Ampeg B-410 DC On Output Diagnostics Help

[misterc57]

R206 reads 12 ohms, should be 470R
R202 reads 1.2K, should be 470R

[misterc57]

Replaced R206 and R202. Still had only 12 ohms across R206. Which led me to Q205 having shorts. Replaced Q205 and now see 470 ohms across R206.

[misterc57]

After all that I still have about 4.5V DC on the output.

[misterc57]

Still have V that appear too high.

Another round of readings with a bulb limiter, which is shining pretty bright (60 watt bulb).

I noticed that the wire sending V to Q305 Collector was disconnected at the board. I reattached.

Q206 CBE 3.5V, 2.9V, 2.9V
Q207 CBE -5.4V, 2.2V, 2.85V

D204 C,A -5.4V, -5.3V
D205 C,A 3.1V, 3.3V

Q205 CBE -6V, -6.5V, -6.8V

R206 -6.3V
R202 5.2V

[misterc57]

Q201 CBE -6.2V, -6.1V, -5.9V
Q202 CBE -5.7V, -5.9V, -5.6V

What could be causing V to be too high?

[misterc57]

Q201 and Q202 C to B shows a short in circuit

[misterc57]

Replaced Q201 and Q202 which tested bad on the bench.

Still have about 4V DC on output.

[g1]

If C203 blew, it must have got too much voltage. Are R207 and R208 ok?

[misterc57]

If C203 blew, it must have got too much voltage. Are R207 and R208 ok?

They are okay.

[misterc57]

Calling it quits on this one. I want to thank everyone for their help. Although I could not fix the amp, I learned more about finding transistor shorts in circuit which is a good walk away for me.

BTW I discovered that the entire output section can be removed from the chassis which makes it easier to access the components. Remove the two wire harness and then there are two bolts holding the heatsink to the chassis, remove those and the entire output section with heatsink easily pulls out.

Thank you! MC