Sinclair ZX81 General Maintenance

Sinclair ZX81 main board

The Sinclair ZX81 is a small, simple machine: one supply rail, one custom chip, two electrolytic capacitors and no battery. Most of what fails on one is mechanical — keyboard tails, edge-connector contacts, dry joints — or arrives through the external power supply.

Observe ESD precautions when the board is out of the case: wrist strap, antistatic mat.

What is on the board

Sinclair's own component list.

ZX81 semiconductors and key parts

Ref	Part	Notes
IC1	Sinclair Logic IC (ULA), 40-pin	Called "Sinclair Computer Logic" on the circuit diagram. Does the video, keyboard scanning, tape I/O and chip selects that the ZX80 did with twenty 74LS packages
IC2	2364, 24-pin	8 KB ROM
IC3	Z80A or D780C-1, 40-pin	The CPU. Not the regulator
IC4	MK4118, 24-pin	1 KB × 8 static RAM. The diagram notes the position will also take a 4816 2K × 8 part in a 28-pin pack
IC4a, IC4b	µPD2114LC and µPD2114LC-1, 18-pin	Alternative to IC4: two 1K × 4 static RAMs
REG	7805	The 5 V regulator, bolted to an aluminium heatsink with a 4BA nut and bolt
TR1, TR2	ZTX313	Video output stage and master oscillator
D1–D8	1N4448, 1N4148 or 1S4A	Keyboard matrix diodes
X1	CDA 6.5MC	Three-lead 6.5 MHz ceramic filter; the ULA divides it to the 3.25 MHz CPU clock
MOD	UM1233	UHF modulator, channel 36
KB1 / KB2	5-way / 8-way	Keyboard tail connectors

Resistor packs RP1 (8 × 10 k) and RP3 (5 × 10 k) are fitted on the Issue 1 board; RP2 is not used, and R3, R30, R31 and R32 are not used either.

Board issues. At least three PCB versions exist. Component reference numbers are the same on all of them. The notable Issue 3 difference is that RP1 and RP3 are replaced by individual resistors, R35–R42 and R43–R47 respectively. Work from the issue number silkscreened on your own board.

Power supply

The ZX81 runs from an external unregulated DC adaptor through a 3.5 mm mono jack — the same size as the EAR and MIC sockets beside it, so it is easy to plug into the wrong one.

ZX81 external supply, as specified by Sinclair

Property	Value	Source
Polarity	Positive to the tip of the jack; shaft ground
Type	DC only. Need not be regulated, but should be well smoothed
Voltage	Between 12 V maximum and about 8 V minimum when on load, depending on smoothing. About 14 V open-circuit; roughly 11 V on load at 400 mA. Below about 7 V the regulator will not function correctly
Current	Not less than 600 mA, or 1.2 A if the ZX Printer is to run from the same supply

Get the polarity right. A supply re-terminated the wrong way round is a recognised way to destroy the regulator on this family. The regulator usually dies and spares the rest of the board — treat that as luck, not protection.

The adaptor is a mains transformer unit and is perfectly repairable. The primary side is at mains potential and the reservoir capacitor holds charge after unplugging: unplug it, wait, and confirm the reservoir is discharged before putting a hand or probe inside. Replace a perished lead rather than patching it — on the closely related Spectrum supplies a shorted lead has been known to set the rectifier diodes alight.

Supply, clock and video checkpoints

Point	Expect	Source
Positive end of C3	The incoming unregulated supply
REG (7805) output	+5 V, reaching every IC on the board
IC3 (Z80A) pin 11 Vcc / pin 29 ground	+5 V / ground
IC4 (4118 RAM) pin 24 Vcc / pin 12 ground	+5 V / ground
IC4a, IC4b (2114 RAM) pin 18 Vcc / pin 9 ground	+5 V / ground
IC1 (ULA) pin 35	Connection to the 6.5 MHz ceramic filter X1; the ULA derives the 3.25 MHz CPU clock from it
IC3 (Z80A) pin 26	/RESET, held low briefly at switch-on by R15 (220 k) and C5 (1 µF), then high
IC1 (ULA) pin 16 to the modulator	About 2 V peak, measured from picture peak to the bottom of the sync pulses
Keyboard tracks between the KBD lines at KB1	A few thousand ohms

The ZX81 has no reset switch. The only reset is R15/C5 acting at switch-on, which is why the remedy for a hung machine is to unplug it and wait a few seconds before reconnecting.

Capacitors

There are exactly two electrolytics: C3 (22 µF, 16 V minimum) across the incoming supply, and C5 (1 µF, 5 V minimum) in the reset network. The other ten capacitors are ceramic discs and are not a service item.

Full detail is in the Sinclair ZX81 Capacitor Replacement Guide. See also Capacitor Failure Symptoms.

Checking the ROM

This BASIC program sums all 8192 ROM bytes. It takes a little over a minute.

10 FAST
20 LET L = 0
30 FOR N = 0 TO 8191
40 LET L = L + PEEK N
50 NEXT N
60 PRINT L

• 835106 — correct.
• 854885 — an early ROM version with known bugs, not a fault. Confirm with PRINT SQR .25: the early ROM returns 1.359 instead of .5.
• Anything else — a ROM error.

Preventive maintenance

1. Unplug the supply. Measure its output and confirm its polarity before plugging it into the machine.
2. Remove the four base screws — three are under rubber feet — then the two board screws.
3. Inspect the board for corrosion, cracked tracks and burnt components.
4. Inspect C3 and C5 for leakage or bulging.
5. Withdraw the keyboard tails straight out of KB1 and KB2, never at an angle. Inspect them for cracks across the plastic and for the condition of the metallised end contacts.
6. Clean dust with a soft brush. Clean the edge-connector fingers.
7. Reseat socketed ICs. If some ICs are socketed and others are not, the board has probably been worked on before — Sinclair either socketed all of them or none.
8. Inspect solder joints under magnification for dry joints and for splashes. Pay attention to the tracks that run obliquely under socket KB1, a documented place for shorts.
9. Check the regulator is firmly bolted to its heatsink before reassembly.
10. Refit the tails by laying the board component-side up on the face-down case top with the edge connector at the front left, looping the tails over and pushing them in with a slight rocking motion. Do not force them: the plastic buckles and splits.

Do not use liquid solvent cleaners on the keyboard tails. Some of them do not merely soften the plastic, they disintegrate it.

Things that are not faults

• The regulator heatsink runs hot to the touch — but it should not be unbearable. A regulator that never warms up means the supply is not reaching it; one that is too hot to touch is probably not bolted down properly.
• The display flickers when you press a key in FAST mode, and blanks while a program runs. In FAST mode the ZX81 cannot display and compute at once. SLOW mode is what avoids it.
• Fitting a RAM pack disables the internal 1 KB. That is by design, not a fault.
• PRINT SQR .25 returning 1.359. An early ROM, not a failing one.

Common faults and first checks

Symptom	Where to look first
Dead, regulator cold	Supply output and polarity; is the plug in the power socket rather than EAR or MIC; the jack's solder joints; C3
Works then cuts out, regulator very hot	Regulator not properly bolted to the heatsink
Excessive current drawn from the supply	A fault on the board, not in the supply
No picture	Tuning first — sweep UHF around channel 36. Then the +5 V rail, then ULA pin 16 to the modulator, then the modulator's own supply and its tuning screw (about 3 mm down inside the former)
Screen clears but no cursor	Unplug, wait a few seconds, try again; then the R15/C5 reset network at IC3 pin 26
Cursor present, keyboard dead	Tails seated and untwisted; the D1–D8 matrix diodes the right way round; a short across the KBD tracks — a short holds one key permanently on and disables the whole keyboard
One row or bank of keys dead	A cracked tail. If the crack is near the end, a clean square cut may recover it
Horizontal black-and-white stripes through the picture	The supply: inadequate smoothing, or output too low on load
Random crashes or resets with a RAM pack fitted	Edge-connector contacts and physical movement of the pack — "RAM pack wobble" — before anything on the board

For a symptom-by-symptom walk-through, see the Sinclair ZX81 Troubleshooting Guide.

Tools

See Recommended Tools. ZX81-specific additions: a multimeter for the supply check that should precede every session, a logic probe for the IC checks, and an oscilloscope if you intend to chase clock or video faults.

Related pages

• Sinclair ZX81
• Sinclair ZX81 Capacitor Replacement Guide
• Sinclair ZX81 Troubleshooting Guide
• Sinclair ZX81 Service and Assembly Manual
• Sinclair ZX80 General Maintenance
• Capacitor Failure Symptoms
• Recommended Tools