DC / RF voltmeter

[K9STH]

My old Motorola TEK-7A DC / RF voltmeter, which I have had for right at 45 years, got knocked off the bench and the meter bezel was shattered. The unit still works fine, but I have to be careful not to damage the meter movement. So, I went on-line looking for 50 microamp meters. I found some import meters that were priced at $5.90 each including shipping. So, I ordered 3 of them. It took a little over 2-weeks to get them in the mail.

These new meters are almost exactly the same size as the original Simpson meter in the TEK-7A. However, the mounting holes were not quite the same requiring some modification to the TEK-7A. Also, the RF / DC probe for the TEK-7A has been "through the wars" and is getting a bit "ragged". The result was that I decided to build basically a "home brew" version of the TEK-7A and build new probes (separate probes instead of a single "switched" probe).

Most of the resistors in the TEK-7A are pretty common, with a couple of exceptions. As such, for a couple of the resistors I substituted 2-series resistors which came very close to the original values. Since "stock" resistors do vary, within tolerance, from the absolute value, I chose combinations that came the closest to the "ideal" values checking with my DMM.

The aluminum mini-box, sold for $3.29 at Radio Shack, was just the right size, so that was the basis for the unit. The optimum rotary switch is a 1-pole, 6-position. However, I didn't have such a switch in stock so I used half of a 2-pole, 6-position, switch. The stereo jack and stereo plugs for the probes are also available at Radio Shack if one doesn't have such on hand.

The necessary holes were cut in the mini-box and the wiring is straight forward.

I used old ball point pen housings for the probes. The DC probe is the body from a really cheap "Bic" pin. The probe tip was made from a 3d nail with the head cut down so that it fit inside the pen housing. Since the negative voltage lead has to be isolated from the mini-box (so that you don't get a shock!), 2-wire, shielded, wire was used. One wire, the wire that goes to the "tip" of the stereo plug, was put through the pen housing and soldered to the head of the 3d nail. The nail was then epoxied into the end of the pen housing. A piece of braid from some old RG58/U was attached to the "other" wire at the end of the pen housing opposite from the "tip". Heat shrink was then put over the probe.

The RF probe was made from a larger ball point pen housing. The ink cartridge was removed and the parts were run through the housing. The parts used for the RF probe are small enough so that, if you are careful, they will "slip" into the pen housing. Attach the shielded wire to the parts before running them into the pen housing. The lead from the coupling capacitor is run out through the end of the housing and then soldered to the head of a 3d nail. The nail was then run through the metal tip of the pen and epoxied in place. After the epoxy has set, screw the metal tip back on the pen housing. Again, a piece of braid from RG59/U was attached to the negative side. However, in the case of the RF probe, the shield is attached to the "ground wire" at the probe end and it is attached to the "ring" on the stereo plug instead of going to the "barrel" as with the DC probe. Before the stereo plug was attached, the wire was epoxied at the probe end where it comes from the pen housing. Also, a small piece of heat shrink was put over the end of the probe to help retain the wire.

An alligator clip was put on the end of both pieces of braid.

With a 50-microamp meter, the unit is basically a "20,000 ohms per volt" meter which was the "olde tyme" standard for VOMs for decades. The accuracy is, of course, no where near as good as a "modern" DMM. However, the accuracy is in the 5-percent range which is not bad for general troubleshooting.

The approximate full scale voltage readings are as follows:

Position 1: 0.1 volts DC / 0.5 volts RF

Position 2: 1 volt DC / RF

Position 3: 5 volts DC / RF

Position 4: 15 volts DC / RF

Position 5: 100 volts DC

Position 6: 200 volts DC

I verified the calibration with my DMM using batteries, a variable power supply, and fixed power supplies (which just happen to put out almost exactly 100-volts and 200-volts). The calibration was very good. Of course, you have to "mentally" convert the 0-50 microamp reading from the meter to volts by dividing the full scale reading by 50 to get the reading for each "tick" on the meter. In reality, just use either the 5 or 10 division marks and you will soon get to know just where various voltages read.

Attached is a photo of the meter with the probes and a scan of the schematic.

Glen, K9STH

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[AF6LJ]

Nice looks like a fun project.

[K9STH]

I forgot to mention that the purpose of the DPDT switch is to reverse the polarity of the meter so that both negative and positive voltages can be read. Also, the RF probe produces a negative voltage so the switch has to be put in the reverse polarity position to read the RF voltage.

Attached is a better photo of the unit.

Glen, K9STH

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