Heathkit V-7 Vacuum Tube Voltmeter

I bought this cute little guy for $10.  It's a vacuum tube voltmeter.  It can measure AC and DC voltages up to 1500 Volts, and can also measure resistance.

Like the Eico 950, the Heathkit V-7 contains a bridge circuit.  The bridge is used slightly differently in each device, however.  The Eico uses a variable resistance to achieve a balanced bridge, and the tuning eye indicates when the bridge is in balance.  With the bridge circuit balanced, it is then possible to determine the resistive or capacitive value of an unknown component.

The Heathkit V-7 uses a bridge circuit in a slightly different manner.   The bridge circuit is balanced before a measurement is taken, it then uses an analog meter with a scale to show how far out of balance the bridge is.  The deflection of the meter is proportional to imbalance in the bridge, and the magnitude of this imbalance is shown on various scales on the meter face.

One important specification for a meter when measuring voltage in an operating circuit is "input impedance".  "Input Impedance" is a measure of how much resistance in Ohms the sensing circuit of the meter has to current.  When you take a voltage reading, you are putting your meter in parallel with something in the circuit.  If a meter has low impedance, it can electrically load the circuit, and drag down the circuit's voltage, just by taking the measurement.  You don't want that, so a high impedance meter is better.  This old vacuum tube V-7 has an 11 Megohm input impedance, which even today is a solid specification.  The modern digital Fluke 87 digital multimeter has a 10 Megohm input impedance.  So there!

Below, the schematic of a V-7A, slightly different from the above V-7. 

Both the Heathkit and the Eico were sold as electronic kits, so the assembly may leave a bit to be desired.  The Eico was certainly assembled a little poorly, but the Heathkit appears fairly solid, aside from a minor issue with the pilot light.

Pilot light jewel lens was missing. The case was pretty dirty as well.

Inside:  Solder trace had lifted off the circuit board and broken.  Note the solder joints at the left that have not filled in the holes completely.  Was solder expensive back in the day, or was it just a workmanship issue?

Fixed and cleaned up!  Works fine.