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Fender guitar amp started humming, can't figure out why

Started by brodie1600, April 27, 2015, 04:57:55 PM

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brodie1600

Alright, I understand now. I edited my last post, so the measurements should be correct this time.

DrGonz78

Okay that makes sense finally. For the life of me and everyone here, we were brain farting on how that collector of Q11 could have 42v and not negative voltage. So I would first unplug the amp and test D23 to make sure is not shorted. Do you know how to test a diode with your meter?
"A person who never made a mistake never tried anything new." -Albert Einstein

brodie1600

I don't have a diode test setting, so I measured resistance across it. It was 13.5 kilohms, no measurement the other way.

DrGonz78

#33
Try measuring the resistance of the other diodes D21-24 to see a difference. Also try reversing the probes as well to see if there are any differences.
"A person who never made a mistake never tried anything new." -Albert Einstein

brodie1600

These were measured with the negative lead to the cathode, and positive lead to the anode:

D21: 13.7 kilohms
D22: 13.7 kilohms
D23: 13.7 kilohms
D24: 12.85 kilohms

Couldn't measure anything with the leads flipped.

DrGonz78

Well that looks fine actually and just had to make sure. I have been reading up a bit on this amp and there is many problems with this amp. First off the mute circuit causes many problems. So again measure the DC voltage on the speaker output, but this time insert a guitar cable in the input jack. We want to see if this makes any changes to the DC voltage on the output of the amp.
"A person who never made a mistake never tried anything new." -Albert Einstein

brodie1600

Measured the voltage on the output again and it read -400 mV. This is strange because right after I installed the new transistor, it was still delivering the same hum to the speakers. Output voltage measures 10 mV with a guitar plugged into the amp. I plugged the speakers in and played the guitar a bit and got sound from the amp.
Safe to say that it's fixed? Or should I check anything else before putting everything back together?

DrGonz78

So the hum is gone? Now unplug guitar cable with the amp just at idle, is the hum there now? Even better hook up the voltmeter to observe voltage on the speaker while you unplug the guitar cable from the jack.
"A person who never made a mistake never tried anything new." -Albert Einstein

brodie1600

Yeah, no more hum. Just normal amp hum. Had the leads on the speaker outputs and watched the voltage drop from 10 mV with the guitar plugged in, to -400 mV when it got unplugged.

DrGonz78

Well that is as good as it gets and it seems to be back to normal. Still there is a concern that Q11 had nothing to do with the problem and as you said it did not change the hum earlier. Something triggered this problem and something triggered it to go away. Thus meaning the problem could return. I would put the amp back in the cab and play it for a while. Also, get a rubber mallet or use your fist to hit the top of the amp to see if the hum comes back. In other words this could still be an intermittent problem down the road. 
"A person who never made a mistake never tried anything new." -Albert Einstein

brodie1600

#40
That's true. All I remember is that I still heard hum at one point after I installed Q11, but not anymore - the amp seems to be working fine. I'm betting it was Q11 that was causing the issue.
Everything's put back together and we're testing all the knobs and buttons.
If the issue comes back, I'll know it's an intermittent issue caused by something else, and I'll post another thread on the forums.

You've all been a great help, and you saved my friend and I loads of money. And I learned a thing or two in the process. We're both real grateful - thanks again everyone!