[sdiy] Debugging problem with 4-track

[Corey K]

I was hoping someone here might have some suggestions for me.

I recently purchased a Fostex x18 4-track cassette recorder. Cosmetically
it is in mint condition, and it came with the original box and power
supply. When I plug in the power supply the green power LED illuminates.
The play, fast forward, and rewind controls seem to work. So far so good,
however, when I push the record button, the record LED does not illuminate.
I connected a few different sources to the machine and raised up the input
gain, but the level meters do not light up, and I can't hear anything from
the headphones output...

I noticed something strange which might point to the problem: if I unplug
the power supply (while the record button is depressed) the record LED and
level meters light up momentarily. I wondered if there was a problem with
my power supply, so I tried using an adjustable one. The machine is
supposed to use a DC supply in the range 11-15V. When I set my adjustabe
supply to 7.5v the machine actually seems to work correctly! But when I set
the adjustable power supply to 12V (which is the same voltage as the
original supply), the machine exhibits the same bad behavior I describe
above.

What might be happening here? I'd love to hear any hypotheses. Seems like a
problem with the power distribution system, but what would be the first
things to check? (I have a multimeter and scope). Is this a case that might
require recapping? What would be the best way to investigate this? Any
ideas would be much appreciated.

Thanks,
Corey
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