I recently acquired a family of six Guildline 9330 oil\air bath standard resistors, in the values of 1 ohm, 10 ohm, 100 ohms, 1K ohms, 10K ohms, and 100K ohms from a scientific surplus instrument company. The resistors have serial numbers in the low 27,000s ( a few have sequential numbers ). The resistors were originally immersed in oil, and they were mounted on a Plexiglas base, with a Plexiglas top. On the top were four Superior 5-way binding posts for each resistor, two for potential and two for current. Pure thin copper strips were used to join each resistor's terminals to the binding posts. Both Plexiglas sheets were joined together by six brass support rods. Testing with a 34401A showed all the resistors were fine.
Once I took the resistor system apart down to just the resistors that I wanted, I cleaned each resistor of the old oil with a brief soak in petroleum ether, following all chemical and fire safety rules. Two washes were enough to remove all the old oil. I began to use a small amount of Brasso to clean the darkened copper current terminal bars. The potential terminals were clean copper however. But, I found the current posts of each resistor to have what appears to be a strange deposit of a silvery colored metal when I began to clean them. The copper oxidation was too strong for Brasso, so I sealed up each resistor in masking tape, and then I tried a drop of 30% HCl, which quickly dissolved the dark copper oxidation. But, underneath it, was an uneven silvery colored metal deposit. I wondered what this deposit was, since all the standard resistors I have seen have either all pure copper posts, or a special low thermal plating over the copper ( e.g. actual silver ). Then I washed off the area to which the 30% HCl was applied.
However, the deposit was strange. It didn't look like a factory treatment, since all of the potential posts were completely free of it. Also, it looked uneven in deposition, and like an electrochemical migration of some sort. I suspected this because all current bars had a coating which transitioned randomly into cleaner copper on the other surface side where the bars went inside each resistor. The deposit completely covered all other parts of the current bars, but oddly enough, the copper screw terminals were not coated, except for one. That screw terminal was also completely plated by this strange deposit. Also, the strange deposit was also present on the lower part of the copper strips connecting the current bars to the binding posts on the Plexiglas top, but stopped completely halfway up at what must have been the oil level.
I tried to clean one of the current bar screw terminals of a resistor with a brass brush, far away from the resistor. The mysterious deposit came off, leaving clean copper below. On a resistor's current bar, the next chemical I tried was a small amount of strong NaOH. NaOH did slightly dissolve the deposit, but it was slow and quickly became exhausted. The deposit then became purplish-green and I washed it away. The last chemical I tried was full strength ferric chloride for etching printed circuits. It was much more effective, but still slow, and had the weird effect of changing some of the deposit into a pasty room temperature alloy, which later hardened. After washing that away, I found that 200 grit sandpaper removes some of the deposit, but again slowly. The deposit looks plated thickly onto the copper.
Does anyone know what this strange deposit is, and if so, what chemicals will safely remove it down to the bare copper? One of the 9330 resistors of a later serial number has black painted current posts, and was spared from the deposit. That resistor has no plating under the contact areas for the current terminal contact screws- only clean copper. I can post some pictures of the strange deposit on the other resistors if needed.
On another topic, I wonder why most of the volt-nut items have left eBay, and where they went. Items like the 732A don't really appear anymore. Also, the 732B, which I have wanted for a long time, hasn't appeared on eBay for years. I have been trying to locate a 732B in good condition, without any success. Does anyone know where I can get a 732B for a reasonable price? It's fine if the 732B needs a new 12v 7Ah SLA battery, recal, cal expired, or someone reset the in-cal switch.
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