Sinclair ZX81 General Maintenance
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
[edit | edit source]Sinclair's own component list.[1]
| 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[2] |
| 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.[1]
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.[2] Work from the issue number silkscreened on your own board.
Power supply
[edit | edit source]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.[3]
| Property | Value | Source |
|---|---|---|
| Polarity | Positive to the tip of the jack; shaft ground | [1][3] |
| Type | DC only. Need not be regulated, but should be well smoothed | [1] |
| 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 | [1][2][3] |
| Current | Not less than 600 mA, or 1.2 A if the ZX Printer is to run from the same supply | [1] |
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.[2]
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.[4]
Supply, clock and video checkpoints
[edit | edit source]| Point | Expect | Source |
|---|---|---|
| Positive end of C3 | The incoming unregulated supply | [5] |
| REG (7805) output | +5 V, reaching every IC on the board | [1][5] |
| IC3 (Z80A) pin 11 Vcc / pin 29 ground | +5 V / ground | [5] |
| IC4 (4118 RAM) pin 24 Vcc / pin 12 ground | +5 V / ground | [5] |
| IC4a, IC4b (2114 RAM) pin 18 Vcc / pin 9 ground | +5 V / ground | [5] |
| IC1 (ULA) pin 35 | Connection to the 6.5 MHz ceramic filter X1; the ULA derives the 3.25 MHz CPU clock from it | [2][5] |
| IC3 (Z80A) pin 26 | /RESET, held low briefly at switch-on by R15 (220 k) and C5 (1 µF), then high | [5] |
| IC1 (ULA) pin 16 to the modulator | About 2 V peak, measured from picture peak to the bottom of the sync pulses | [2] |
| Keyboard tracks between the KBD lines at KB1 | A few thousand ohms | [2] |
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.[5][1]
Capacitors
[edit | edit source]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.[1]
Full detail is in the Sinclair ZX81 Capacitor Replacement Guide. See also Capacitor Failure Symptoms.
Checking the ROM
[edit | edit source]This BASIC program sums all 8192 ROM bytes. It takes a little over a minute.[2]
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.[2]
Preventive maintenance
[edit | edit source]- Unplug the supply. Measure its output and confirm its polarity before plugging it into the machine.
- Remove the four base screws — three are under rubber feet — then the two board screws.[2]
- Inspect the board for corrosion, cracked tracks and burnt components.
- Inspect C3 and C5 for leakage or bulging.
- 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.[2]
- Clean dust with a soft brush. Clean the edge-connector fingers.
- 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.[2]
- 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.[2]
- Check the regulator is firmly bolted to its heatsink before reassembly.
- 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.[2]
Do not use liquid solvent cleaners on the keyboard tails. Some of them do not merely soften the plastic, they disintegrate it.[2]
Things that are not faults
[edit | edit source]- 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.[1][2]
- 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.[6]
- Fitting a RAM pack disables the internal 1 KB. That is by design, not a fault.[2]
PRINT SQR .25returning 1.359. An early ROM, not a failing one.[2]
Common faults and first checks
[edit | edit source]| 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[1] |
| Works then cuts out, regulator very hot | Regulator not properly bolted to the heatsink[1] |
| Excessive current drawn from the supply | A fault on the board, not in the supply[2] |
| 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)[1][2] |
| Screen clears but no cursor | Unplug, wait a few seconds, try again; then the R15/C5 reset network at IC3 pin 26[1][5] |
| 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[1][2] |
| One row or bank of keys dead | A cracked tail. If the crack is near the end, a clean square cut may recover it[2] |
| Horizontal black-and-white stripes through the picture | The supply: inadequate smoothing, or output too low on load[1] |
| 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
[edit | edit source]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
[edit | edit source]- 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
References
[edit | edit source]- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 Sinclair Research Ltd, ZX81 Service and Assembly Manual, section 6 (fault finding), section 8 (component lists) and section 9 (power supply specification). Hosted on this wiki as File:Sinclair ZX81 Service and Assembly Manual.pdf. Source for the component list, the supply specification and Sinclair's own first-line fault checks.
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 2.20 2.21 "Servicing Sinclair Computers, Part 2", Television magazine, 1986. Trade servicing article devoted to the ZX81, mirrored in the Sinclair ZX Computers Archive at k1.spdns.de. Secondary source: on-load supply figures, board issues, the ULA clock derivation, keyboard diagnosis, the ROM check program and Sinclair's socket-fitting practice.
- ↑ 3.0 3.1 3.2 Sinclair ZX81 Connectors, The Sinclair ZX Computers Archive, k1.spdns.de. Secondary source: connector types, supply polarity and open-circuit voltage.
- ↑ "Micro Clinic", Television magazine, 1986 — collected Sinclair repair notes, mirrored in the Sinclair ZX Computers Archive at k1.spdns.de. Secondary source: Sinclair power-supply lead failures.
- ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 ZX81 Circuit Diagram, drawing SRC 049, Sinclair Research Ltd. Hosted on this wiki at full resolution as File:Sinclair ZX81 circuit diagram.png. Source for pin numbers, designators and circuit positions.
- ↑ Differences between ZX80, ZX81 and Timex 1000, The Sinclair ZX Computers Archive, k1.spdns.de. Secondary source: the ZX81's FAST and SLOW display modes, and the ZX80-to-ZX81 change from discrete TTL to a single ULA.
