Crate FXT120 Loud Hum Problem - strange one

[toob_amps]

Hello,
I have a Crate FXT120 with a loud hum. A link to the schematic is attached below. I figured it was the filter caps, replaced them and the problem remained. I was able to get the humming to go away when I disconnected the +40v from power transistor Q11. I have replaced the power transistors and the problem persists. The 200v emitter diodes and 1N914 diodes are new as well. Disconnecting the DC supply from the collector of Q11 is the only thing that makes the hum go away. Any help is appreciated.
Thanks-

http://music-electronics-forum.com/a...c-194sch_a.pdf

[Enzo]

Your loud hum is probably DC on your speaker. That will damage the speaker, so disconnect the speaker until there is no DC coming from the amp.

Is -40 present too?

You have four output transistors, are any shorted?

The schematic has a list of test points, how do your readings compare to those?

[toob_amps]

Enzo,
Thanks for the reply. There is about 8v DC on the output when Q11 is connected. There is only a few mV on the output when the collector of Q11 is disconnected. I should have included that in my first post. The readings I get are pretty consistent with that shown for each test point. The + and - voltages are present at each collector. The actual voltage for each DC rail is +35v and -35v as shown for test points 14 an 15. I installed 4 new power transistors. None of the new or original ones are shorted. I'm puzzled.

[mboessen]

Enzo,
Thanks for the reply. There is about 8v DC on the output when Q11 is connected. There is only a few mV on the output when the collector of Q11 is disconnected. I should have included that in my first post. The readings I get are pretty consistent with that shown for each test point. The + and - voltages are present at each collector. The actual voltage for each DC rail is +35v and -35v as shown for test points 14 an 15. I installed 4 new power transistors. None of the new or original ones are shorted. I'm puzzled.

I'm an old guy. You may have tried some of this stuff, but maybe a couple of old fashioned ideas would help.

If a scope shows no ripple on your power supply, and you haven't had any luck tracing the source of the hum with a scope, you might try the old trick of "half splitting" the circuit. Go in the middle of a string of amps and remove a coupling capacitor, or cut a trace, if possible. If the problem remains, it is after your cut, if not, it is before it. You can narrow something down faster than you might think this way. If it is 60 cycle hum it seems like it should be a bad ground or cap somewhere. Old amps used to get this from having the hot and neutral reversed. I guess it is far fetched that you would have an incorrectly wired outlet or power cord? Probably none of this is applicable, but since I am asking for help elsewhere on this forum, I thought I should try.

[Jazz P Bass]

Enzo,
Thanks for the reply. There is about 8v DC on the output when Q11 is connected. There is only a few mV on the output when the collector of Q11 is disconnected. I should have included that in my first post. The readings I get are pretty consistent with that shown for each test point. The + and - voltages are present at each collector. The actual voltage for each DC rail is +35v and -35v as shown for test points 14 an 15. I installed 4 new power transistors. None of the new or original ones are shorted. I'm puzzled.

I have never heard of disconnecting half of an output section to troubleshoot.
Now that your power rails are both up, can you redo the voltages from the first post.
Please keep in mind how a solid state output section works.
The TIP 142 & 143 transistor bases should be just at the edge of being turned on.
About + 0.65 volts for the 142 & -.065 volts for the 143.
That puts the driver bases at + & - 1.3 volts (as per the schematic).
This being so, the output will sit at zero volts.

[Enzo]

OK, so you have reasonable voltages on TP14 and TP15. And unwanted 8v on TP13. Now what do you find at TP10,11,12? I am going to wager you find about 8v on TP11,12. And what is coming into the base of Q14? A leaky Q16 (the preamp out on the other page) could put the DC balance out of whack.

[toob_amps]

Enzo,
Thank you very much. You're dead on correct. I was getting about 7.8 volts on test points 10, 11, and 12 and on the base of Q14. I harvested a J175 out of a parts unit and replaced Q16 in the preamp. The amp is now working great. I feel a little silly for not catching the DC voltage on the input from the preamp since this isn't my first rodeo. But I am still confused as to why the amp would only act up when output transistor Q11 was disconnected. Why wouldn't it react the same when Q5, which is in parallel, was disconnected? The education is appreciated.

[Enzo]

If disconnecting Q5 didn;t change the operation of the amp,m and you test it as good, look at the resistors around it. There is a 4.7 ohm with parallel diode, check those parts, and the 0.47 ohm 5w ballast, any of those open?

[toob_amps]

I checked all of those and they were good and within spec. I guess it's just one of those strange things where the incorrect voltages on transistors upstream created some strange return paths.