Fixing old Nixie Clock

Father’s clock

My father built a Nixie clock in ‘ye olden days‘, over forty years ago… It was a relatively simple design compared to what we build nowadays.

Design and simplified workings

  • The clock derives it’s clock-pulses from the mains power, 50 Hz here in the Netherlands
  • Dividing it by 50 yields the seconds pulse
  • after that it goes into a four of SN7490 divider chips with various reset’s for minutes, tens of minutes, hours, and tens of hours
  • the outputs of the devider chips go into another four SN7490 counters
  • the outputs of the SN7490 counters go into four SN74141’s which are the Nixie drivers
  • it features two buttons on the back, one to reset the minutes and one to advance the hours

See (part of) the schematic below:

Problem

Lately, the clock started to malfunction, it erratically jumped twenty minutes ahead at seemingly random intervals.

Investigation

I decided to investigate the circuitry to see whether I could find something out of the ordinary. So I opened it up to have a close look:

Rework

First I decided to rework some things. For this purpose I had to detach the upper circuit board:

Detached upper circuit board

Then I replaced two wires that were preventing the circuit board to rotate out of the way; they are the red and green twisted wires on the picture below:

Longer wires for ease of access

Now that the upper board was out of the way I had access to the big lower board and took out all the IC’s, sticking them in a piece of foam board in the same order as they came of the board. My hopes were that re-inserting them would improve the contacts that where perhaps causing the erratic behaviour.

IC’s taken out

Mains connection

The connection to the mains had a terminal block connecting the outside wire to the transformer:

Terminal block for mains

This, in my opinion, is not such a good idea. So I got rid of that, see below:

Mains directly connected to transformer

Reassembly and test

I put everything back together on the bench and connected it. No smoke and it showed time:

It still skips twenty minutes, but I haven’t caught the exact condition. Now I’ve set a timer for 9,5 minutes because I first want to check whether the minutes reset is flawed and sometimes gives to many pulses…