors ?
 

Mike33

Joined Feb 4, 2005
349
Resistors and ceramic caps don't have a lifespan (not measurable by human years, anyway....); electrolytic caps do. After anywhere from 10 to 40 yrs., they dry up and become less efficient (noisy), then finally die. A bad bypass cap will drop the gain, hence a lower signal level, so studiot's observation is right-on! Now, if something shorted, a transistor could still be blown, too, but look at the caps 1st.

And, expect the others to do the same at some point in the not-too-distant future.
Good amp maintenance requires replacing ALL of them every 10-20 yrs., LOL. Not fun, you might opt to just do the one at a time route, it's up to you!

This is the point where you might try audio probing (or scoping) that board, listening to the signal (comparing w/working board). The stage it drops at will be where the bad cap is. Be careful due to possible presence of high voltage!!!!!!!!
 

Thread Starter

lubnaan90

Joined Jan 13, 2010
198
Resistors and ceramic caps don't have a lifespan (not measurable by human years, anyway....); electrolytic caps do. After anywhere from 10 to 40 yrs., they dry up and become less efficient (noisy), then finally die. A bad bypass cap will drop the gain, hence a lower signal level, so studiot's observation is right-on! Now, if something shorted, a transistor could still be blown, too, but look at the caps 1st.

And, expect the others to do the same at some point in the not-too-distant future.
Good amp maintenance requires replacing ALL of them every 10-20 yrs., LOL. Not fun, you might opt to just do the one at a time route, it's up to you!

This is the point where you might try audio probing (or scoping) that board, listening to the signal (comparing w/working board). The stage it drops at will be where the bad cap is. Be careful due to possible presence of high voltage!!!!!!!!

Thank you for your reply ,

I get your point ,
The amp having a manufacturing year of 1985 :) , i think it definately needs replacement in Electrolytic capacitors , as for the transistors i checked them , they are fine.
 

Audioguru

Joined Dec 20, 2007
11,248
I have an American made HH Scott FM stereo receiver that was made with good quality parts in 1964 and it still works perfectly. I replaced its dial light bulbs a few times.
 

Thread Starter

lubnaan90

Joined Jan 13, 2010
198
You have quite a few electrolytic capacitor cans on those boards.

75% chance it is one of these. Look for an emitter bypass one gone open circuit.

25% chance a transistor has failed.
Replaced all the Old (soviet) electrolytic capacitors to Modern ones ,
The amplifier is now working Fine, offering equal output in both the channels.
 

Thread Starter

lubnaan90

Joined Jan 13, 2010
198
The amplifier was working fine with the capacitors updated.
I did some further updating in the amplifier that includes, replacing the power amplifier transistors to new ones & some resistors.
Now i wish that shouldn't have done the above mentioned, Since i was kind of careless while removing the power transistors from the giant heat sink.
While removing the transistors , i noticed a thin sheet of plastic between the transistor & the heat sink (which i thought was totally unnecessary & lost them).
Well, I installed the new transistors (without the plastic sheet) with new thermal paste, Finished installing, Switched the amp on & boom , the transistors blow up & the transformer begins to vibrate.
Note: the heat sink is connected to the chassis which has good ground supply.

Now i think that the thin sheet of plastic played important role, since i dont have that plastic sheet which was originally placed there , is it ok to use ordinary plastic sheet between the heat sink & the transistor.
 

SgtWookie

Joined Jul 17, 2007
22,230
No. They are usually mica, or another material that is engineered to be both a good electrical insulator and a good conductor of heat.

You need to obtain transistor mounting kits, or at least the proper insulators.

If you try to make them out of something different, you will probably wind up with more burned up transistors.
 

flat5

Joined Nov 13, 2008
403
It's not April first yet so I guess you're not kidding.
When you order new transistors make sure you get the correct insulators for them.
No, don't just use plastic. The real ones are made to conduct heat and insulate.
You may also need to use special screws or insulating washers that keep the screw/nut from electrically mating with the heatsink.
 

Thread Starter

lubnaan90

Joined Jan 13, 2010
198
Hello,

Thank you for your reply.

Luckily the power supply is Fine , since it uses 5A protection fuses which blew up during the Short out process (the transformer vibrated for about 2 seconds) .
I have replaced The Fuses & power transistors , The amp seems to be working fine. Presently the amp circuit board (along with the heatsink) is out side the chassis so the Heat sink doesn't contact the chassis.

Regarding the insulation washers between the screw heads and the transistor mounting tab, you are absolutely correct , The washer is present on all screws for all the four power transistors.

I have ordered some Insulators for TO220 Package (which i will collect tomorrow).
 

Thread Starter

lubnaan90

Joined Jan 13, 2010
198
It's not April first yet so I guess you're not kidding.
When you order new transistors make sure you get the correct insulators for them.
No, don't just use plastic. The real ones are made to conduct heat and insulate.
You may also need to use special screws or insulating washers that keep the screw/nut from electrically mating with the heatsink.
Thank you for your advise & noted your points.
Your guess was correct, I was definitely not Kidding.
 

Thread Starter

lubnaan90

Joined Jan 13, 2010
198
I would just leave the update with the Capacitors...
The age old trick to troubleshooting a low output audio amp is to jumper accross the electrolytics with a new one. The bad caps almost always jump out at you with increased gain.
..
In the Automotive Power amp service industry a DC Ammeter is used in line with a hi-amp 13.8VDC power supply. Caps are jumpered and the dc current increases and gain noticeably jumps up when a bad cap is bridged with a new one. Hence, troubleshooting is quick and dirty - but extremely accurate.
..
I have done the same with high-end audio amps (both tube and transistor) with predictable and success. As for the semiconductors? Are you going through the pain of reverse engineering the entire amp? Why not just build one outright?
..
dataman19
Thank you for your recommendation & advises , will definitely note them for now & future consideration .
 

Thread Starter

lubnaan90

Joined Jan 13, 2010
198
Hello to all ,

Completed renewing the amplifier , Changed the power transistors , changed some Capacitors & some resistors , Also installed some New A/V type connectors replacing the old Soviet Connectors which were so irritating & impossible to use.

A new problem arose though, The amplifier Board has two detachable boards in it (plz check the attached pic) Each one for individual channel.
The problem is with the left channel part, a 100 Oms Resistor gets extremely heated while the music is being played , its takes around 3 seconds for it to heat up & create irritating distortion (bass) in the left channel , I tried Switching the boards , then too the same problem only in the left channel.
Note : Detachable board from the left channel when placed in the right channel slot , it works absolutely fine. Also to be noted that the resistor only heats when there is Load on left channel (when the Loud speaker is connected)
 

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beenthere

Joined Apr 20, 2004
15,819
Something the 100 ohm resistor feeds current to is bad, causing too much current. You need to trace the wiring to see what part of the amp is involved. Do you have a meter to investigate voltage readings in the two channels?
 
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