First nixie clock build, struggling to set the time - help, please.

Four pages into the thread and I realize I never asked you which kit you have! Oops From the pictures it could be either an All-In-One (tubes soldered to clock board) or a Modular (tubes soldered to little tube carrier boards that plug into main board). Since R17 and R23 are 4.7K on the Modular I'll assume for now that's what you have.

I see no reason for either resistor to be optional or whatever so it looks like one of them got missed when packaging the kit. R17 is part of the HV feedback divider - definitely needed. R23 drives the decimal points on the tubes - arguably optional but not likely to have been left out on purpose.

I agree with your decision to use a resistor from your stock. Wattage doesn't really matter since both resistors are carrying minuscule amounts of power,

As for the power supply, using that old Sinclair would be good as a test if it's not too much of a hassle to hook up. For long term use a good switcher is certainly fine, that's what I use, it's just that getting a "good" one may take a few tries.

Walter John wrote:
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perhaps there is an issue when you use an external switching power supply combined with the switching high voltage power supply circuitry within the clock. Again, I am not sure of this theory, I can only rely on my observation.

Hmmm. Interesting theory that I never though of before. If the external power supply has really poor filtering there could be some interesting interactions between the external and internal switchers' switching frequencies. The clock kit has a 220uF reservoir and a handful of 100nF decoupling caps but if the input power was noisy enough there could still be problems IMO. Would have to be a pretty awful power supply though!