UREI 1176 rev C opening the potted module, possible ?

[andre tchmil]

Has anyone succesfully opened the potted amp section on an original Rev C ?
I want to improve the THD and suspecting the caps in it.
I don't know where to start, heating or hacking or ....

 

[12afael]

you can try acetone it will soften the epoxy and you have to carve little by little. you will have to do it several times because it does not soften the epoxy deep enough. it is a lot of work but possible.

 

[gyraf]

distortion in any 1176 is usually by far dominated by the attenuator jfet - try first to lift that one and evaluate dist again..

 

[andre tchmil]

I know Jakob. Unit is working within specs. It’s just the THD that is 3% whereas should be 0,5.
All other caps have been replaced and they where BAD, so it can’t be anything else than the two in the epoxy, considering their age

 

[andre tchmil]

you can try acetone it will soften the epoxy and you have to carve little by little. you will have to do it several times because it does not soften the epoxy deep enough. it is a lot of work but possible.

You mean on the top, where the epoxy is ? Will the can not melt and the components ?
I also wonder how much epoxy there is in it. Just sealing the top or also down deep, glueing the parts together

 

[gyraf]

You sure it's an amp section?

It looks very much like a retrofitted potted -LN extension

/Jakob E.

 

[andre tchmil]

Yes it's this one. I'm confused , I thought that all the C versions had them potted because of not yet clear patents ?
I don't have the schematic at hand right now, what section are we talking about ?

 

[gyraf]

Was potted in the hope of landing a patent, but did not succeed

Mentioned pp 31 here: https://media.uaudio.com/assetlibrary/1/1/1176ln_manual.pdf
Schematic pp 51 here: https://thehistoryofrecording.com/Manuals/UREI/Urei 1176LN 01.pdf

 

[andre tchmil]

thanks ! will look at it when home.

 

[andre tchmil]

I think I'm gonna rebuild the little circuit on a breadboard, keeping the black box aside. We'll see.
btw , anyone knows if the original meter has a resistor hidden inside, meter doesn't mirror the ouput very well. 3.6k on the PCB is OK

 

[12afael]

go for the epoxy side, plastic will melt too I suppose. in my experience the acetone did not damage the components. in my case was a bogner compressor pedal that was epoxied but the synths guy do this very often to the voice chips on the juno 106. the epoxy start collecting humidity or something and conduct and mess with the electronics so you need to get the epoxy out and everything is fine.

 

[andre tchmil]

Had a further quick look today. I compared the oldie side by side with a hairball rev D.
Tracking, GR, levels etc are pretty much comparable except for the THD which goes up way too much when engaging compressor.
Swapped the FET's, replaced C1 and C2. No change.
Pictures included of my unit and a web picture of an open module ( or rebuild )
ignore the missing C2, it's on the underside of the pcb. If you see anything suspicious let me know, thanks.

 

[beatnik]

This might be obvious but with the compressor disengaged (switch on the attack pot) the signal is still going trough the input and output amplifier sections so if the problem is in caused by the potted module, wouldn't it be high THD even with the compression off ?

If you want to clear the potted module from epoxy this solution is probably a lot less work than heating and scraping the epoxy, although quite expensive

https://electrolube.com/product/rrscured-resin-remover-solvent/

 

[gyraf]

..are we all agreeing that the "Dist. Trim" trimmer is the one located in the potting: Does adjusting this not solve the problem (enough)?

/Jakob E.

 

[andre tchmil]

I'm aware of that small trimmer adjustment on this is minimal.

 

[andre tchmil]

This might be obvious but with the compressor disengaged (switch on the attack pot) the signal is still going trough the input and output amplifier sections so if the problem is in caused by the potted module, wouldn't it be high THD even with the compression off ?

Exactly my thought as well.,I'm a bit limited in measuring equipment here but would the distortion not simply multiply when engaged.

 

[Disco Volante]

UV as in ultraviolet radiation is deadly to epoxy... I've had some success slow-baking potted transformers. A few days at moderate heat and the epoxy crumbles. Of course, you'd want to not melt the plastic pot, so YMMV.
Good luck,
V!

 

[andre tchmil]

Here’s the bad boy

 

[gyraf]

What part failed in there? Electrolytic?

 

[andre tchmil]

Yes, and some loose solder joints