Amplifier repair

Discussion in 'General Electronics Chat' started by lubnaan90, Mar 21, 2010.

  1. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    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.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    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.
     
  3. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    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.
     
    Last edited: Mar 21, 2010
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    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.
     
  5. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    Thank you for your reply ,

    What about the Resistors & the Ceramic capacitors ?
     
  6. Mike33

    AAC Fanatic!

    Feb 4, 2005
    349
    25
    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!!!!!!!!
     
  7. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2

    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.
     
  8. Kamala

    New Member

    Mar 20, 2010
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    Nice piece of gear. Good luck with it!
     
  9. studiot

    AAC Fanatic!

    Nov 9, 2007
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  10. Audioguru

    New Member

    Dec 20, 2007
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    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.
     
  11. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    Replaced all the Old (soviet) electrolytic capacitors to Modern ones ,
    The amplifier is now working Fine, offering equal output in both the channels.
     
  12. studiot

    AAC Fanatic!

    Nov 9, 2007
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    There you go.

    Now you have time to tell us more about your collection.
     
  13. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    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.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    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.
     
  15. flat5

    Active Member

    Nov 13, 2008
    403
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    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.
     
  16. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    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).
     
  17. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    Thank you for your advise & noted your points.
    Your guess was correct, I was definitely not Kidding.
     
  18. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    Thank you for your recommendation & advises , will definitely note them for now & future consideration .
     
  19. lubnaan90

    Thread Starter Active Member

    Jan 13, 2010
    196
    2
    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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  20. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    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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