Hewlett-Packard HP-312A

I love a piece of test equipment that can act as its own radio receiver (see also the Rider Chanalyst).

The Hewlett-Packard HP-312A is a frequency selective voltmeter designed to measure the strength of a radio signal from one kilohertz to 18 megahertz. The bandwidth is selectable among 3100 Hz, 1000 Hz, and 200 Hz. It has a Nixie tube display showing the frequency being monitered to within 10 hertz. Selling for around $4000 new in 1967, these can be found at hamfests for well under $100 each, sometimes in the $10-20 range. They are true boatanchors in weight but are competely solid-state with the exception of the Nixie number-display tubes. They are also extremely well made. The sensitivity is excellent. The lowest meter scale is 3 microvolts full-scale with a 50 ohm impedance antenna connected.

The unit shown is monitoring a 40 meter SSB transmission. The 10K ohm line-out audio (right lower front panel) is amplified by a cheap powered speaker. The 312A is a favorite for low-frequency (VLF) monitoring. It also is an excellent tool for servicing the usual boatanchor receivers, calibrating a cheap signal generator, etc. The main drawback for use as a radio is the lack of AGC, thus having to manually select the proper input level for a signal. The bandwidth selection is a bit limited for use on AM, but is excellent for SSB and CW.

The 312A is labeled as either "Selective voltmeter" or as "Wave analyzer". Both labels are used for the same device. Intermittent switch contacts are typical for long-inactive units. Using de-oxit on the switches will typically restore a set to full function.