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[SOLVED] Strange thing happened to my YGM3 - no sound

blueworm

Member
Messages
3,203
I had just changed the neck pick-up on my guitar and was playing it through my YGM3. Sound was pretty good, no sign of loss of volume etc... then I messed up with a screwdriver to adjust the pick-up position (is an old strat pick-up put in a wilshire reissue, and it did-not fit so well). The screwdriver accidentally came into contact with one of the pick-up pole and then ... bang! (well.. the usual one when you do that) ... but then right after no sound at all from the amp. Picked another guitar, changed cable, checked the speaker connection, plugged to another cab... No sound. And not the slightest hum/hiss/crack from speaker. Amp is now dead silent.

I might have fried one of the tranny :(

Or could it be something else ? What I don't get is how this could have happened suddenly from that guitar pick-up - because that really seem to have cause the problem.

Any thought ?
 
Last edited:

Blue Strat

Member
Messages
30,192
Transformers are the least likely component to go...EVER! Unless there was smoke and/or bad smells, that's not it. You'd probably have to drop a screw driver INSIDE the chassis and have a much higher than spec'd fuse to kill the power transformer.

First guess: It's not the amp at all. Try a different guitar, cable, etc.

Second guess: Amp fuse, as Drew mentioned.
 

blueworm

Member
Messages
3,203
Amp fuse is OK. I've swapped tubes (preamp, PI). The pilot light glows normally and tube filaments are heated.

I've checked with another guitar, other speakers as well. Amp dead silent.
I think that anything else non related to the amp can be ruled out because usually there is always audible hum with my YGM3, and is no longer the case.

I've open it up and took some measures. Plate voltage is OK, bias too - same as last measure I've written down. I've checked the OT windings too and it seems correct - but I'm not sure of the exact correct procedure for that. While I'm at it I've checked the speaker internal connection wires. Also gently poked with a chopstick here and there to check for bad connection and if I can get some noise - but with no result. Only sound that came to the speaker was when I did measure plate voltage, an almost inaudible hiss that was there anyway.

There was no smell nor smoke when this happens.
 

mmrxcsr01

Member
Messages
140
Just to be clear- is this a ygm reissue, or one of the originals? Single speaker, or multiple?
 

blueworm

Member
Messages
3,203
Original one (72) but converted to to 2x10. Filter caps and other electrolytic caps are quite recent.
 

OlAndrew

Member
Messages
2,345
Hate to say it, but this is one of those things that takes maybe 10 minutes to find with a scope or a signal tracer, and can be nearly impossible without.
 

mmrxcsr01

Member
Messages
140
I ask because my '72 has a circuit breaker, not a fuse, and the main speaker out is an RCA jack on the underside of the chassis. Broken wire there? Corrosion? I've had tubes that still light up, but pass no signal. Again, to be clear- a different guitar, a different cable, played through a different cab, with a known good cable, and replaced all tubes? Tried all two (or three) input jacks?
 

blueworm

Member
Messages
3,203
On mine there's a fuse, between AC input and PT. And it's OK.
The main speaker is indeed RCA jack underneath. I've checked the connection and cleaned the plug. I also tried with the external jack speaker on the back panel (which I know for sure worked well recently).

And yes: guitar, cable, speaker, tubes changed. With no result. The amp is silent whatever I do with it.

I've had tubes that still light up, but pass no signal.
What was the issue with yours ? the speaker RCA jack you mentioned ?
 

mmrxcsr01

Member
Messages
140
Not in that particular amp, but I've had 12A..7's that had one half die- recently in a 5E3...sorry, but at this point, I'm out of suggestions...please post when you or your tech finds the issue...
 

blueworm

Member
Messages
3,203
I've been testing signal with a DIY audio probe and here are some results:


  • Blue star = I can pick up signal
  • Orange star = I was able to pick up signal then lost it. Could be bad connection here.
  • Red star = No signal

Also the audio signal sent to input can be heard at very low volume in speaker.

Any suggestion to what further test I should do ?
 

DrewB

Member
Messages
676
Did you happen to check plate voltages at V2? If you're getting a little bit of signal through, it could be the plate resistor connected to that tube.
 

blueworm

Member
Messages
3,203
Did you happen to check plate voltages at V2? If you're getting a little bit of signal through, it could be the plate resistor connected to that tube.
Plate voltage on V2 (PI) looks OK = 180/187 V

Also: there is no audio signal at the output tube plate (pin 7 of V3/V4)
 

blueworm

Member
Messages
3,203
Here is where I am tracing the signal :



Preamp stage has signal >> PI stage has signal (coming out V2 plates) >> Output stage lost signal at both V3/V4 output (plates).

So I'd say: something is wrong with either both output tubes V3&V4 (and I did test with two pairs of EL84s ) or tube sockets. Is that correct ? What could be other possible cause ? Could it be a short somewhere ?

Now I need to make sure of this result, because the small amp I'm using together with the probe is in not working so good condition and has loose contacts. But let's assume these are correct.
 

Blue Strat

Member
Messages
30,192
ANYTHING could be wrong. Having 2 out of 2 bad EL84s is a long shot.

More likely a circuit problem that affects both tubes. Check the ground connection that goes to both of the EL84s. Also the plate voltage.
 

blueworm

Member
Messages
3,203
Do you have signal at the grids of the output tubes?
Yes. But not on the plate.

Anyway I found the issue thanks to Mike: there is something wrong with output tube grounding. More specifically pin 3 of V3. With a chopstick I was able to briefly get a normal tone back in speaker. So I tried to fix the soldering here but when I powered the amp on I got a big spark right away on that socket .... some nasty arcing going on here :mad: I might as well change that tube socket.
 




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