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

Stan Katz stan.katz.hk at gmail.com
Wed Mar 11 01:42:58 EDT 2015

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?


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