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.
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.
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.
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.
Two of the leads on the capacitor here (it's a 600 volt cap) are from the IT-18, and two are from...
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.
Then you reverse the RCA's leads, change it to DC- and calibrate it for DC-.
Straightforward to do.
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.