Substitute transistors

Discussion in 'Electronics Resources' started by zvir, Oct 22, 2012.

  1. zvir

    Thread Starter New Member

    Oct 22, 2012
    Dear everyone,

    I would like to ask a question. I burned out a few transistors in a Peavey PV-8.5C amplifier. Specifically, the transistors are MPS-6531 and MPS-6533, which are quite hard to find for a relatively small amount of money. Peavey lists 2N3642 and 2N3638 as substitutes, but those are also quite rare.

    I would like to ask if anyone can help me with an advice about these. I've been googling for a few hours, and I can't find any useful stuff. Which common transistors can be used as replacements for these burned ones?

    Thank you in advance.
  2. Audioguru

    New Member

    Dec 20, 2007
    I used the old MPS6531 transistor 37 years ago because it was cheap. They were made by Motorola then.
    Fairchild's old datasheet says to see their PN2222A datasheet for characteristics.
    PN2222A transistors might still be available.

    A transistor burns out because there is something wrong in its circuit. Then a replacement transistor will also burn out.
  3. zvir

    Thread Starter New Member

    Oct 22, 2012
    Audioguru, thank you for your answer. However, it leaves me pretty much exactly where I left off. In my country, PN series of transistors has rarely been seen, and local electronic stores do not have them.

    I found a website with a cross referencing database:
    There, I found that the appropriate substitutes for my faulty transistors would be BC637 and BC161. I checked the datasheets, and they seem fairly similar to the original transistors. However, I am not very experienced in this kind of things, so I would like to ask you, or anyone else who has more knowledge than me, to double check the specs of those for me...

    I am aware that other things might be burned out too. Even though everything else seems okay, I plan on changing every transistor and op-amp in that section, just in case. However, I am certain that the cause of these transistors burning out is not within the circuit itself.

    The thing is, a few weeks ago I used a faulty cable to connect the speaker to the amplifier, and accidentally burned out the entire power output stage of the amplifier. I then replaced pretty much everything in the output stage. Since this amplifier is pretty tightly packed, I had to disassemble half of it to be able to disconnect the output stage. Among other things, I disassembled the mains input board, and made a grave mistake of not photographing it to make it easier to put back together.
    I had no idea whatsoever of how to put it back together, so i made a few guesses. I was completely wrong a couple of times, nothing worked. Once, i managed to activate only the ventilator, the other time I managed to activate the amplifier, but the ventilator was dead, and the last time, i burned out the amplifier, which means I must have connected something wrong.

    I intend to fix what I burned, hook the amplifier to a laboratory supply, and check everything. And lastly, I would try a few more times to connect the mains input board correctly. The problem is that Peavey's service manual says absolutely nothing about how to connect that board, or I am unable to find the needed information.

    Would anyone be very kind and check that for me, and possibly give a few hints? I would be more than happy to provide the schematics.

    Regards, Z.