18 October 2022

HP 3734A Electronic Counter

I really enjoyed Mombarone HAM Fest (near Asti, Italy), October 2022. The visitors flow was relaxed and I could spend time looking above and below stands.

In a corner I spotted a device that supposedly contained Nixie tubes. The seller, who had a stand of regular wristwatches, said: "it's for the Nixies". It was wrapped in plastic, I could see it had physical damage. He asked 15€ but had no change, so I raised the swap to 20€, which is closer to the current ZM1022 quotation (2022AD). That's 4€ per tube, plus the transformer and .... a true HP device!

So I got an "HP 3734A Electronic Counter". The Nixies tested OK for gas but one side of the case has heavy damage with broken aluminium brackets! The plastic display bezel is broken as well as a corner of the protective glass (it is composed of an orange plastic sheet over a clear glass).

As looked from above it is also not square but a bit rhombic. Nothing a light hammer and a 3D printer cannot fix, right?

Since I had nothing to loose, I powered it up (with the right "163" cable). This was the pass mark to decide on fixing or scrapping the whole device (minus the Nixies).Well, the verdict is "fixing" since it did power up! Without smoke.

I haven't found a free PDF manual online, but it's not that hard to understand how it works, showing either a frequency or its period. Unfortunately it doesn't work even if the internal 100 kHz oscillator is runnng.

There is also a pulse counter function controlled with start/stop/reset buttons which obviously bypasses all signal conditioning and averaging circuitry. It counts either an EXTernal or and INTernal source. Well, selecting "INT" on the front knob combined with "INT" on the backside switch makes the counter use its own 100 kHz clock as source. Et voilĂ , Nixies were running as soon as I hit "START".

If the case can be re-aligned and rebuilt, there is enough working electronics to make a five digit clock. A big plus is that input/start/stop controls are available on the back panel too so everything can be hidden inside or behind. Nice!

HP 5512A-4A display boards

Each Nixie is mounted on a removable board. The concept of this 3734A is similar to HP 5212A, 5512A, 5232A, 5532A counters from the same lustrum (1960-1965) as they share the display architecture. Earlier 5512A had the option to install a board with 10 neon lamps or with the "inline" indicator (a Nixie).

I will look for a manual of these other devices to grab an idea of the circuit diagram.



HP 5512A-4A insideEach display board uses a mix of transistors and a rudimentary integrated circuit as visible in the last picture (click to enlarge as usual).

Next step is to remove all boards and try to straighten the case.