Another good deed headache Crate VC3112

[rake]

Hey guys, one of my pal's amps. Back story, this amp had issues and went shop hopping for 2 years and finally he was told, "this can't be fixed" with no explanation. So I get the please check it out for me????
I checked for issues and found the usual. 1 weak tube, a couple leaky caps and some bad solder joints.
Amp then sounded great. We sat around the shop and jammed for a couple hours and had no issues. He takes it to a gig and in the middle of set 2 it loses all power wide open and it as quiet as headphones. Next day it's fine again until you push it hard like stage volume. Master is up around 7. I'm thinking the power supply could be on it's way out???? Thoughts? Ideas? If it's the PT they're discontinued. The St. Louis Music stuff was mediocre at best.

https://el34world.com/charts/Schematics/files/Crate/Crate_vc3112_poweramp.pdf
https://el34world.com/charts/Schematics/files/Crate/Crate_vc3112_preamp.pdf

Thanks all

[Madjackhatter]

My first suspicion would be in the power rail. The filter caps and dropping zisters. If you have it out of the chassis push it hard and test the B+ back from power tubes to the rectifier. Transformers are "usually" an is or aint part and the hsrd thing about Crate PT replacements are the logistics of fitting it on the chassis.

[Platefire]

When it looses power does he look in the back to see if the power tubes a red plating?
Or check to see if any tube has become unseated. When it fails is the time to do some checkingto try to catch it in the act

[rake]

When it looses power does he look in the back to see if the power tubes a red plating?
Or check to see if any tube has become unseated. When it fails is the time to do some checkingto try to catch it in the act

The tubes can't be seen. chassis hangs on the back panel and the tubes face forward.
I've had them out and no signs of burning. Tester said all 4 were good. A 12AX7 tested weak.

[rake]

My first suspicion would be in the power rail. The filter caps and dropping zisters. If you have it out of the chassis push it hard and test the B+ back from power tubes to the rectifier. Transformers are "usually" an is or aint part and the hsrd thing about Crate PT replacements are the logistics of fitting it on the chassis.

We replaced the filter caps, one puked. the resistors tested okay when the caps went in but who's to say
that one didn't crap the bed after the caps were changed???? 
He's gonna bring it up by the weekend so I guess I give it another peek.
I'll have to run the piss out of it with the chassis out. Anyone got any good earplugs? 

thanks guys!

[nandrewjackson]

I remember hearing about the crate blue voodoo amps having an overheating protection system built in. I don't know any more than that, but those schematics say 1994, that's very close to the blue voodoo years.


If it has a similar overheating protection system, try searching about fixes for the blue voodoo to get some ideas.


✔️✔️✔️✝️🎸🎸🎸

[Madjackhatter]

Good point, sir ! Thermistors are the devil ! Also check those dropping resistors to see of they get hot and go on a wild drift...unless you replaced them too. I always treat filter caps and dropping zisters as peanut butter and jelly and replace the whole rail. The wild drift thing is more of a carbon comp Fender thing in my travels though. But whudda I know, I'm an ATM repair man that works on bank stuff all day...not the seasoned expert

[rake]

The Blue Doo-doo amps were junk!
I worked for a Crate dealer back then and the first 4 came in DOA!
I re-checked the schematic and there aren't any thermistors in these.
I reached out to a co-worker from back then and he seemed to remember
an issue with the cathode bias resister and cap in these after a few years of
abuse so another thing to check out.

[Madjackhatter]

I woulda assumed it had one of those long gig grenades in it, lol. I sure hope it's not one of those tunnel of hell folded boards ! Fix 1 problem and break 2 connections in the process 😬😵‍💫😆

[rake]

For a circus board amp it isn't all that bad.
One flat board that isn't all that bad to get out.
Downside is they aren't worth all that much.
They sound a lot like an AC 30.

Some of these new amps are so bad ya want to kick the engineers in the
Jimmies for designing such a rat's den!

BTW, I found a layout/parts .pdf if the boss wants to add it to the crate files.