"The Ballantine Model 421A is an accurate voltage source for the calibration of voltmeters, recorders, oscilloscopes and other high input impedance voltage sensitive devices. It provides dc output of both polarities and ac outputs of 400 Hz and 1 kHz calibrated in terms of either rms or peak-to-peak value. Any output voltage throughout a wide voltage range from 100 uV (1 mV full range) to 111 V may be easily selected and displayed on a clear in-line readout with at least four digit precision. For 400 Hz a separate protected output of up to 1110 V ia available.
These features, along with small size, rugged construction, easy operation and low cost, make the Ballantine Model 421A an ideal calibration tool for production line, repair shop and field use." (Ballantine Labs., Manual 421A, 1973)
BRIEF SPECS
| DCV Range | ±1.11 mV to ±111 V |
| DCV Accuracy | ±(0.02 % of setting + 0.007% of range) |
| ACV Range | 1.11 mV to 111 V rms or pp |
| ACV Accuracy | ±(0.015 % of setting + 0.005% of range) |
| AC Frequency | 400 Hz or 1000 Hz |
| Digits | 4 |
| Power | 40 watts |
| Dimensions | 7" x 19" x 10" (rack mount) |
| Weight | 21 pounds (9.5 kg) |
| Technology | tube, transistor |
| Price (for 1966) | $ 650 |
When you maintain a lot of meters later or sooner the question come up which of them tells you the truth. If you also have some high resolution dmm, let's say 4,5 and more digits, a trial of repair without a stable & precision voltage source must fail. Used voltage standards are still in high demand which keeps prices fairly high. I bought this ac/dc-calibrator from a surplus dealer because was a good compromise between its accuracy and price tag.
When looking on the data you may notice that the AC accuracy is superior to DC. This is because the 421A is in fact a constant leveled ac source, where the main 100 V ac is divided with precision winded transformers having 'digit' taps. DC is just the rectified ac output, nothing else (o.k. there is some more circuitry to compensate any unlinear effects of rectification). During calibration I was not able to get the expected results. A more closer look on the parts list stated, that Ballantine tried out resistors to find out the right value. Despite the costy time of try-out, this method payed off as fixed resistors have much better specs than pots. After changing the resistor which sets the oscillator output to the design center, all following adjustments were no more problem. Ballantine Labs. is still in business - they provided me a copy of the manual for free.
