03 July 2016

Success with the Philips PM2421 U4 board!

At Friedrichshafen 2016 I bought a board with 5 Nixie tubes (ZM1000 and ZM1001). It is a U4 board from a well documented Philips PM2421 instrument. Even if a 1974 circuit is easy to follow and understand, having access to the schematic diagram saves a lot of time!

Before buying the board just for the tubes and their sockets, I tested them in case they were outgassed with my HVAC circuit (see my posts about CCFL laptop inverter in end-2015), and they all happily glew.

The board on a printout of the schematic diagram for U4 board.

Looking at the schematic diagram I realised that the symbol Nixie will display only plus, minus or tilde (AC) and that the thousands tube is wired only for 0 or 1: out of the box I cannot make a 9'999 counter, or a 24h clock with it. On the other hand a 12h clock will fit.

Nevermind, I have to light it up first.

Everything on the board is clearly marked, and it matches what is drawn on the diagram I found.

The digital logic needs a +5 Vdc while Nixies call for +180 Vdc. 5V/GND are clearly marked on the board, while HVDC can be reverse engineered to be fed at pin #20. If I was to use the analog part I would have to provide more voltages, like +/- 12 Vdc, but I am not going down that route.

So, 5V is easy. 180V were picked up from my biNixie clock PSU. Fire both and ... there you are!

The board greeting me with a reassuring quadruple 0 AC :)

Then I grounded one of the AC/DC selector pins and a minus sign appeared:

DC configuration without input signals.

According to the shematic, points P and /P are where a counting signal is supposed to be, the output of the ADC process. I will throw in a square wave and see what happens.