"Mark D. Zacharias" wrote in message
t...
wrote:
wrote:
Dave wrote:

Practically all the amps I work on are dc coupled. Not a problem in most
cases.
Virtually all your Kenwood, Pioneer,Sansui, etc going back to 1970 or so
are DC coupled.
Modern amps like Denon, Yamaha, Onkyo, etc are all pretty similar in
topology. Certain components tend to go out along with the outputs
(resistors and driver transistors for example) and symptoms such as
premature clipping, DC offset etc have predictable causes.
As to the OP, it does seem his piece is quite possibly a "piece" all
right, but I'm not at all sure I'd classify it as such a difficult repair
simply because it may be a "DC" amp. As I say, I fix such amps all the
time. No problem.

Mark Z.

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. 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.

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?

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. 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.

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

Dave