Amp came back with hum

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Dogged
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Amp came back with hum

Post by Dogged »

I built an 18 watt lite back in 2007. I used the amp from that time until 2011, when I loaned it to a friend who gigged it. I got it back a year later and put it on a shelf. Another year went by and I had occasion to use the amp, and wouldn't you know it, the thing wouldn't work. It had a loud hum (120 cycle always the same level and independent of the volume control) and wouldn't pass signal. :cry: It sat there for years. I had other amps. Now I've retired and I broke this thing out the other day and started to go through it. It went from humming and not passing signal, to humming and passing a attenuated signal, and, as you turn the volume up the amp kicks over into a grating, loud, distorted oscillation.

I took all the voltages and with the exception of Pin 2 of V3 and V4 (20v and 24v respectively) everything else is spot on. :D

I got rid of the hum by reworking the grounds. Now the amp is quiet as a mouse. : :D

So I have one problem left. If I have a guitar plugged in, the amp sounds great and I can turn the volume to the pin without getting the distorted oscillation, but if I have nothing hooked to the jack, i.e., nothing is plugged in, and turn the volume clockwise, I again get the distortion (although not as loud) in ,perhaps, the last tenth of the pot's travel. :?

These are Marshall type jacks. Aren't they supposed to short to ground when nothing is plugged in?

Any ideas? I'm not confident enough to take this thing out to a gig and expect it to work. Sounds great otherwise.

I admit that I am mystified as to why the thing would work so long and then have a grounding problem. Why did it ever work?
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JMPGuitars
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Re: Amp came back with hum

Post by JMPGuitars »

Dogged wrote:
Tue 06/16/20 3:33 pm
I admit that I am mystified as to why the thing would work so long and then have a grounding problem. Why did it ever work?
lots of possibilities. For example, a cold solder joint could have worked for a while, and then broke.

More importantly, I would highlighter test your build and make sure everything is in spec, connected where it should be, and with good solder connections.

Are the tubes recent?

You should also test your filter caps, as they may need to be replaced.

Thanks,
Josh
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Dogged
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Re: Amp came back with hum

Post by Dogged »

What is a highlighter test?
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JMPGuitars
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Re: Amp came back with hum

Post by JMPGuitars »

Dogged wrote:
Tue 06/16/20 8:16 pm
What is a highlighter test?
Print both the schematic and layout. Start at the input jack, and highlight each connection and component only after verifying the values and quality of connections.
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Modern Ground Schemes
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