10 Aug
2014
10 Aug
'14
1:52 p.m.
Another discovery is that the tubes in the power supply are not triodes,
and I also miscounted the pins. These are 9 (not 8) pin mini tubes, and
OK, the 9 pin miniature ('Noval', B9A) is very common.
I was able to get a part number off of the second tube
by examining it
very closely with a bright light while it was still plugged into its
socket. The tubes are 1V2 diodes. I'm not sure what they are for,
but on closer inspection of the circuit board that they plug into, there
are a number of transistors that I suspect are wired as an oscillator,
but what the diode tubes do is a mystery. The datasheet for the 1V2
tubes says they are primarily used in the flyback circuitry of
television sets, so perhaps they are used in a similar way in the HV
power supply in the calculator.
THaey are simple rectifier diodes -- that is they will pass a current one
way (when the anode (plate) is +ve wrt the filament) and not pass a
current the other way. The thing about them is they will stand a high
inverse votlage without breaking down
The anode is the top cap, the filament (IIRC these are directly heated,
so the filament is the cathode) is wired ot 2 of the base pins. The
filament is nomally powred from a widing on the HV transformer (I susepct
it will be here)..
As to what they are for in this circuit, well, a transformer will give
you AC, you need DC for the CRT anode. With 2 of them, it's quite likely
you have a voltage doubler circuit. This means the transforme can operate
at a lower voltage (about half the eneded votlage for the CRT) reducing
the chances of breakdown. You hope...
-tony