Mark D. Zacharias wrote:
wrote:
I suspect the OP is having access difficulties. If I'm not mistaken
that unit has the amp built around in like a "box" and is fan cooled
through the middle. Ugh.

A shop I worked at previously actually took to replacing the whole
assembly, though I have no idea how they worked the cost of it into
the estimate. I think MCM sells those assemblies. I don't remember
how much they cost.

I've also noticed class D amps in subwoofers, and if that's what you
got this method won't work. You can tell by a pi filter at the
output. Class D amps lend themselves well to subwoofers because they
can get away with a lower chopping frequency. With their efficiency
they are singularly suited to the massive current needed to drive
some woofers. You mentioned bias, so it's not class D, consider yourself
lucky,
class D amps are a whole different animal.

JURB

This is a pretty conventional design, a KEF PSW-2150. 2 ea. 2SC 5200
and 2SA1943. I was just trying to do some troubleshooting with the
outputs removed. The board was upside down at the time or I would
have seen the smoke sooner. :-(


Mark Z.

This one just gets worse and worse. I suppose I'll just order a replacement
amp plate from KEF, cost permitting.

35 volt DC offset. Outputs NOT biased on, measure NO millivolts across the
emitter resistors, yet the outputs get hot. No HF oscillation, no AC signal,
no load, no DC bias (B-E voltage no more than 50 mV), yet they get hot.
Replaced the outputs just in case there was some leakage I had missed - no
change. I'm pretty much out of ideas, and I'm way upside down time-wise on
this one. Time to cut my losses.


Mark Z.