Wednesday, March 8, 2017

Canon P170-DH Teardown, part 1

I decided to start really tearing down my Canon P170-DH calculator. Here's the main board with the vacuum fluorescent display detached:

Some time ago I'd determined that the filament of the VFD was being driven with a 5V (pk-pk) square wave. Since square-wave AC is pretty much the same as DC from a power perspective, I connected the VFD filament to my bench supply and slowly brought up the voltage while monitoring the current.

Voltage Current
4.5 Vdc 150 mA
5.0 Vdc 170 mA
5.5 Vdc 190 mA

Thus it appears the hot filament has a resistance of about 30 ohms. As I expected, this is quite a bit more than the cold resistance, which measures about 7 ohms.

How is the 5 Vac filament voltage generated? The round, gray device just to the right of the top center of the board is a transformer with three isolated windings. One lead of the primary connects to the unregulated positive output of the power supply (~7.5 VDC under normal load), while the other is driven through a transistor to ground in some manner. A center-tapped secondary winding connects to the two ends of the VFD filament, and the center tap is connected to a Zener diode and a capacitor.

That looks a LOT like the configuration found in the excellent description of VFD operation provided by Noritake-Itron:

That explains two of the transformer's windings; what about the third? This board drives the grids and anodes to +5V to illuminate a segment of a digit, and drives them to -25V to keep the segment or digit dark. I expect that the third winding also provides these voltages, with the filament transformer center tap connected to the -25V through the Zener. I'm guessing the logic also runs from the +5V supply, but that's a guess. Why are there four taps on the transformer? I won't know that until I trace the rest of the circuitry.

Another area to examine is the circuitry driving the transformer. This appears to be a flyback configuration, but I haven't ruled out a forward converter circuit. I also want to determine how the output voltages are regulated, as the unregulated output from the transformer will vary with the line voltage and load.

There's always more to investigate!

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