"fruittool" schreef in bericht
...

Dave Platt;2949536 Wrote:
I've just been done on ebay. Both output transistors on a Kenwood A-97-
amplifier had blown. Ordered the pair from ebay, installed them,
working, great. Turned it off to put it back together and noticed the
heatsink was really hot on the repaired side. I found that the base
voltage on both transistors was low. Swapped both transistors from the
good side to the repaired side and its now fine. Put the new ones in
the
good side the and now the good side has got hot and all the voltages
have dropped the same as they did when they were on the repaired side.

Does anyone know where i can get a genuine pair of these transistors at
a reasonable price?-

What was the base-to-emitter voltage on the questionable transistors,
when the amp is idling? How does it compare to the working parts?

These parts (when genuine) seem to be Darlingstons, not just single
transistors. You may have been sold a pair of mislabeled standard
transistors instead - this would probably result in over-biasing and a
high idle current.

Or, possibly they're genuine parts, but have a higher beta than the
ones for which the amp was spec'ed? The beta bin would be a
one-character A, B, or C, possibly stamped on the lower left corner of
the plastic. If that's the case, they might work OK with some
adjustment to the idle/bias settings.

TIP142/TIP147 *might* be acceptable substitutes although their gain
seems lower.


The design uses an STK350-030 ic to drive the output transistors.
According to the service manual, at idle, the output from the driver ic
to each transistor base should be around 1.2 volts. These new ones are
causing that voltage to drop to around 0.6 - 0.8v, and this is with the
bias adjustment full up.
I tried using one of the new ones with an old complimentary one from the
good side and this caused a massive imbalance in the idle bias 1.4v for
the good one and 0.6 for the suspect one. Can i assume from these
voltages that the replacement's are not darlingtons? 0.6v is right for a
single transistor and 1.2v (approximately double) for a darlington?

Regards




--
fruittool

No doubt you were sent single transistors instead of Darlingtons. The latter
have two diode drops Vbe so 1.2V-1.4V instead of 0.6V-0.7V. If you did not
find out already you could check using the diode tester of a common
unimeter.

petrus bitbyter