"James Sweet" wrote in message
news:VXNQi.979$GM2.581@trndny02...

wrote in message
ups.com...
On Oct 10, 10:32 pm, "James Sweet" wrote:
If you can lay hands on a variac, use that to power the unit. This
kind of
fault then becomes a breeze to locate (the basic cause of). If it
turns
out to be related to the output stages - and I'm not quite sure of
what
you are measuring to come up with your "800 ohms" figure - then how
simple
or not the *actual* problem is, is a whole new ball game with DC
coupled
amps like this ...

Use a large incandescent lightbulb wired in series with the unit for
testing. It's cheaper than a variac and in this case is a better
solution.
200-300W should do it.

Ok, I dug a bit deeper and found that the two center channel power
transistors were shorted. I replaced them with the rear channel
devices to see if it would work.
The fuse no longer blows but the standby relay still kicks it out.
The difference is that the display comes on momentarily and no fuse
blowing.
So either I have more issues on the center channel or the protection
circuitry knows that the rear channel transistors are missing.



There's a good chance the latter is the case.

Agreed. This is common for Yammies, and is a most frustrating characteristic
when you want to just check that all the other channels work before putting
in a firm quote. I have one sitting up the corner of the workshop waiting
for transistors right now, that does exactly as you describe, with the bad
ones removed. You will be quite lucky if there are no issues other than bad
output transistors on your originally faulty channel.

Arfa