Running It

This volume covers the finished clock. The figures come from the manual and the schematic until my own build reaches them.

4.1 Power

The clock runs from a 12 VAC wall transformer and draws about 14.4 W (1.2 A at 12 VAC), per the manual’s specifications.¹ There is no battery and no backup: a power cut stops the clock, and it must be set again when power returns.

Because the time comes from the mains frequency, the transformer must be an AC adapter. A 12 V DC adapter would power the logic but give the 60 Hz extractor nothing to count. The 50/60 Hz jumper must match the local mains: 60 Hz in North America.

4.2 Setting the time

Three buttons, H, M and S, step the hours, minutes and seconds; Seconds Hold (SW3) freezes the count by grounding the 60 Hz line.¹ To set it to the second: set the hours and minutes, hold the seconds at the right value with Seconds Hold, and release it on the beat of a reference clock.

4.3 Accuracy

The clock counts the mains, so its accuracy is the grid’s. Mains frequency wanders slightly from moment to moment as load changes, but grid operators steer it back over the day, so a mains clock drifts a little in the short term and keeps good time over weeks. The hub’s LED dive, built around this clock’s LED sibling, covers mains timekeeping in detail.

The 60 Hz extractor is there to stop power-line noise from adding counts: a noise burst that arrives mid-cycle discharges the timing capacitor and costs at most one count instead of adding several (Volume 1, §1.2). A clock that gains time, rather than losing it, points to noise getting past the extractor.

4.4 The hour shut-off

Loading a diode in an hour’s shut-off position holds the HV supply off for that hour, and the tubes go dark (Volume 1, §1.4). The counters keep running, so the time is right when the display returns. On a version 5 board each loaded diode affects only its own hour; earlier boards needed Mod 3 for that.²

Remember that the 120 µF capacitor takes minutes to discharge after the supply stops (Volume 2, §2.3). Dark tubes during shut-off hours do not mean the board is safe to touch.

4.5 Troubleshooting

The manual’s troubleshooting section starts with a line worth keeping: “The most useful tool you have when troubleshooting is your brain.”¹ Its questions for a digit that is not counting properly:

Table 1 — The manual's troubleshooting section starts with a line worth keeping: "The most useful tool you have when troubleshooting is your brain."[^man] Its questions for a digit that is not counting properly

Question	What it tells you
Is it changing at all?	Whether the input clock is arriving
Is it changing at the right rate?	Whether the previous stage is dividing correctly
Does it go through the right number of states, even if the numbers shown are wrong?	Counter fault vs decode/display fault
Does it show only proper digits, or odd characters and blanks?	One-of-n decoder or display wiring

Symptom by symptom:

Table 2 — Symptom by symptom

Symptom	Likely cause	Check
Nothing counts	No 60 Hz	About 12 V on C1; the first comparator’s output; the ramp on C5; the second comparator (manual, Power Supply Debugging)
One stage counts wrongly	That stage’s flip-flops or steering	Find the last working flip-flop; check the clock arriving at the next one’s 10 kΩ input resistor
A counter skips states	Often the decoder, loading the flip-flops	Check the decoder before replacing counter parts
Two digits glow together, or odd characters	A short between decoder select lines	Look for solder bridges in the diode field
A digit never lights	Its decoder, its STX13003, its 22 kΩ, or its wire	Trace the one cathode path
A tube is dark	Its COM wire, or HV	Measure HV (carefully); check the COM lug
Clock gains time	Mains noise getting past the extractor	The extractor’s capacitor and comparators
Counters misbehave while you touch the board	Normal	The manual notes the flip-flops are sensitive to fingers and scope probes; it cannot hurt them

The manual adds a caution about probing: touching a flip-flop output with a scope lead can make it change state, so a flip-flop that looks stuck with both outputs low may in fact be toggling as the probe moves.¹

4.6 Upkeep and the case

The manual is frank about lifetime: electrolytic capacitors may need replacing after 5 to 15 years, and nixie tubes eventually fail as contaminants coat the numerals and segments stop lighting fully.¹ The large electrolytics, especially the 120 µF HV capacitor, are the parts to watch.

The finished clock is a 10 × 14 inch board, about 20 oz, needing about ¼ inch clearance behind and 1.5 inches in front for the parts.¹ The designer’s own clock is mounted on a wooden plaque (Volume 1, Figure 1.0). With 1.6–1.9 J on an exposed board, the manual’s advice is the right one: display it “up high, behind glass”.

4.7 References (Vol 4)

Footnotes

1. Keith Bayern, KABtronics, Nixie-Transistor Clock Assembly Manual, document 1.7 for PCB version 5, 2016: specifications, theory of operation, In Case of Difficulty, final assembly, https://tube-clock.com/neon_man.pdf. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

2. KABtronics, 24hour_mod_21.pdf (hourly display shutdown modification, “Applies to KABtronics Nixie clock PCB version 4 and earlier”), linked from https://tube-clock.com/index_002.htm. ↩