Alec S. wrote:
Hi,

I have a Matrix auto chromatic tuner-to tune instruments-model
SR-4000.

A while back I killed it by-accidentally, I swear!-plugging in a
power cord which was set to a few volts too high (12v instead of 9v)
backwards (reversed polarity).

I would like to fix it. I would like to diagnose the problem and if
I'm lucky repair it (I assume it's a dead component that needs to be
replaced.)

While I am no electronics expert, I am also not a novice. I have
repaired computer hardware (and built a few parts of my own), and
even
our home's wiring (although that's really just dangerous, not
complex.)

I've got a digital multimeter, variable power supply, all kinds of
wires, alligator clips, etc, hopefully pretty much any tools I might
need. I've also got a large box full of electronic part potpourri
that
I got from various places including a supplier a long time ago and
from
various electronic devices I've cannibalized.

I have scanned the circuit board and they can be found he
http://img378.imageshack.us/img378/4...erfrontbo8.png
http://img230.imageshack.us/img230/8...nerbackag3.png

I've tried running a couple of small tests, but I don't really know
how
to test something like this. For example, I'm not certain how to
test
if a transistor is good (I've got an idea but I'd rather hear from
experienced people), or a quick connectivity test reveals nothing
useful since pretty much everything seems to be connected to
everything
else, and testing for power returns all kinds of variable voltages
that
tell me nothing.

Does anyone have any tips or advice (or maybe a complete
walkthrough?)
on how to test and repair this?


Thanks a lot!

--
Alec S.

1: The 80C51 is a pre-programmed microcontroller. Tektronix used bugs
like this in the 1730/1/5 scopes. The point is they didn't set any of
the protection features so I read out the program from a working unit,
programmed it into an 8751 (actually 8752 but the concept is the same)
and brought the other scope back to life. The two-fold hitch is you
need a programmer and a working chip AND the protection bits NOT set.

2: Someone else noted a 78L05 regulator that may have failed. There
certainly is some sort of regulator as the 8051 family is 5 volts and
will let the magic smoke out with 9 volts. There might be a protection
diode on the power input to protect just this mode. If that failed, an
ohm meter on the power input would read nearly 0.

My bet is the processor is still OK and repairing the regulator(s) will
bring it back.

GG