Paco V-70 Valve Voltmeter

The Paco V-70 is a basic valve voltmeter (VTVM), with a double triode used as cathode followers in a differential bridge configuration. The circuitry is much like the Heathkit V-7A I also have, but the components are not mounted on a circuit board but on a metal chassis. And of course, the Paco V-70 comes in a larger case with a slightly more elegant styling.

The Paco V-70 uses two valves:

• a 12AU7 as the amplifier
• a 6AL5 as rectifier for AC measurements.

This valve voltmeter is neatly built. All components have enough space and it is easy enough to reach the callibration controls.

I bought this valve voltmeter in June 2009 on the quarterly NVHR radio swapmeet. It looked like it was in good condition and the original probe and earth lead came with it. But there was no manual. On Internet, I found a 3 page excerpt from the manual on the Boat Anchor Manuals Archive site. The last page dates the manual in 1957. I haven't found any production dates on components in the instrument itself.

When I bought it, I noticed the meter movement was heavily out of balance. Since I have done that before, I trusted I'd be able to fix it. At home, I first opened up the meter movement. As it turned out, this movement has a somewhat special balancing arrangement. In most d'Arsonval meter movements there are three counterweights, in a cross configuration with the pointer. In this meter movement, there are only two counterweights, sticking out at a 120 degree angle relative to the pointer. This makes the balancing procedure less straightforward. The counterweights were soldered, so that they could easily be set. After a few tries I was content and I could go on with the electronic circuits.

The VTVM appeared to work well and I was able to callibrate DC and ohms ranges. But the AC ranges gave a too low reading. Swapping the 6AL7 didn't make any difference. I was in a hurry so I postponed fixing that until later.