Dave wrote:

As I noted in my post, I'm willing to spend some time on this and would love
to learn something.

I've checked all of the driver transistors inasmuch as is possible using the
diode check function of a DMM. The ones that didn't pass were replaced.
I've checked all of the diodes and resistors in-circuit, everything passed,
resistors all "in the zone", i.e. within 40% of their stated values.

Most Rs should be within 5%, a few 1%. Anything as far out as 20 or 40%
needs replacing.

My
multimeter also has a transistor check but it's pretty flaky especially with
small transistors.... I've tested known bad transistors and the meter
happily gives me an hfe value for them.

then why do you say those trs are bad?

I can power up the amp with output transistors removed, as soon as they go
back in the two in-line 5A fuses on the transformer secondaries blow. there
are no dead shorts across the outputs. I get proper voltages at the inputs
to my IC's (really the only values I can check as I have no service manual
or schematic), voltage regulators are working at +15VDC/-15VDC/-6VDC.

what could cause this overcurrent situation?

a fault of any kind anywhere in the power amp that results in the
output trs being turned on.


The new output transistors are NOT fried, they're rated at 10A peak and the
new fuses are fast-blo at 5A. I'm sure they don't LIKE a big pulse of
instantaneous current but they're brand new and likely can handle it a few
times.

very unlikely. If they handle it once you've been lucky.

ALL but two (drivers) of the transistors in the amp circuits are
2SC945/2A733's which should vaporize with anywhere near 5A of current..
theyr'e only rated for 150mA.

Almost makes you think there's a short before the amp section if the fuses
blow but not the drivers/outputs... but the thing runs fine with outputs
transistors removed.

so its your output trs that short. Or the speaker wiring.

I'm going to do something I should have done a few posts ago... install the
outputs one pair at a time which will at least localize my problem to one of
the three channels.

I know there's no substitute for experience, but does anyone know if any
books or online publications that cover DC coupled (audio in particular)
amplifiers and the troubleshooting thereof? I know you're all busy and
probably tend to lose interest when you see posts and think "this guy ain't
never gonna' fix this" but I'm a relatively smart guy with a couple of years
of Electrical Engineering schooling

OK first thing you need to do is put current limiting Rs onto the
collectors and bases of all these big expensive trs. This will stop
them dying instantly, and enable you to measure whats going on. Your
psu is 15v, so if we say 15v 1.7A thats 10 ohms 23 watt Rs for the
collectors. You can make those out of a reel of resistance wire, or buy
high power Rs if you've money to waste.

Base R values will depend on hfe of the TIPs.

Next thing you need is a circuit diagram. Without that its a waste of
time. Then you should be able to follow the cct through, check voltages
all over and narrow down where its going wrong.


NT


(although that was more than 20 years
ago we were still in the transistor age).

Dave