"Mark D. Zacharias" wrote in message
. ..
Dave wrote:
"Mark D. Zacharias" wrote in message
. ..
Dave wrote:
You can swap Q405 and Q406 and see if the problem moves to the other
channel. Resistor R417 (560 ohm) can be checked in circuit. R405, a
68K, would probably need to be checked out-of-circuit. Experience
tells me that high-value resistors can change value or go
open-circuit for no good reason. Worth keeping in the back of your
mind sometimes as you're troubleshooting problems such as this.

R417 and R418 both test around 560 in circuit

R405 and R406 both test 66k in circuit

I'll try swapping Q405 and Q406 and see if the problem follows the
transistor. I found them relatively close (4 hour drive) in stock,
or I can order them in to my local supplier and wait a couple of
days/weeks to receive. How many do you think I'd have to order to
find a pair with DC gain matched to within, say, 15%? The 2SC2603
crosses to NTE289, which is available as NTE289AMP, a matched pair
for use in amplifiers...
For sure Q405 is not turning on. I watched the output of Q406 and,
about 5 seconds after turning power on, it goes from -0.4 to -7.1V. Q405
just stays at -0.4V.

Dave

If they are using a 2SC2603 then it is not critical. The 2603 is a pretty
standard signal transistor - nothing special. A 2SC945 would be fine,
heck, even a ECG / NTE 123AP. Just make sure the basing is correct. An
American type transistor will usually go EBC rather than ECB as viewed
from the front.


OK. I have lots of 2SC945's, they're in everything. But... I swapped Q405
and Q406 and the problem DID NOT follow the transistor.

I kept thinking about C405 and C407. If either one leaked, even a little
bit, that would throw off the base voltage of Q405, maybe enough to keep it
from turning on. So I threw in a new C405 and, voila, Q405 now shows -5.9V
at the collector. Great, I says to myself, I am so smart. (Note to self,
find new source for electrolytic caps, two bad ones out of 30 so far). Set
about checking other voltages in the channel, but HEY! what the ????
Something else is not right.

The voltages that were out we

Base Collector Emitter
Q409 +34.2 +34.2
Q411 +33.0
Q413 +33.9 +33.8
Q401 +0.2
Q403 +0.4

I didn't check the collector voltages of Q401/Q403, I'll do that next but...
I'd be surprised if they weren't +40.5 given that all the other positive
voltages check out.

The negative power rail is fine at -43.5V, and I am getting my -12.5 at the
base of Q405 so it's not the negative supply.

If Q409 fails to turn on, then that would make Q413 not turn on either.
There would be then be no negative voltage applied to the collector of Q411
to get down to the +1.1V we want to see. I'll check out R439.

I wouldn't have expected to see a positive voltage at the collector of
either Q409 OR Q413 given that they're basically connected to the negative
power rail through a small value resistor. Almost makes you think that if
R439 is open, R445 would be too.

But then we're back to my basic lack of understanding of how the transistor
works...

I think I'm learning something here.

Dave