HP-85 tape drive lamp replacement

[MBY]

I have a HP-85 and I successfully repaired the capstan in the tape drive. It worked for a while but suddenly stopped working with the error "STALL" on all tape command. After a bit of troubleshooting I discovered that the lamp in the end-of-tape detection circuit has broken. Physically, the lead is broken at the glass, probably due to vibration. This lamp is in series with the lamp for the position encoder disk.

The lamps seems only to be on during tape operation, and since tape operation immediately stops due to error, I haven't managed to measure the lamp voltage. The lamp for the encoder wheel still works, and it starts to glow at about 1-2 volts. But I have no good idea if the lamp circuit is 12 volts (two 6 volt lamps in series) or 5 volts. I don't know the polarity either, so I don't know if it is worth it to replace with LEDs. I don't know a suitable color as the lamp is lamps for a reason (otherwise the designers probably would have chosen LEDs instead).

Do anyone know the specs of these lamps, the voltage or has heard about any modification or repair? Instructions to replace the capstan is easy to come by, but I haven't found anything about the lamps...

[MBY]

Ok, I've solved it. So, for future reference:

Many hours invested in debugging a circuit containing two lamps and a transistor! It had probably taken the age of the universe if it was three lamps in series...

Anyhow, the driving voltage is 12V and the lamps thus are 6V lamps. The current is about 70 mA. I replaced both with white LEDs but could not get it to work, so I ended up with a circuit with one LED and one lamp and resistors parallel with the LED and its series resistor in order to keep the current at 70 mA (it ended up being 60 mA, but it works) for the lamp and about 5 mA for the LED. First after many hours I discovered that the motor was stuck via its encoder disk. So the two-LED circuit probably should work fine and I switched back to the LED-lamp combo for nothing.

So, if anyone else happens to have a problem with the lamp circuit in the tape drive, the lamps are in series, a replacement lamp should glow and draw ~70 mA at 6 volt. And LEDs probably works just fine.