Sprayberry Academy of Radio VOM

Sprayberry Academy of Radio VOM

I had seen advertisements for the Sprayberry Academy of Radio including the VOM tester offered as part of their course in a few old Popular Electronics magazines (mid 1950's). When I came across one of these testers in an auction, my curiosity was piqued. It has more knobs and settings than the typical VOM.

Sprayberry VOM
SprayVOM.jpg (42k)

Sprayberry Ad in 1/56 Poptronics showing VOM
 Sprayberry Academy of Radio Ad 1/56 Poptronics (42k)

VOM includes resistor-capacitor substitution box
What I found on opening up the VOM and tracing the circuitry was that the switches on the lower vertical section were selectors for resistors and capacitors for in-circuit substitution by way of the two banana jack/ binding posts on the right. Those two jacks were also used for just one VOM setting, the R X 10,000 resistance measurement. That one setting requires an external source of high voltage. The pin jacks on the left are used for all of the other VOM functions.

The R X 1, R X 10, and R X 100 settings need a 7 to 9 volt battery source. The set apparently used a battery with screw terminals, likely 7.5 volts. A 9 volt battery works fine as a substitute. The R X 1000 setting requires adding a 45 volt battery. Since only a milliamp of current is drawn for full-scale zero ohms setting at that range, a set of five partly worn 9 volt transistor batteries in series should work fine. The 1 mA draw at full-scale indicates a 1000 ohms-per-volt meter.

Electronic condition and repairs
The capacitor substitution section will require replacement of several of the higher value capacitors which were leaky to some degree. Several of the resistors in the voltage measurement function will also need to be replaced since the readings were a bit low on several settings.

An earlier Sprayberry ad
Sprayberry ads in the late 1930's and 1940's did not include the VOM tester. They primarily advertised their radio kit, provided by Meissner, that was included in the course.

The Meissner Broadcast TRF receiver kit was the previous item on the bench.

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