I already had a Hewlett-Packard 410BR, the rack-mount version of their famous 410B Vacuum Tube Volt Meters, arguably one of the best VTVMs ever made. However a rack-mount piece is obviously not designed to be portable. This portable version came up for auction at a recent swap meet. Both the power cord and the probe cables were stiff and brittle. The condition kept the price reasonable. The meter was of course made with the excellent build-quality typical of a Hewlett Packard test instrument of the vacuum tube era. However, one never knows what sort of repairs are needed. I figured that in the worst case it could be used as a source of spare parts for my rack-mount version.
I replaced the worn and cracked power cord with a new grounded cord and plug. After this and some safety checks, I powered the VTVM. It was working on DC but not on AC. The AC probe stayed cold when the unit was plugged in. This particular unit uses the special Amperex EA53 diode tube inside the probe. The EA53 filament is 6.3 volts. My rack mount version uses the Eitel-McCullough 2-01C which has a 5 volt filament. It was apparent that some repairs had been made on the probe cable. The separate ground cable and ohms cables had been shortened and had cracked and worn insulation. The special triaxial cable used for the AC probe had also been shortened and was in very poor shape. I suspected that that probe cable had broken connections and was the cause of the probe staying cold, not powering the EA53 tube.
HP originally supplied a complete probe and cable assembly for replacement purposes. The cabling was not considered to be repairable. That was the case with the sealed probe cable for my rack mount version. In an addendum in later HP manuals, the triaxial cable was sold separately implying that the probe and cable assembly was repairable. On this VTVM, it was obvious that someone in the past had tried repairing the AC probe cable, I decided to see what would be needed to repair it myself. I had a roll of yellow Belden 9222 triaxial cable in the garage. It was quite flexible and should work well in the application. Opening the back end of the AC probe as well as the connector on the bottom of the VTVM exposed the soldered wire connections, two of which had broken loose from the solder points inside the probe. The triaxial cable has a center conductor and two concentric shields. The inner shield is grounded and the outer shield carries the 6.3 volts AC for filament power.
After determining which connection was which and some very careful soldering of the replacement triaxial cable inside the back of the metal probe, I used heat shrink tubing and some electrical tape to insulate the solder connections and strengthen the cable at the attachment point. I also soldered the triaxial cable and new ohms and ground cables onto the chassis connector. And the result? I was rewarded with a warm probe and successful repair.
What's it good for?
Very few digital meters can perform as well for AC measurement at radio frequencies as the HP-410B. As noted on my HP-410BR web page, the VTVM can measure AC voltages with a frequency response to 700 MHz within 1 dB. It is useful to around 3 GHz according to the manual. That makes it excellent for measuring radio frequency voltages such as those encountered across a transmitter dummy load. It can also measure DC with an input impedance of 122 megohms on all ranges. Resistance can be measured from 0.2 ohms to 500 megohms. The HP uses an internal low-voltage DC source for the ohms function instead of a battery, therefore battery corrosion is never a problem.
The 410B was introduced in an article titled, The 700 Megacycle Voltmeter and Its Applications
in the November 1950 Hewlett-Packard Journal (Vol 2, No. 3).
A good quality PDF version of the operating and service manual is available on the HP archive site. Here is an index of manual links including the HP Archive site.
Adjusting for calibration.
The HP-410B has several screwdriver adjustable calibration potentiometers. Section IV, Page 4 of the manual has the procedure. I used a high-precision DC meter as the reference for adjusting the DC scales (one adjustment, R-32 at 1 volt) and an isolated variable AC source with a known accurate reference meter to tweak the various AC ranges.
List of other HP pieces I have repaired
Here is an index to my other HP projects.
A Hallicrafters SX-100 receiver was the previous item on the bench.