Calibrating an RCA Model WV-98C Senior VoltOhmyst

In our last installment of work on the RCA VTVM, we rebuilt the power supply.  Probably an overstatement since it's one diode and one capacitor.  But still, doesn't "rebuilt" sound like a major undertaking?

Now we can calibrate it.  This is a straightforward job, and fortunately RCA even gave us instructions in the manual.  I'm not going to show the whole operation, just a couple of highlights so you can get the idea.

Before I get going, I wanted to share a picture of the cool RCA 'meatball' logo that's on the meter face.  This is a variation of the logo that RCA used beginning when the company was founded in 1920.  There were slight modifications to the logo until the 'modern' block letter logo was used beginning the the 1960s.

I like the "lightning bolt" on the tail of the 'A.'  Radio was in its infancy when this logo was created, and the logo reflects the excitement of a new age.

Ok, enough art criticism and back to the calibration.

When the VTVM is turned off, the meter should return to zero.  If it doesn't, you remove the small cover over the meter adjustment lever and gently move the lever until it the meter needle is at zero.

Mine wasn't at zero, so I did this adjustment.

Now we'll proceed with the electronic calibration.  To do this, the VTVM has to warm up "for at least 30 minutes," says the manual.  Turn it on, and go off and do something else, such as watch qualifying for the Canadian Grand Prix.

The calibration procedure essentially consists of using a "known source" at specific voltages, connecting the meter, and adjusting various trimmers to calibrate the meter to the voltage.

Here's how I got my specific voltage readings to use as a source.  I connected my trusty Heathkit IT-18 Capacitor Checker to a 500 volt capacitor as if I were going to check it for leakage.  The checker has a switch for different DC voltages:  3, 6, 25, 75, 150, etc., volts up to 600.   I plugged the checker into my variac - which let me vary the AC voltage (line voltage) the checker is running on.  That way I can choose, say, the 75 volt range, but dial down the voltage until it reads 50 volts.

You can't get any DC voltage right off the outputs of the Heathkit.  However, if you connect a capacitor across it as you would to check it for leakage, you can then measure the voltage the capacitor is seeing.

Two of the leads on the capacitor here (it's a 600 volt cap) are from the IT-18, and two are from...

...my Fluke DMM.  This is a super accurate meter.  I just adjust the Heathkit range setting and adjust the variac it's plugged into until I get the exact voltage I need to calibrate the RCA against.

Then we just follow the RCA calibration instructions.  Basically you connect the RCA's probes to the voltage source at a specific voltage and adjust.

Here's an example.  The first adjustment is to get the RCA's meter to read full scale - exactly 50 volts - on the 50 volt DC+ range.  With 50 volts applied to it, adjust the DC+ trimmer until it reads exactly 50 volts.

Then you reverse the RCA's leads, change it to DC- and calibrate it for DC-.

Straightforward to do.

Here's the reading after the first adjustment is done - full scale (i.e. meter all the way to the right) reading at the "5.0" mark on the scale.

The rest of the calibration is similar - set the voltage, set the range on the meter, and adjust as needed.  It only takes about 15 minutes.  You also adjust for AC volts and resistance (the "Ohms" scale).

This one calibrated perfectly.  I mentioned in the first post about the meter that my other RCA wouldn't calibrate.  I need to troubleshoot that one - I suspect some to the circuit resistors have drifted in value. 

In the meantime, I have a newly serviced VTVM I can use!