Home Stereo Amplifier Overload Indicator

[The Dude]

......On the bad side the Q003 emitter is also 15.7V (obviously, they are connected) while the base is 15.6V and the collector is 14.84V (compared to 5V last night). The Vbe is -.023......

If measurements are correct, either the transistor is bad (leaky/shorted) or it's installed wrong (basing diagram). With only -.023 B-E, the transistor should be off and collector should be at 0VDC.

[bobloblaws]

If measurements are correct, either the transistor is bad (leaky/shorted) or it's installed wrong (basing diagram). With only -.023 B-E, the transistor should be off and collector should be at 0VDC.

On the other hand, changing the transistor has not made a difference.

[g1]

One thing I don't think has been tried. When the LED is on, short out R005. Does the LED then go off?
And which part is currently installed for Q003?

[bobloblaws]

And which part is currently installed for Q003?

Either one. I have it clipped in with alligator wires. I'm not observing any difference between the two.

One thing I don't think has been tried. When the LED is on, short out R005. Does the LED then go off?

Yes it does. And the readings I'm getting on my voltmeter make more sense now. Now it's showing a Vbe of -0.530 and roughly the same differential if I measure the ends of the junction to ground. Not sure why it's different than last night. But, I tried shorting R005 at 2 different times, once with the original transistor, and then with the replacement. On one of my attempts I popped the main fuse but I have another lead of the same color connected to the chassis for when I need it to measure voltage to ground and I may have grabbed the wrong lead and actually grounded the base of the transistor (Edit: it was actually the emitter side). In any event I still have to figure out why the right side is on when it's not supposed to be.

[bobloblaws]

And now the LED is off with a Vbe of -0.3V. So I think it's time to pull the board and check the solder joints etc.

[bobloblaws]

Well, success to some degree at least. The LED is staying off now after replacing R005, R004, D007 and D204 (they had all been snipped at some point) and soldering Q003 back in. The right channel overload indicator lights up briefly sometimes when first powering the amp on, maybe after it has been off for awhile? Also, if I shift R243 (R143 on the drawing) in either right or left direction with my chopstick the voltage on D204 anode drops about half of a volt and the LED lights up. The same does not happen on the left channel. I took the board back out (for the umpteenth time) and re-soldered R204 and a couple of nearby solder joints as well. Still does the same thing. I'm prepared to leave well enough alone at this point. However, there is another oddity. When I turn up either channel to the point of overload, both overload LED light up. This amp sure has some pesky gremlins hiding in it!