I’m getting closer to having a reasonable calibration setup, so I now need a null meter for comparisons between my lab voltage standards and my HP 6920B Meter Calibrator.
So, I thought I’d restore my HP 413A Null Meter. The Boatanchor Manual Archive (BAMA) has a scan of the manual.
The meter movement is stamped with a date of April 4, 1964.
It’s one of those individually calibrated HP meters. HP printed the scale photographically on an emulsion coating the meter card. The process is described in the March 1961 issue of the HP Journal. (The HP Labs archive of the HP Journal has been somewhat unreliable lately. Worldradiohistory.com also has a copy of the March 1961 issue.)
Unfortunately, this emulsion sometimes blisters, cracks, and lifts from the card. Often it lifts high enough to interfere with the movement of the needle. Since the scale is individually calibrated to the non-linearity of the meter movement, you can’t just replace it with a card from another meter movement.
I thought the only solution was to scan the meter card and print a new one.
To do that, I had to remove the meter bezel. The bezel is attached by four 5/32” hex head machine screws. The heads of the screws are in a recess in the meter back. The recess didn't have enough clearance to allow my Xcelite nut-driver to fit.
So I had a good excuse to buy a Moody Tools nut driver set.
Their nut drivers have very thin walls, so I bet that they would fit.
And they do, just barely. There’s no clearance to spare.
I cheaped-out and bought their 55-0142 set, which has one handle and five nut driver blades. They also sell a five-piece set that comes with a handle for each of the five blades, the 58-0150. I bought it direct from Central Tool (now the owner of the Moody Tool line). Central Tool is located just south of me in Rhode Island, so they arrived fairly promptly. I bought one of the JIS crosspoint screwdriver sets along with the nut drivers.
I scanned the meter card. You can download a full 1600 dpi image of it in a PDF, although it probably won’t match your own meter movement very accurately.
I happened to leave the meter card in the scanner for a few days after scanning it. When I inspected it afterwards, I found that the lid of the scanner had compressed the emulsion back into place. It was no longer raised from the card, and wouldn’t interfere with the meter pointer.
You can still see some bubbles, but hopefully they will stay put for a while.
I thought about masking off the mirror and spraying a coat of lacquer to fix the emulsion in place, but my spray booth is out of commission at the moment. Also, I wanted to use Microscale’s Micro Mask liquid masking film, rather than trying cut a piece of paper to match the mirror cut-out and rubber cement it in place. I’ll have to order some as I don’t have any in stock.