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

fala at gmx.net fala at gmx.net
Wed Mar 11 03:04:54 EDT 2015

Hello Stan,

since KOH is water soluble, thorough cleaning with a moist cloth (di 
water) should do the job as well. Just wear proper protection, i.e. 
safety goggles and gloves. I myself would use disposable latex gloves 
from the grocery store in this instance, since you are dealing with very 
dilute chemicals (beware: latex gloves do not protect against chemicals! 
If in doubt, use (disposable) nitrile gloves).

Furthermore, instead of bromthymol blue, you could check your progress 
with a "universal indicator" or "pH indicator" paper as well (ref. 
Wikipedia "Universal indicator"). Should be easier to obtain.Indicator 
paper is a disposable roll of thin paper, treated with several indicator 
chemicals (e.g. bromthymol blue and others) which change color based on 
pH. After wetting the paper with di water, observe if there is a color 
change after contacting the polluted parts (KOH is basic).

- Firat

Am 11.03.2015 um 06:42 schrieb Stan Katz:
> 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
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