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Cleaning vintage PCB - Damaged by isopropyl?

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TERRA Operative:
I found the same with an old oscilloscope I refurbished a while back, the PCB looked identical in construction.
I didn't take as much of the coating off, so I just left it as-is in the end, but if you wanted to make it look real nice, I'd clean the coating off as suggested above and use a modern spray coating after.

It depends on why you are doing the restoration. 

If appearance matters,  I would remove and recoat.  Apparently, there are still shellac based conformal coatings still available for authenticity.  Otherwise I would switch to a lacquer or another varnish based coating.  Removing it could be a tedious job or very simple.  Depending on what else is on the board, a few minutes in an ultrasonic bath with denatured ethanol or pure ethanol will work.  If there are transformers and relays, etc. on the board, I wouldn't do that.  Then it would require gentile cleaning with a brush/swab.  The nice thing about shellac is that it is simply solvent based in contrast to enamels that are reactive.  Thus, applying a good shellac solvent/thinner, letting it level, and slowly evaporate may repair the finish cosmetically.  All of that assumes it's shellac.

If appearance doesn't matter, then I would leave as it is. 

An Avo meter? Certainly it's Shellac as anything that was quality and built to last was Shellac coated. Leave in situ and focus on the parts tarnished with time. What about the wiper contacts on the other side of the board?

The PCB is resin bonded paper, essentially the same material as stripboard/Veroboard. It has that distinct 'old school electronics' odour when warmed. I should add, avoid using any volatiles such as ethanol in ultrasonic tanks, unless you feel your life needs a lot more danger.

Good luck with your restoration.

ehh conformal coating is tough. I had it damaged with IPA, denatured alcohol and even PCB flux remover, it all causes it to wrinkle. Your best bet is to spot clean carefully and then apply new coating. Even some modern conformal coats from the last 20 years will delaminate/weak from alcohol and solvents .

I say using anything but soap in a ultrasonic is asking for trouble from plastics.

Shellac is still very widely used in furniture restoration and sometimes used to finish new furniture. Something to keep in mind is that the shelf-life of premixed shellac is limited: after months in alcohol solution the resin starts to break down, with the result that it never really dries. The longer lasting form is dry shellac flakes, which come in various grades and types of processing. Some of them have had the wax removed and there are different levels of bleaching to get progressively lighter colors. They are dissolved in warm ethanol with or without methanol but without any water etc., and shaken/stirred for a few days to dissolve as much lac resin as possible, then filtered through a paint strainer or coffee filter to remove insoluble debris.
Shellac is also soluble in mixtures of petroleum solvents, where it is referred to as "padding lacquer".


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