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MFJ-993 Radio Tuner

Discussion in 'Electronic Design' started by Jon Slaughter, Oct 5, 2008.

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  1. My father has an MFJ-993

    and it doesn't work. Here are the symptoms:

    All tests pass OK, Frequency Counter doesn't seem to work but LCD does, Can
    manually tune it. (I don't know if all these are exactly true or not but
    just from what I've been told)

    I'm guessing that it's the frequency counter part of the circuit that is bad
    and the pic cannot get the frequency for some reason.

    The frequency section consists of a 74HC14, 2 transistors, 2 resistors(103
    and 562), and a few caps(4 to 5). (I'm guessing that this is the frequency
    counter section)

    The two transistors both seem to have holes in them. I'm not sure that they
    are bad or not though as they are sot-23's but the tiny holes don't seem to
    be on any of the other transistors. (I'd bet they are bad)

    Also, they seem to be used as diodes. I only say this because they are
    marked DXX instead of QXX like all the other transistors. Again I'm just

    Any ideas? I'd like to replace these transistors but I have no idea what
    they are... NPN's? PNP's? Nch fets? etc... I think the numbers on them are
    587, 5D7 or 507. The 7 is sidewise too.

    This is what I'm guessing they are:

    I'm going to go ahead and replace them with normal diodes and hope for the

    Any other ideas?

  2. Well, it's not the diodes... or at least wasn't the only problem ;/
  3. Guest

    Hey Jon,

    Did you note the warranty info in the MFJ manual???? NOT void if you
    try to service it. They also seem to offer repair consultation if you
    call them. I'd give it a try.

  4. wow, I didn't see that but I've never seen that before either. In any case
    it was the hex inverters.. after replacing with the proper chip it worked.
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