loss of signal

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loss of signal

Postby nyazzip » Wed Sep 06, 2017 11:52 am

i have a simple SE guitar amp that suddenly got very quiet. no buzzing or anything, just barely any output, overnight(worked one day, didn't the next). tube swapping doesn't help...if i crank it to 10 i can hear it, but of course it isn't really useable like this.
i looked briefly at the PCB and didn't notice any fried or bulging components. i'm thinking maybe simple corrosion at an input or output jack? anything else i might look for?

regards.
been busy the last few years and haven't been tinkering with amps at all. hope everyone is well
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Re: loss of signal

Postby mike567 » Fri Sep 08, 2017 8:13 am

Is your cable good?
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Re: loss of signal

Postby nyazzip » Mon Sep 11, 2017 11:50 pm

yep, went through all the possible external reasons... speakers, jacks, cables, instruments, tubes, my imagination, ears, etc

:/
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Re: loss of signal

Postby Gingertube » Tue Sep 12, 2017 12:47 am

Check preamp tube heater wiring - is it lighting up?
Check screen supply to the output tube - is it getting volts.
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Re: loss of signal

Postby nyazzip » Wed Oct 11, 2017 5:16 pm

finally got to poking around...this a single ended EL84 amp, from the factory
cathode:9v
plate: 334v
screen(pin9; g2):321v
g1=0....not sure if that is normal. i've forgotten so much over the past 5 years :(


the heaters all glow fine, but for some reason i don't get a reading off of pin 4. pin 5 is 6v(on my DMM's 600v setting...i didn't bother to try to get a more accurate reading [filaments are DC])

plates on the 12ax7 are both about 190, and nothing on the grids
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Re: loss of signal

Postby mike567 » Fri Oct 13, 2017 6:03 pm

get a signal source and inject to control grids of tubes starting with the output.
should find where you are losing your signal.
old cheap transistor radios can work as a signal source.
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Re: loss of signal

Postby nyazzip » Wed Oct 25, 2017 4:31 pm

injecting audio signal into the grids is a bit beyond my comfort level...what would i be looking for? if i hear it coming out of the speaker then it's ok? i don't have a scope or any bench testing gear

it is a PCB build; everythig looks ok on the board. it just went from working perfectly to not working one day. in other words, i used the amp, turned it off, then a day or two later it barely made any sound when i powered it up again. it just sat in place- it wan't transported or anything. no lighting storms. it's a mystery
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Re: loss of signal

Postby mike567 » Thu Oct 26, 2017 1:49 pm

If you are not comfortable not doing more than just changing some tubes you might want want to get some professional help with the amp.
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