Sup folks, I'm gonna start off here like most other forum noobs begging for help.
I have a 90s tremoverb on the bench with multiple issues.
It started out with a blown screen resistor, and a couple of bad caps in the tremolo circuit. bad corrosion around the leaky caps. replaced, and repaired, but now I'm getting very low signal, and horrible distorted sound only when I strum hard.
I'm getting 430vdc on pin 6 of the PI, and proper vdc on pin 1.
I understand that this means that half of the PI is not conducting. I tested all plate/cathode connections, and even sacrificed (Broke, clipped, and tested leads to PCB connections) a 12ax7 to test the tube socket.
Here's where it gets weird. The voltage on pin 6 of the PI will sit steady at 430 VDC until I put a .1VDC 1KHZ signal into the input. The high voltage breaks through the blocky distortion, and starts something that causes the voltage on pin 6 to start falling (PI starts conducting) once it warms up, the amp is fine. Just needs that nudge to start that half of the PI. I've tried more than 5 different tubes in the spot.
Any help to be had here?? I'm fu**in stumped broskies. could it be LDR issues? I know that those are suspect in this amp, but channel switching and stuff seems to be OK.
The OT tests ok as far as secondary and primary windings being intact, and no shorts between. plate and cathode caps and resistors are good.
WTF?!?!?!?
mesa tremoverb issues
Moderators: pompeiisneaks, Colossal
Re: mesa tremoverb issues
Sounds like a bad connection alright. Have you reheated all the solder joints in that area? Do the circuit boards use ribbon cable also?
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Stevem
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Re: mesa tremoverb issues
when the amp is acting up do you get clean signal out of the effects send jack?
How much AC voltage do you read across the output side ( output tube side) of the PI coupling caps.
How much AC voltage do you read across the output side ( output tube side) of the PI coupling caps.
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Re: mesa tremoverb issues
https://el34world.com/charts/Schematics ... moverb.pdf
That triode looks to be the signal input / non-inverting side of the pair.
It seems that triode may not be conducting for some reason; maybe its grid is floating (eg lost Vdc reference to its cathode) or pulled down (getting referenced to a mush lower Vdc than its cathode, putting it into cut off). With it in the fault mode, try measuring the Vdc across the pin 7 grid leak resistor.
Or perhaps the plate resistor is getting shorted somehow, eg bad 120pF bypass cap.
Or one of the coupling / decoupling caps either side of the trem LDR is bad.
I don't think that the actual LDR itself could be causing the triode bias to go off, unless the insulation between the optoresistor and LED sides was failing, which should be very unlikely.
That triode looks to be the signal input / non-inverting side of the pair.
It seems that triode may not be conducting for some reason; maybe its grid is floating (eg lost Vdc reference to its cathode) or pulled down (getting referenced to a mush lower Vdc than its cathode, putting it into cut off). With it in the fault mode, try measuring the Vdc across the pin 7 grid leak resistor.
Or perhaps the plate resistor is getting shorted somehow, eg bad 120pF bypass cap.
Or one of the coupling / decoupling caps either side of the trem LDR is bad.
I don't think that the actual LDR itself could be causing the triode bias to go off, unless the insulation between the optoresistor and LED sides was failing, which should be very unlikely.
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tubesinside
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Re: mesa tremoverb issues
I've had this very issue recently. It was a leaky low voltage cap near the PI valve. Remove the cap and clean the board thoroughly.
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kilgoretrout5447
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Re: mesa tremoverb issues
Thanks all for the replies. I'll be back at it tomorrow night, and I'll answer your questions.
I changed a leaky 100uf over by the 555 chip, which I assume is the one in the tremolo circuit, and it got the tremolo working (Which it wasn't), but I see no other obviously leaky/corroded/swollen caps.
The only other electrolytics nearby are the 47uf 100v axial near the edge of the PCB, and the weird fat 100uf over by the trem circuit again. I removed the fat one and checked it, and it was OK.
What does mesa have against reference designators dammit!?
Which cap was it tubesinside?tubesinside wrote: ↑Wed May 22, 2019 11:52 am I've had this very issue recently. It was a leaky low voltage cap near the PI valve. Remove the cap and clean the board thoroughly.
I changed a leaky 100uf over by the 555 chip, which I assume is the one in the tremolo circuit, and it got the tremolo working (Which it wasn't), but I see no other obviously leaky/corroded/swollen caps.
The only other electrolytics nearby are the 47uf 100v axial near the edge of the PCB, and the weird fat 100uf over by the trem circuit again. I removed the fat one and checked it, and it was OK.
What does mesa have against reference designators dammit!?