Darlington pair shorting in communist amp

Discussion in 'The Projects Forum' started by nikolatesla, Sep 5, 2015.

  1. nikolatesla

    Thread Starter New Member

    Sep 5, 2015
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    Hello, I recently bought an old commie amp from a thrift shop with a burnt output channel. I replaced the faulty transistors with equivalent ones ( the second transistors in the darlington pairs were fully shorted), also checked the rest of the components wich seemed ok. The problem is that the new transistors shorted instantly and I cannot figure out the reason. Also, while running it with the functioning channel only with the balance not fully on the good output, a resistor (R435) started smoking on the faulty channel ( with the shorted transistors removed). Any suggestions? The electrical diagram is attached.
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Make sure the pin-out of the new transistors were the same as the originals. Seems like some may be non-standard in that old amp.

    Also, make sure you have correct power (bridge rectifier works ok, and power supply filtering capacitors work ok). It is possible that you are getting no steady DC supply and causing issues.
     
  3. absf

    Senior Member

    Dec 29, 2010
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    R435 smokes could be due to T413 conducts heavily or CE shorted, even though T417 was removed.
    T413 conducts heavily due to T407 was conducting heavily.
    T407 conducts heavily due to T403 was conducting heavily.

    In this type of direct-coupling amplifier you have to trace it all the way to the front end to pinpoint where the source of trouble is .......

    Allen
    schema complet am CROPPED.jpg
     
  4. nikolatesla

    Thread Starter New Member

    Sep 5, 2015
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    I will check all the transistors mentioned above. The new transistors have the sane pin layout as the old ones, the DC bridge is ok and the filtering capacitors are brand new as the old ones were shot.
    On both output channels i measured the voltages across the transistor junctions without the transistors mounted and there is a difference in voltage of about 3 volts. Peculiar for me is that T409 is ok, eventhough I was expecting it to burn as well
     
  5. nikolatesla

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    Sep 5, 2015
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    Any other thoughts about why both t417 and 415 get shot?
     
  6. Kermit2

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    Feb 5, 2010
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    Checking voltages may not be the best approach. Use an ohm meter and start at the speaker outputs with power off and compare the readings between channels. One of your small value caps may be shorted as well. I see a .22u going to ground through a very large low ohm resistor which could easily be a source of the problem. Start removing components from both channels to maintain balance and continue until you get identical resistance readings. Then start replacing parts. If the readings don't remain the same between channels swap the component with a new one.
     
  7. nikolatesla

    Thread Starter New Member

    Sep 5, 2015
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    What number did it have on the diagram? If you meant C421 with R441, both have been changed.
     
  8. nikolatesla

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    Sep 5, 2015
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    The fact that on one channel i've got bd911/912 and on the other 2n6491 and 2n6488 couldn't be by default a cause of difference in output resistance measured on the ohm meter, so making it difficult diagnosing this way?
     
  9. Kermit2

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    Feb 5, 2010
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    Not if they are equivalent replacement transistors. How large of a difference and what is your resistance reading?
     
  10. nikolatesla

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    Sep 5, 2015
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    On measurement the values on both channels were rising to maximal value, I should take all the caps out, right? I found also an original replacement pair of transistors, so we can scratch the idea of the difference due to different transistors. Suspecting the quality of bd911/912 I installed the replacement originals, and fired the amp up. Strange thing happened, it worked for a bit, then blew the fuse out, leaving the 2n6488/2n6491 pair intact, without shorting them, I find it quite peculiar
     
  11. Kermit2

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    Feb 5, 2010
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    Are you feeding any audio signal in, or just turning on and letting it idle with no input? Do you have a scope?
    Does the amps front end feed the power stages via traces on PCB or from a wired connection?
    Thinking you may need to verify operation of all other subcircuits to make sure you only have the one problem with this amp.
     
  12. nikolatesla

    Thread Starter New Member

    Sep 5, 2015
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    I always test it first without any imput or output. If it fires up without blowing the 630mA fuse then I add imput and output. I don't own a scope. The feeding goes with cables to the transformer and the on/off button, then some short cables make connection between the tranformer and the pcb. The second transistors in the darlington pair are also connected to the pcb with some wires, being mounted higher on the heatsink.
     
  13. Kermit2

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    no load on the output? surely I am mis-understanding you? You need a speaker or a high wattage dummy load resistor.
     
  14. Kermit2

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    The scope was to look at your output waveform and "see" if there is any high freq oscillation happening.
     
  15. nikolatesla

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    Sep 5, 2015
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    I didn't connect anything to output on the faulty channel because the final transistors are taken out. If I hook up the final transistors (T2 from the darlington pairs) the safety fuse, the one placed in the back where ac enters the device blows. The only way I got the machine operating is with only the right channel intact and the second transistors from the darlingtons removed.
     
  16. Kermit2

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    The term "It worked for a bit, then the fuse blew..." from post #10 confused me. I assumed the amp operated properly with both channels going using a test input signal and a load such as speakers or ballast resistor.
    You are powering on the output stage while components are removed and without a load? I'm pretty sure you are not helping and may even cause more component failures by doing that.
     
  17. nikolatesla

    Thread Starter New Member

    Sep 5, 2015
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    The only components wich were removed were T415 and T417. If they are mounted, the device switches on for like 3 seconds then the 630mA fuse blows, with or without imput or output. I suspected faulty resistors initially, causing the final transistors to burn up, but all of the resistors in the darlington pair amp circuit are in normal limits.
     
  18. Kermit2

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    Feb 5, 2010
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    You will need to supply more information. Guessing what you are doing and have already done, render this forum to no more than a suggestion board. Please supply close up, detailed pictures of the amp. Top and bottom. Highlight or point out the problem areas you suspect and what you have worked on already. This is sounding more and more like a short. Also detail the mounting arrangements of transistors on the heat sinks and if the mounts require mica isolation washers. An improperly mounted device that has a "hot" metal heat sink mount requires a good isolation washer and heat sink grease. If one of those was missing when you started and you have not corrected it when mounting new components it would explain all of your problems. I am almost sure you have a short and the heat sink area is a good place to start the search.
     
    absf likes this.
  19. nikolatesla

    Thread Starter New Member

    Sep 5, 2015
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    All the transistors from the heatsink except T409 and T410 require mica istolations. All of them have isolating washers. Heat sink grease got rubbed off in some areas, but it didn't affect the working channel (the grease got rubbed off at T416 but works properly). Here are some pictures of the device, I will make more on request.
     
  20. Kermit2

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    Feb 5, 2010
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    I'm studying the pics. Thank you for hanging in there.
    First suggestion would be to take a toothbrush and rubbing alcohol and clean away all the excess Flux from the back of the PCB. it would really help us see the connections.
    How extensively had the board been "worked on" before you started? There appears to be many melted wire connections and solder splatter there.
    Don't give up yet. You are to be commended for your prompt responses. Unfortunately I am at work and will be slow in providing help, but I do check this thread regularly.
    If anyone else spots a problem, feel free to jump in. I'm sure the TS wouldn't mind.
     
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