Any more recent successs stories to brag about?

C'mon, don't we all enjoy patting ourselves on the back, really?


Mark Z.

**I recall the first time (1980-ish) I discovered those fusible resistors
that go high after a few years. With no obvious signs of distress. Now I
just head straight for the buggers.

Then there's those low value (/=47 Ohms), 1/4W cracked carbon resistors
that go O/C when subjected to ca. 60+ Volts with no signs of burning
(Marantz 1200b, 240, 250M, 500 models). Over the years, I learned to
suspect any resistor over the value of 100k, if the circuit is displaying
some kind of mysterious fault that cannot be explained by a semiconductor
failure or cap leakage.

--
Trevor Wilson
www.rageaudio.com.au

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A brand new "war story".

As I am nearing the end of my career, I wonder if any one unit will be the
high water mark so far as feeling the satisfaction of fixing a really tough
one.

This may be it.

A Yamaha M-80 power amplifier. I've worked on a few in the past - difficult
but doable.

I've always thought that "M-80" was a humorously ironic model number for an
amplifier so flammable.

Initial inspection:

Burned resistors on both channels, a vented 1000uF 100v cap (one of a pair)
on the main board, lots of brown glue, some green corrosion visible on
component leads.

Strangely - only one output transistor was bad. I knew this was going to be
a tough one but I figured I could do it - just give the customer a pretty
high estimate.

Replaced those larger caps, lots of bad drivers, pre-drivers, signal
transistors, several burned and corroded resistors, one bias transistor.

Replaced the one bad output and it's mate. I figured the same current ripped
through both, so I wanted at least do that.

I knew the speaker relays would need service, so I took them out of order
and did that job.

Bringing up on a variac, the fires were out, bias adjusted OK, but no sound.
Another bad resistor.


Replaced this, but now there was a -86 volt(!) offset. Couple more bad
resistors.

Each time a component replaced it was necessary to monitor bias when
bringing it up.

Bring it up again, no offset but one channel oscillates. Fine. Trace down
and replace the bias transistor on that channel that was breaking down.
Replace it and: one channel low in gain, approximately 1/2 the other
channel.

Replace a bad 3.9K 1/2 watt resistor in the feedback loop. That was easy.
NOPE.

Now BOTH channels oscillate like crazy. Apparently a larger 3.9K 2W resistor
was corroded and got nudged while replacing the other. Replaced that. No
more oscillation. NOW:

Still no change on the gain problem.

Bad 430 ohm resistor hiding UNDER a power resistor, and not even visible
until the other was removed.

Unit now repaired and functioning properly.

This thing took approximately five whole days worth of bench time.

I'm going to spend a very generous amount of time patting myself on the back
for this one.


Mark Z.