The HP 410A was HP's first VTVM (Vacuum-Tube-Voltmeter) introduced after the Second World War. It went through several design iterations, before being replaced by the 410B. One notable feature of this meter
was the diode rectifier, specially manufactured by Eimac, in the AC probe. The first revision (the model that I had) actually used two of the diodes with one intended to cancel the quiescent current in the other. Unfortunately, this arrangement
was rather unstable, and HP abandoned it in a later revision.
The front of the meter after I acquired it
The repair process for this meter was quite intensive. All of the old paper and oil filled capacitors were replaced due to their inherent unreliable nature. While testing the meter, I found that it was quite inaccurate, and I ended up having to replace most of the precision reference resistors. I also installed new probes and probe cables on the meter, as the old ones were dry rotted. After that, I still had problems calibrating the DC range, which turned out to be partially caused by some mica capacitors on the cathode follower tube. This was quite surprising, as mica capacitors are usually quite stable. I even tested them with a General Radio capacitance bridge, and I was unable to detect any DC leakage. After all the work I did on the meter, I was still unable to bring the AC range into calibration. Given the unstable nature of its design and the fact that I fixed a GR 1806-A Voltmeter, I ended up selling the 410A.
New capacitors installed
Frozen potentiometer that had to be disassembled and repacked with grease
Experimenting with new capcitors on the cathode follower
The meter in its final, mostly operational state
