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@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.