PDP-8/a cleaning

allison allisonportable at gmail.com
Tue Apr 25 09:01:49 CDT 2017

On 4/25/17 6:06 AM, Pete Turnbull via cctech wrote:
> 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
Considering your also trying to remove possible hazardous material and 
leave a noncontaminating
surface its overkill to the max for a backplane.

> 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.
Again you are striving to neutralize the chemistry used.

> 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.
Considering a fan(s) has been blowing dust and who knows what else at it 
for years
cleaning is good and any residue is nothing compared to the crud that 
was formerly there.

In the pre-freon days the standard cleaner (when they did!) was a 
modified dish washer.
The usual mod was reshape the racks to hold the boards and lower the 
heater temperature
used for drying.  A week rarely went by that someone had open it mid 
cycle to see if
it was done.

>> 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.
> Ouch!

Plastics and elastic materials require more care for compatibility.
Water is usually the safest but, none can remain.


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