Sourcing the Parts

I have the bare version 5 board, not the kit, so every part has to be bought. This volume is the shopping list. It starts from the parts list in the KABtronics manual (document 1.7, BOM updated 2015)¹ and gives, for every line, the original part, the part to buy now, and why it changed.

Table 1

Tag	Meaning
Same	The original part is still a standard catalog item; buy it as listed
EOL	The original is obsolete; a replacement is needed
Spec	The original BOM line does not say enough (no voltage rating, no part number) or the part is under-rated
Conflict	The schematic and the BOM disagree; the BOM (newer) wins unless noted
Choice	A change I chose for this build

Prices and stock change daily, so none are given; part numbers are examples of parts that meet the spec, and any equivalent that meets it will do. Check each footprint’s lead spacing on the board before ordering, especially for capacitors.

5.1 Semiconductors

Table 2 — 5.1 Semiconductors

Qty	Where	Original	Buy now	Tag	Why
150	Flip-flops, gates, decoders	2N3904 NPN	2N3904	Same	Still made by several manufacturers
30	Logic	2N3906 PNP	2N3906	Same
46 (BOM: 50)	45 nixie cathode drivers + Q129 for the AM/PM neon; 4 spares	STMicroelectronics STX13003G (400 V, 1 A, TO-92)	MPSA42 (300 V, 500 mA, TO-92), after the pinout check in §5.5	EOL	The STX13003G is listed obsolete by distributors.2 Each driver switches 1–2 mA and stands off the HV when its digit is dark; 300 V covers the 165–180 V rail. Positions: Volume 3, Figure 3.4. The board uses a separate outline for these transistors, so check the lead order first
1	Q139, HV power switch	”Transistor, NPN Power, TO-220” (schematic: “8AMP”; manual: “actually a Darlington”)	MJE5742G (onsemi, NPN Darlington, 400 V, 8 A, TO-220)	Spec	The BOM never names the part. It must be a high-voltage Darlington: it sees the full HV plus turn-off overshoot. A 60 V TIP120 would fail at once; a BU806 (200 V collector-emitter) is too marginal. The MJE5742G’s lifecycle is unclear across distributors3, so buy two while stock exists
1	Q69, switch driver	MPSA56 PNP (80 V)	MPSA56	Same	Still stocked; if it disappears, a 2N5401 (150 V PNP) is the usual substitute, pinout checked
~500	Logic gates, decoders, diode matrix	1N4148	1N4148	Same	Buy the full 500: the kit page counts 518 diodes, and the hours matrix uses many
4	Bridge rectifier	1N5818 Schottky (schematic shows 1N4002)	1N5818	Conflict	The BOM’s Schottkys drop less voltage and run cooler; 30 V reverse rating is enough on 12 VAC
1	D99, HV rectifier	”Diode, 3A, 400V” (schematic: onsemi MR854)	UF5404 (400 V, 3 A, 50 ns, DO-201AD)	EOL	The MR854 is obsolete.4 The UF5404 is the same size and rating, and recovers about six times faster (50 ns vs about 300 ns), which wastes less in a boost converter
2	HV reference; hourly shut-off	Zener 4.7 V 0.5 W	1N5230B (4.7 V, 0.5 W, DO-35)	Same
16	Counter test LEDs	Red 3 mm LED	Any red 3 mm LED	Same	Used to watch the counters during the build
5	Colons NE1–NE4, NE5	Neon lamp (about 70 V, 0.5 mA)	NE-2 type neon lamp	Same	The type the designer budgeted for5

5.2 The display

Table 3 — 5.2 The display

Qty	Where	Original	Buy now	Tag	Why
6	Tubes	IN-12A nixie	IN-12A, new-old-stock	Same	Plentiful; IN-12B adds a decimal point the clock does not use
6	Tube sockets	”Socket, Nixie, IN12A”	IN-12 floating-pin sockets with solder lugs	Same	The lugs must stay unsoldered from the PCB and are wired by hookup wire (Volume 3, §3.5)
22 ft	Tube wiring	Hookup wire	Stranded hookup wire, insulation rated 300 V or more	Spec	Every cathode lead carries up to the HV when its digit is dark

5.3 Passive parts

All resistors 1% metal film, 1/4 W, unless noted (1% is my standard).

Table 4 — 5.3 Passive parts

Qty	Where	Original	Buy now	Tag	Why
180	Logic	10 kΩ	10 kΩ	Same
180	Logic	100 kΩ	100 kΩ	Same
75	Logic	1 kΩ	1 kΩ	Same
50	45 cathode resistors + others	22 kΩ	22 kΩ	Same	Sets about 0.9 mA per digit at 165 V (Volume 2, §2.6)
5		220 kΩ	220 kΩ	Same
3	Comparator hysteresis	1 MΩ	1 MΩ	Same
2	R332 (Q139 base drive); FUSE stand-in	22 Ω	22 Ω	Same	One is the temporary FUSE stand-in for the HV bring-up
1	PWR stand-in	10 Ω	10 Ω	Same	Temporary, for the HV bring-up
1		100 Ω	100 Ω	Same
1	SNS stand-in	47 kΩ	47 kΩ	Same	Temporary: sets about 25.5 V for the first power-up
1 set	SNS, sets the HV	348 kΩ, 357 kΩ, 390 kΩ (schematic shows 380 kΩ)	348 k, 357 k and 390 k, 1%	Conflict	Buy all three; the right one depends on the zener (Volume 2, §2.2)
1	R333, PWR (current sense)	0.47 Ω (schematic shows 0.5 Ω)	0.47 Ω, 1/2 W metal film or metal oxide	Conflict	The BOM value; 1/2 W gives margin on the 2 A peaks
1	Bleeder across C73	(none)	150 kΩ, 1 W, rated 350 V or more (or two 75 kΩ 1/2 W in series)	Choice	Cuts the HV bleed-down from about 90 s to about 25 s (Volume 2, §2.5)
1	C73, HV reservoir	”Capacitor, 120uF” (no voltage given)	120 µF, 250 V or higher, 105 °C radial electrolytic, long-life	Spec	It holds 165–180 V; the BOM never says so. It is the “long electrolytic” held with a tie wrap: check its diameter against the board
2	C1 logic reservoir; C72 HV input	6,800 µF (no voltage given)	6,800 µF, 25 V, 105 °C	Spec	The rail reaches about 18 V unloaded
1	C59, Q69 supply	10 µF 16 V tantalum	10 µF 25 V aluminium electrolytic (or a 35 V tantalum)	Spec	A 16 V tantalum on a rail that reaches 18 V breaks the usual rule of running tantalums at half their rating or less, and they tend to fail short
90	Flip-flop speed-up and trigger	220 pF ceramic	220 pF C0G (NP0), 50 V	Same	C0G keeps the timing stable with temperature
2	HV oscillator timing	”470” ceramic (PCB marks one 470 pF, one 0.001 µF)	470 pF and 1,000 pF C0G, plus a few spares of each	Same	Swapped as needed to tune the oscillator to about 22 kHz1
2		0.001 µF ceramic	0.001 µF C0G, 50 V	Same
3		0.01 µF ceramic	0.01 µF X7R, 50 V	Same	Includes the HV pulser capacitor
13	Bypass	0.1 µF ceramic	0.1 µF X7R, 50 V	Same
1	L1, boost inductor	220 µH toroid, 2 A	220 µH, saturation 3 A or more, DCR ≤ 0.12 Ω, toroid; for example Bourns 2200LL-221-V-RC (4.4 A, 0.063 Ω)6	Spec	The designer reports peaks of about 2 A,5 right at the original part’s rating. Bourns rates the LL series by temperature rise, not saturation; powdered-iron toroids saturate softly, but check the part fits the board’s IND outline before ordering

5.4 Power, protection and hardware

Table 5 — 5.4 Power, protection and hardware

Qty	Where	Original	Buy now	Tag	Why
1	F1, FUSE	”Fuse, 3A, Fast, pico” (schematic shows 2 A)	Littelfuse 0251003 Pico II, 3 A (choose a suffix: .MXL, .NAT1L…)7	Conflict	The BOM’s 3 A; a 2 A fuse would sit at the ~2 A peak current
1	Q139 heatsink	”Heatsink, Dual TO-220” (schematic note: Digi-Key 294-1080-ND)	A TO-220 heatsink matching the board’s pad and hole	Same	The manual notes the supplied heatsink varied
1	Power	12 VAC wall transformer (US mains)	12 VAC, 2 A (24 VA) or more, AC output	Choice	The clock draws about 1.2 A; the extra margin keeps the transformer cool. It must be an AC adapter: the clock counts the 60 Hz
1	Power input	2-position terminal block	2-position terminal block, pitch to match the board	Same
3	H, M, S	”Switch,“	Momentary push buttons matching the footprint	Same	The BOM gives no type; match the board’s footprint
13	Mounting	4-40 screws, nuts, washers	4-40 × 0.5 in screws, nuts, washers	Same
10	HV capacitor, wiring	Nylon tie wraps	Small nylon tie wraps	Same
6 ft	Power cord	2-conductor cable	2-conductor cable	Same

5.5 Before buying the cathode drivers: check the pinout

TO-92 transistors do not all share one lead order. The 2N3904 and the MPSA42 are both normally emitter, base, collector, but this board draws the high-voltage transistors with their own outline, different from the NPN outline used for the 2N3904s.¹ That strongly suggests the original STX13003G’s lead order differs, so an MPSA42 may not drop straight in.

With a bare board this takes two minutes with an ohmmeter, at any one cathode-driver position:

1. The pad that connects to a 22 kΩ cathode resistor is the collector.
2. The pad that connects to a 10 kΩ resistor (the base resistor) is the base.
3. The pad that connects to ground (the negative side of the large 6,800 µF capacitor) is the emitter.

Write the order down against the silkscreen outline, then compare it with the MPSA42 datasheet’s drawing:

Table 6 — Write the order down against the silkscreen outline, then compare it with the MPSA42 datasheet's drawing

If the footprint reads (outline orientation)	Then
E – B – C, same as the MPSA42	Fit the MPSA42 as the outline shows
C – B – E	Fit the MPSA42 turned around 180° (rotation reverses the order and keeps the base in the middle)
Base on an outer pad	The middle lead would have to be crossed on 50 parts; instead look for a ≥ 300 V TO-92 NPN with the matching lead order, and confirm it from its datasheet

5.6 Not needed

• IC sockets. There are no integrated circuits on the board.
• The published mods. Version 5 already includes them, or they apply only to earlier boards (Volume 3, §3.1).

5.7 References (Vol 5)

Footnotes

1. Keith Bayern, KABtronics, Nixie-Transistor Clock Assembly Manual, document 1.7 for PCB version 5, 2016: parts list, high-voltage build steps, component silkscreen symbols (p. 86), https://tube-clock.com/neon_man.pdf. ↩ ↩² ↩³

2. Distributor listings marking the STX13003G obsolete: https://ic-direct.com/product/stmicroelectronics/stx13003g/ and https://www.microchipusa.com/product/stmicroelectronics/single-bipolar-transistors/STX13003G (checked 2026-10-29). ↩

3. MJE5742G listings disagree on lifecycle: Octopart shows production, https://octopart.com/part/onsemi/MJE5742G; ic-direct shows the non-G MJE5742 obsolete, https://ic-direct.com/product/onsemi/mje5742g/ (checked 2026-10-29). ↩

4. DigiKey lists the onsemi MR854 as obsolete, https://www.digikey.de/en/products/detail/onsemi/MR854/608181; UF5404 ratings from distributor listings, e.g. https://www.jameco.com/z/FUF5404-Major-Brands-Glass-Passivated-Ultrafast-Recovery-Rectifier-Diode-400V-3A_2285375.html (checked 2026-10-29). ↩

5. Keith Bayern, “Return to Transistor Land,” Nuts & Volts, April 2012, pp. 36–39, https://www.nutsvolts.com/magazine/article/april2012_Bayern. ↩ ↩²

6. Bourns 2200LL series datasheet, 2200LL-221-RC: 220 µH, rated 4.4 A (30 °C rise), 0.063 Ω, https://cdn.hackaday.io/files/1896368113386624/Inductor%20-%202200ll_series-776025.pdf (copy of the Bourns datasheet; confirm against Bourns). ↩

7. Littelfuse 251 Series PICO II, 251003 (3 A, 125 V), https://m.littelfuse.com/products/fuses/axial-radial-thru-hole-fuses/pico-fuses/251/251003.aspx. ↩