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I get around 2 volts across pins 1 and 9.

Sorry, I should have been more specific. Could you measure the voltage to ground on each pin?.

Hi Tony,

Using the ground on the USB connector, I get a steady fluctuation between zero and 0.4 vdc at pin 1 and a steady 2.1vdc at pin 9.
I used a better meter this time because my cheap one was giving readings with only one lead touching.
With the good meter, I get nothing at all on the pads where C6a and C6b would go if they were there.

Could you measure the voltage across R1 and from the tab of T12 to ground?. It might be worth making sure T12 doesn't have any dry joints - if not it looks like it may be o/c.

I'll check when I get home. Thanks

BTW What do you mean by o/c?

Hi Tony,
with one lead on the tab of T12 and the other on one end of R1, I get fluctuation between 2.5 and 5 volts.
At the other end of R1, I get fluctuation between 4.6 and 5 volts.
Another worrying development. I noticed D17 flickering. When I took the PCB out of my PCB clamp, the LED went out. If I flex the board a little, it turns on and off.
There can't be any dry joints, I've re-soldered everything.

D17 is connected, through R29, across the 5V supply. If it's flickering either the resistor, diode or the supply is the problem. Try pushing the resistor and diode to see if there's an internal bad connection causing it to flicker. I doubt it is so once you've confirmed the LED is OK you'll need to trace where it's going missing. Try pushing on the USB socket and R28.

Hi Tony,

I'm getting nowhere with this and think I should start replacing components.
Without a circuit diagram I can't figure out why the nixies won't work (Even if I had one, I'm not sure it would help lol).
The LEDs seem to be working now, though.
Where do you think I should start? Should I replace T12 first?
I can order most items from RS, but I'd prefer to just order once to save postage.

The nixies aren't working due to the missing HV supply, from your faultfinding I'd say T12 is faulty but before replacing it you'll need to find out why the 5V supply is dropping out as this may have caused it to fail. The only parts that would cause D17 to flicker are the USB socket/plug and R28. D17/R29 could be faulty but that would only be an indicator problem, the 5V supply will still work.
How many spare components do you have on hand? Other than T12 there may not be much you need to order. If you want to cover everything a couple of each NTD5865, MPSA92, MPSA42, 10x 1N4148 diodes and a coule of spare fusible resistors. The only other parts you might need to replace are the PIC and the 74141/KM155ID1.

Hi Tony,
Thanks for replying at this busy time of year.
As I half expected, I need to source these components from all over the world. The only items I can get in Australia are the diodes and some of the transistors.
I'll get the T12s from RS and the rest off eBay from Hong Kong.
It seems that almost no-one has the fusible resistors (at least not in 0.47ohm). Do you think this eBay item is compatible? It's the only one I can find anywhere.
Cheers, Dale