Earlier, I speculated that my test bench might have room for "just one more" meter, probably in the form of an analog FET VOM. In the last day or so I was lucky enough to pick up a nicely kept HP410C lab meter (see e.g
http://www.barrytech.com/hewlett-packard/meters_analog/hp410c.html), which is in a class well above those I mentioned in the earlier post. I took care to ensure that the meter I was buying had the "new" amplifier board, introduced around 1970 and based on a FET/op amp combination. The earlier board used a devilishly clever low-ish tech chopper based on neons, photocells, transistors and a tube. While it worked well, the choppers are now quite difficult to repair.
The specifications in the link above show that the meter is very capable in terms of accuracy, has a 100 M ohm input impedance on many ranges, and comes with an ac probe which works from a few hertz to over 700 MHz, and from millivolts to hundreds of volts. To get the sensitivity, frequency response and accuracy HP chose to keep an acorn tube diode in the probe, rather than go to a solid state detector. The probes can be something of an Achilles heel but, fortunately, mine is in good shape and works well. If the tube expires and no replacement is available, the probe can be re-built with a solid state diode to give tolerably good results.
The meter was in good condition and, checked against my Keysight 6.5 digit 34461A DMM, was within specification on all dc ranges. The "ohms infinity" adjustment pre-set was at one extreme of its travel and investigation of the circuit showed a 68 ohm resistor in series with the pot had gone high. After resistor replacement and pot adjustment the ohms meter was also impressively accurate. The last fix was to cure a very noisy ac 0.5 V FSD position. The active probe was clearly working but the meter was very erratic. After a bit of investigation the lowest range ac calibration pot turned out to be very touchy and the rather specialized 6.5 mm probe socket contacts were a bit the worse for 50 years of use. Some cleaning cured both problems and, after resetting the ac coarse zero and calibrating the low range against the 34461A, the meter was well within low-frequency specifications, with minimal drift.
A quick check of the probe showed an ac response within 1 dB from audio to about 500 MHz, a little down on the original response and accuracy specifications but still very good. The acorn tubes are still around and, with the rest of the meter working well, I may treat it to a new detector to see if the original specification can be reached. I was also lucky enough to get the HP11042A coaxial line probe T adapter with the meter, making coaxial in-line measurements easy. So, in the end, a very nice analog electronic meter for not much effort.
A closing question for HP aficionados. Does anyone know if the full manual for the "new" HP410C is available on line? I have the schematic for the newer A3 solid state amplifier PCB, and most of the remainder of the meter is the same as the earlier version, the manual for which is published widely. Still, it'd be nice to get a full manual for my version and a quick look around the HP Group didn't provide any clues (although a couple of people had asked the same question).
73, Peter