Need help with Mitsubishi DA-A30 repair

Discussion in 'Solid State' started by kotofei, Aug 31, 2017.

  1. elnaldo

    elnaldo Addicted Member

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    EDITED: I think you need a brief test at full power, to check voltage drop at R 146.

    Something is wrong at Q114 base. It could be shorted or those -34V coming from somewhere. Please check Q114 or replace it before doing a full power test. Q112, even if not perfectly symm to Q111, has -19V at the emitter and Q111 has 17.8, I don't see a short there.

    You could also check in this same condition (with DBT), the good channel at that same Resistor and Q114 equivalent.
     
    Last edited: Sep 11, 2017
  2. kotofei

    kotofei permanent gear evolution Subscriber

    Tested R146 out of circuit and it's fine, 152 Ohm. Soldered out Q114 and tested with diode test. It's shorted C to B.

    That's strange since I removed it working from the other channel and it was good before measurements. Furthermore, all measurements were done with DBT i.e. not at full voltage. The Q114 is 2SB536, Vcbo =130V, Vceo=130V, Vebo=5V, Ic=1.5A (pulse 3A) and Ib=0.3A. That was only around +/- 34V in the amp at the moment of testing and so the only way to short it was to apply more than 5V between base and emitter or more than 0.3 A to the base.

    I don't have anymore Q114 substitute so I need to get some to continue tests; however, why this Q114 failed three times?
     
  3. elnaldo

    elnaldo Addicted Member

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    Perhaps it took the current when the output was shorted. ..

    I think you can try another transistor in that place, the 130V rating I guess is to hold the full PS voltage (like 118V) in case of a short. If you have some other transistors at hand post the numbers, perhaps you have a replacement.
     
  4. kotofei

    kotofei permanent gear evolution Subscriber

    They (drivers) were installed with no outputs present and tested also without outputs.

    The complete storyline is as follows: a pair of original drivers was gone when I "tuned-up" the working amp in the first place. Then I removed them and replaced with MJE15030/15031 which I had on hands. These were gone when I tested the amp without outputs and accidentally plugged it into 117V AC. THEN I removed working drivers from another channel, installed them in the wrong channel and tested voltages-and one of them, Q114, is gone now. Again, no outputs were present. So now I need to find a new output(s) to continue work.

     
  5. kotofei

    kotofei permanent gear evolution Subscriber

    Q114 is 2SB 537 PNP. I have 2SA1370, FJN4303RTA and MJE253G. The two former are small signal transistors, the MJE253G is referred as "power transistors" by Onsemi: Vceo=Vcbo=100V, Veb=7V, Ic=4A (peak 8A), Ib=1A, Pd=15 watts, Fr=40 mHz, TO-225 case.

    I think these could be used as Q114.

     
  6. elnaldo

    elnaldo Addicted Member

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    I suspect you had a physical short when testing. I think if you redo all from scratch it will be fine, but you need to be obsessive, make sure nothing is shorted before powering up again.

    BTW, it's not a good practice to swap parts from a working unit/channel to a non working one, since you can end with 2 non working units/channels, and you loose the reference of a good channel to take measurements or compare readings. As you see, sometimes there is a 3rd thing making the suspect component to blow, so when replacing the component it blows again.

    Can you use 2sa1370 at q114?

    Edited: I see your original transistor is 1.5A, I think you can try the MJE253 (4A) but just for a test. I wouldn't increase the Ampere rating that much in that application, some transistors act like fuses in some parts of the circuit. Up-sizing them can make other things to blow under a short circuit.
     
    Last edited: Sep 13, 2017
  7. kotofei

    kotofei permanent gear evolution Subscriber

    Soldered in new MJE253 in Q114 place. Powered up the amp using DBT and checked voltages at Q114. In the very beginning B was +44V, C was - 34.4 V and E was - 34.4V. Then re-measured the voltage at base and it became -20.3V. Voltage around R146 was 0.0 mV.

    I stopped testing and soldered out Q114. Diode test revealed E-C junction is shorted, 0.00 mV drop. E-C and E-B are OK. Another transistor lost.

    So I will solder back original Q113 in the good channel and 2SA1370 as Q114 in the good channel to have it working. That good channel will be used for voltage testing and in the bad channel I'll be doing MASSIVE shortgun change of all transistors and diodes I can get my hands on and testing all passive components in between.
     
  8. elnaldo

    elnaldo Addicted Member

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    I don't think the 2sa1370 can be used. It's 100mA.

    Be careful q113 can be burned too. We need to see what is doing the transistors to blow in that position. If base was +44V something is shorted in the positive rail, doing that +V to reach that point.
     

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