[volt-nuts] OCD About My HP419A Attenuator Switch Gold Plated Contacts

Chuck Harris cfharris at erols.com
Wed Mar 11 02:26:35 EDT 2015

Does KOH really evaporate, and spread in the wind?

I don't think it does.

In my experience, the damage done by nicad's is limited to
where the electrolyte dribbles, spatters, or wicks.
Powdered electrolyte can get all over things, but usually
it can be just brushed away, as it is not too corrosive
when dry.

If the switches are not covered with KOH dust and crust, I
truly doubt that they have any measurable amount of KOH
inside of them.


If you want to be really sure, go to your grocery store and
buy yourself a gallon of steam distilled water for $0.50, or
so, and an empty spray bottle, like is used for window cleaner
and liberally hose out the switches and any other area you
are concerned about.

When you are done, shake, blow, or simply let drip as much
water as you can off of the unit.  Set the unit up next to
a fan, and let the fan blow air on it for a couple of days.

It will dry out.

If you want to be doubly sure, a convection oven is the
answer.  There are several ways you can get one on the cheap.
If your house has an electric convection oven in the kitchen,
you can use that set to 150F (50-60C).  Or, you can put a
muffin fan on a simple stand, and put it in an electric oven,
with the door closed, and the oven light turned on.  (Gas
ovens are fine too, if they do not have a pilot light.)

And, if you want to stay away from the kitchen, you can fashion
a simple convection oven using a cardboard box, a 40W lamp,
and a small muffin fan.  Make sure the box is big enough so
that the lamp is no closer than 12 inches from any side of the
box, and make sure that the fan and lamp cannot fall over, or
the box move and cause issues... don't want to make a fire

-Chuck Harris

Stan Katz wrote:
> Now that I've restored my HP419A to running order for the meantime, I have
> nagging questions regarding the continuous use of the attenuator switch.
> The main board of the instrument, A4, was badly polluted with an invisible
> layer of potassium hydroxide from the 30 years of being in the same box
> with dead/mummified nicad batteries.
> In accordance with
> http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC43-206part2.pdf I
> prepared a 3% boric acid solution and flushed the affected circuit board
> several times, while scrubbing with a soft nylon toothbrush. I then flushed
> the board with hot water. After leaving the board on my furnace plenum for
> a few days to thoroughly dry out, the board problem was sorted out. If the
> KOH deposited an invisible layer on A4, then certainly, there must be a
> layer of KOH on the attenuator switch's gold contacts, as well. I posted my
> concern about the KOH on a premier chemistry forum. The administrator
> himself answered my query. He indicated that the only way to remove the KOH
> from the switch is to thoroughly flush with di water.  There's no place for
> a flushing stream of water to exit the instrument safely.   I'm not about
> to remove the switch! Now, I don't know the hardness of KOH,  but Potassium
> has a very small .5 mohs of hardness. I presume, that KOH is similar. In
> any event, I would tend to believe there will be some small abrasive wear
> from the KOH. The attenuator on a null meter is heavily utilized, and even
> a light abrasive can cause trouble eventually.  Proper gold plating, that's
> kept clean, never suffers from abrasive wear. Improper plating, can suffer
> from adhesive wear, but I have confidence that the HP of the 1960s-80s was
> using the finest gold plating available.  I thought about applying one the
> the Deoxit products. A careful reading of the Deoxit literature claims that
> the D-series treatment is for "non-critical metal surfaces". I think that
> the gold plating in the HP419A qualifies as "quite critical". The Deoxit G
> series literature leads me to believe it may not have enough cleaning
> action to flush out the KOH. Also, please note, only the removal of oxides
> is mentioned as a feature of the Deoxit line. What other corrosion products
> that may be removed are not mentioned.
> A proper "science project" would be to obtain Bromothymol blue indicator
> solution, and prepare it with Phosphoric acid in accordance with the above
> citation. I could then apply it to the switch contacts ( in itself a PITA
> job ), and then try the Deoxit G series, and perhaps other brands of
> contact cleaner to see if the KOH is truly removed without resorting to
> water. I'm actually willing to take on such a project ( it's a hobby, after
> all ), but I don't even know how to procure a proper concentration of
> Bromothymol blue from over the internet. Without access to purchase orders
> through a university, or corporation, even innocuous chemicals are
> unobtainable from US chemical supply houses directly by individuals. There
> are lots of offers on Ebay, but I have no way of knowing if what's sold is
> a the proper concentration. I don't even know how to specify what that
> concentration should be.
> Yes, I should get on with my life, but I've invested quite a few hours
> restoring this instrument, and I just can't let this situation fester
> without a resolution.  Can anyone on the list help put me out of my misery?
> Stan
> _______________________________________________
> volt-nuts mailing list -- volt-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
> and follow the instructions there.

More information about the volt-nuts mailing list