Toshiba BC-1212 Predictable Failure

Here is a suggestion which BC-1212 owners may wish to consider for the sake of preserving their units. My unit (Serial No. 165632z) was manufactured with some capacitors with an inadequate voltage rating. The consequence if these capacitors fail is the destruction of integrated circuits which are now unavailable, although it possible to construct a substitute from modern CMOS.

In more detail:

The unit developed a problem whereby one of the digits would stay on and bright at all times. After a while the calculator failed more completely.

The problem was traced to a shorted capacitor in one of the digit driver circuits (see schematic below). These capacitors require a high voltage rating in order to isolate the high Nixie anode voltage (180V) from the low-voltage logic circuits. When one of these capacitors shorts, 180V is fed to a PD10 IC in the digit-timing ring-counter. The diode in the circuit, although reverse-biased, is inadequate to block the 180V.

The capacitor must be able to tolerate 180 + 24 = 204 volts. The rating on the installed capacitors is 200V.

There are 12 of these capacitors, orange, "puffed-pillow" shape, located in a row on the smaller of the 2 logic boards. You can get physically smaller capacitors with a higher voltage rating today. I replaced them with 0.022F at 400V and they fit better than the originals.

Should you choose to replace them, you may wish to take MOS handling precautions while doing so as the ICs are MOS technology.

PD10s are unavailable unless you have something to scavenge from. I originally scavenged from a non-functioning Commodore DAC-612. Later, when the DAC-612 was functioning, I was back in need of a PD10 and so ended up constructing the substitute mentioned above.

  Calculators | Integrated Circuits | Displays | Simulations
Aug 2000