PDP-8/a cleaning

Pete Turnbull pete at dunnington.plus.com
Tue Apr 25 05:06:45 CDT 2017

On 25/04/2017 10:08, jim stephens via cctalk wrote:
> On 4/25/2017 1:39 AM, Pete Turnbull via cctalk wrote:
>> "Little residue" would be more accurate, and some of that residue
>> will be water (look up "azeotrope") - plus you need a lot of
>> alcohol for something the size of a PDP-8 backplane.  Blow dry,
>> even after an alcohol rinse.

I should perhaps have mentioned that the idea is to flush the remaining
water or alcohol out by blowing, not evaporate it like your hairdresser
would :-)  And you ought to use dry air, ideally - most compressors have 
water in their air.

> In the process of cleaning optics indeed you need air and other
> means to do that, you are right.  But in this case I'm suggesting
> the alcohol as a way to displace water out of internal parts.  The
> spotting or such is not much to worry about in the cleaning job on a
> computer part.

Except those spotty residues are usually hygroscopic, which can lead to
corrosion later.

> But in optics the process is much longer and elaborate, but still
> needs the ventilation to be sure you don't have a problem with
> fumes.

Sure.  Outside of electronics, my experience is in a chemistry lab 
needing clean dry glassware.  The process would go something like this:

- preliminary clean with whatever is best, often water and a little
   detergent/surfactant, then drain most off
- rinse with distilled water
- rinse with ethanol to flush out remaining water, then drain
- rinse with acetone to remove the alcohol/water residue
- air dry

In photography, on the other hand, the final rinse would just be water - 
tap water if not too hard - with a tiny amount of a wetting agent (eg 
detergent) in it.

For a backplane or some PCBs I'd compromise, but closer to the 
photographic example than the chem lab.  In fact I've done that with my 
PDP-8s, rinsing the backplanes then blowing out most of the residue.

> We had a booboo in assembly that required cleaning and we no longer
> had freon cleaner we wanted to use in that quantity, so we went with
> the water / alcohol process.  A switch had defective sticky seals on
> it and they had all gotten waterlogged.  Vendor claimed they would
> survive water process wash and they were wrong.  Paid us quite a bit
> in credit for messing up a couple hundred boards before we caught
> the problem.


Pete Turnbull

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