Re: blown output transistors

Dave wrote:
"Mark D. Zacharias" <spammenot@xxxxxxxxxxxx> wrote in message
news:h1xeh.9003$wc5.6960@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
meow2222@xxxxxxxxx wrote:
meow2222@xxxxxxxxx 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

This is basic and may have already been covered, but DO check the DC
resistance between the collector of the outputs and the heat sink. Should be
open-circuit, an insulator could be damaged or missing.
Also check resistance between each emitter resistor and both the + and -
supplies. Should not be a short - should show open or cap charging. If you
show a short here, then an output transistor is probably bad, despite
earlier testing.

The hFe showing on a leaky transistor is common with these checkers. Lift
the base lead. If the transistor still conducts with only the E-C connected,
it's bad.

Mark Z.


.

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