[Elecraft] KPA100 resistance check shows negative resistance

[Bob]

Hi Tom,

Several years ago I had a problem with a new PC board that was generating about 1V between some points even when the power was
completely disconnected.  This turned out to be due to contamination on the board left over from the etching process, although it
was not visible.  After throughly cleaning the board with acetone, it was ok.   Acetone might be too strong for this board, so try
some 99% isopropyl alcohol or pure ethyl alcohol from the liquor store.
(About a month ago there was a series of messages about board cleaning).

Here are some tips posted earlier:
=================================
From Ron, AC7AC:

"I just want to point out, in case you missed it, this statement in all of
the Elecraft manuals I have seen:

"DO NOT use ... solvents of any kind. Use of any of these will void your
warranty."

I can't offer any other suggestions for common home cleaners because I don't
use solvents on boards. A number of builders have used various solvents and
(apparently) gotten away with it. And then there are some owners who have
had to chase down odd intermittent problems and other issues, or who are
still chasing them.

One of the concerns is NOT what the cleaner does (or might do) to the board,
but what it can do to components like relays and other parts. Even handled
on a swab, it's amazing how far a good solvent will travel along wires, pins
and other surfaces to get in places we thought were "sealed".

The most popular cleaner that I've seen (both on here an commercially) is
alcohol. The problem with the sort of alcohol we can pick up at the store is
that it also contains lots of water, so you are sort of putting the board
"under the faucet". That sort of treatment of electronic devices never
appealed to me. The stuff I have used commercially was 100% pure (200 proof)
grain alcohol... and the company had to pay liquor tax on every drop we
failed to "recover" after using it. "
=================================
From Don Brown, KD5NDB:

"First Elecraft does not recommend cleaning the boards. If you feel you must
use only 99% isopropyl alcohol. Mineral spirits or acetone may attack some
of the plastic parts on the board like the relay housings and BNC
connectors. I have used denatured grain alcohol in a pinch but you need to
be careful with some of them because of the different additives used to
denature it. Drug stores carry 99% isopropyl alcohol. Do not use rubbing
alcohol it is 30% water and will leave a white powder residue on the board
and it does not clean very well."

=================================

HTH.

73/ Bob - AG5Q


----- Original Message -----
From: "Tom, W6WW" <tom@cwx.com>
To: "- Elecraft" <elecraft@mailman.qth.net>
Sent: Thursday, July 17, 2003 9:31 AM
Subject: [Elecraft] KPA100 resistance check shows negative resistance


> I need help with my KPA100 construction.
>
> Performing the resistance checks on page 41 just after assembling the KPA100
> board to the heat sink, I find a NEGATIVE resistance ( -2 meg) on my DMM at
> the J3 + lead.  Further checking shows a persistent voltage of -0.35 volts
> at that J3 + lead with no power connected to the board.
>
> With an analog meter, the resistance check varies greatly depending on the
> scale used, but the negative voltage persists.
>
> I have tried shorting the leads to ground in an attempt to dissapate any
> charge on the electrolytic  capacitor, but the results stay the same.
>
> Any ideas about how to proceed would be greatly appreciated.
>
> 73,
> Tom, W6WW
> K2 #3453