07/22/88: FD-502 mod by Delphi, | Category: Delphi - Hardware Hacking | 1 comment - (Comments are closed)

FD-502 mod

43224 22JUL88-2319 Hardware Hacking
FD-502 mod
From: DEANL To: KDARLING

I took your suggestion and changed the jumper of my FD-502 drive 1, so
that both drives spin when one or the other is accessed. The mod went well,
and it has, in fact, produced the results you said it would.
My problem now is that I have an LED that is on continually now, as long
as there is power to the unit. I thought maybe you might be able to give me an
idea where the problem might be located. I used all the pokes you suggested,
and
the drive works absolutely correctly, except for that LED. The mod was clean,
without any problems, and I didn’t think I had changed anything else, but
obviously I’m getting a crossover somewhere. I thought maybe with you’re famil
iarity with the FD-502, you might give me a place to start, other than sending
it to Tandy for a repair. I don’t have schematics, or anything.
I’d appreciate any help you could offer.

Dean Lawrence (DEANL)

1 comment to FD-502 mod

  • pucc_unknown

    43237 23JUL88-0314 Hardware Hacking
    RE: FD-502 mod (Re: Msg 43224)
    From: MARTYGOODMAN To: DEANL

    Unless you shorted something out some where, the only other explanation
    of your persistantly on LED wuld be a blown gate in the logic
    powering it. You’d need to check that out out by tracing
    back the circuit from the LED to a small scale logic chip,
    and then using a logic probe on first the input and then
    the output of that chip’s gate. If the output stayed one wa
    even when the input varied, the problem is proved to be a blown
    gate, and the chip needs to be replaced.

    Now the FD502 uses surface mount chips, so is not easy to do
    chip level repairs on. One can in such cases sometimes
    just remove the circuit to that gate, and ADD a whole
    new full size chip, and just use one of its gates.

    —marty