15 Dec
2001
15 Dec
'01
9:35 p.m.
It might be worth noting whether your water is "soft" before applying it to
your
boards. The minerals in may municipal water supplies might be sufficient to
cause problems. Getting them off might be a bigger problem than with "normal"
mineral content.
Dick
----- Original Message -----
From: "Pete Turnbull" <pete(a)dunnington.u-net.com>
To: <classiccmp(a)classiccmp.org>
Sent: Saturday, December 15, 2001 1:50 PM
Subject: Re: [PDP8-Lovers] how to clean a PDP8/A, dishwasher?
On Dec 15, 12:28, Matthew Sell wrote:
Typically, the time spent "in the
water" isn't long enough to damage.
Even
items made of steel and iron won't rust if
the water is removed after the
cycle is complete. If they sit overnight, well, that's a different story.
The only production problem I saw with untreated water was with an
electronic test instrument that had a lot of high-impedance signal
interconnects throughout. Many signal lines ran for long distances next
to
each other. While the design of this piece of
test equipment was
questionable, it was our duty to get it to work.
The two biggest problems were contaminants from the water supply used in
the washing process (city water - switched to using a commercial
filtration
system), and humidity (had to paint a sealant on
all of the boards).
That's a well-known problem. Some of the residues from a domestic water
supply -- especially in hard water areas -- are mildly hygroscopic, and as
a result, the boards would acquire very small amounts of moisture on the
surface, especially when exposed to a humid atmosphere. In combination
with the salts in the residue, this makes for leakage across the board,
which could easily upset very high impedance circuits.
I heard of someone who had the opposite problem. He designed a CMOS
circuit which worked fine when forst contructed, but stopped when given
anti-environment protection or was potted. He'd inadvertantly relied on
the normal leakage across a PCB to hold the unused inputs of a CMOS gate at
a particular level. Remove the leakage current and the gate stops working
properly. Solution: add the pullup resistor that should have been there in
the first place.
--
Pete Peter Turnbull
Network Manager
University of York