Utc 3ax reconditioning

[tardishead]

I have 2 of these they sound really great but they are out of calibration with each other.
Components must have drifted out of tolerance.
So the eq circuitry is potted in cans with what I presume is shellac.
Does anyone have any experience of similar builds?

Does it melt with heat or do I soak in meths?
Any pitfalls/dangers?

Does anyone have a schematic. I have never seen one I should probably trace it out hey?

Once I have accessed components replacing PIO caps will probably be an issue.
And getting vintage bumblebees etc with matched tolerances will probably be a PITA
Does anybody recommend a current manufacturer of pio caps that compares with the old ones in terms of sound and reliability.
When I put it all back in shellac I certainly don't want to do it all again in a hurry.

 

[emrr]

How badly do they compare?  Do you have comparative plots you can show?  I would question the original tolerance as part of the procedure.

I put potted pieces in an otherwise unused oven (fume residue) at about 220F for an hour, usually everything runs right out. 200F is frequently the melting point.

The caps may be custom values requiring parallel or series connection when replacing.

 

[tardishead]

Thanks Doug
Is it shellac? It should disolve in methylated spirits no?
The difference is very audible
In high mids one might be 4k and the other 5k or more
Can't use as a stero pair which is what I'd really want to do

 

[emrr]

You may be able to externally parallel something on one to get it closer to the other.  That'll give you the closest 'decayed original' sound.  I'd try it first just to see.  I would expect it is some sort of shiny tar, but I don't know not seeing that one.  I can't think of seeing what I thought was shellac in anything like that. 

 

[tardishead]

And would you melt the contents and put it back in again? Is tgat what you did Doug?

 

[emrr]

I've de-potted transformers that way, yes, and you could also put them back in and reseal the same way. 

 

[lassoharp]

Once I have accessed components replacing PIO caps will probably be an issue.
And getting vintage bumblebees etc with matched tolerances will probably be a PITA
Does anybody recommend a current manufacturer of pio caps that compares with the old ones in terms of sound and reliability.
When I put it all back in shellac I certainly don't want to do it all again in a hurry.

The vintage ones are out there if you must have them.  The 200V versions of less common values usually go cheaper than the 600V versions. Lots of different brands.

I would avoid using any BumbleBees pulled from HV blocking use.  They seem to have a record for going leaky and way off tolerance spec.

The Russian PIO caps were a good buy at one time.  Bunch of US guitar supply guys went hogwild and bought most of the ebay sellers out a few years ago.  Now they're not such a good buy.  I've still seen some nice looking smaller voltage precision caps from the Russian vendors that looked like they might be very good for filter use.  Jensen PIOs can be easily bought but they are in BOO-tique land price wise.