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Rewrite from Sinclair ZX81 Service and Assembly Manual and circuit diagram. Corrects PSU polarity (tip POSITIVE, not centre negative), regulator (REG, not IC3 - IC3 is the Z80A), electrolytics (C3 22uF and C5 1uF, not C5 1000uF/C6/C7), RAM test point (IC4 pin 24 or IC4a/b pin 18, there is no IC5), clock (pin 35 is the 6.5MHz filter) and the Issue 1 ULA claim. Adds Sinclair ROM check, refs; clears Unverified data
 
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[[File:Sinclair ZX81 PCB Top.JPG|thumb|300x300px|Sinclair ZX81 Mainboard (Issue 2)]]Maintaining your Sinclair ZX81 ensures reliable operation and extends the life of this classic computer. This guide covers essential care, preventive steps, and troubleshooting for both Issue 1 and Issue 2 boards.
[[File:Sinclair ZX81 PCB Top.JPG|thumb|300x300px|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.


'''Electrostatic Discharge (ESD) Warning:''' 
== What is on the board ==
Always use proper ESD precautions when handling the ZX81 PCB. Wear a grounded wrist strap and work on an anti-static mat to prevent component damage.


'''Maintenance Intervals:''' 
Sinclair's own component list.<ref name="zx81man">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.</ref>
Perform visual inspections and cleaning every 6–12 months. Replace electrolytic capacitors every 10–15 years, or sooner if instability is observed.


<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:95%; text-align:left;"
== Board Revisions ==
|+'''ZX81 semiconductors and key parts'''
{| class="wikitable styledtable"
! Ref !! Part !! Notes
! Revision !! PCB Marking !! Key Differences !! Common Issues
|-
| 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 [[Sinclair ZX80|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<ref name="tv2" />
|-
|-
| Issue 1 || "ZX81 ISSUE ONE" || Early production, discrete logic ULA, different RAM layout || Poor video sync, RAM instability, ULA overheating
| MOD || UM1233 || UHF modulator, channel 36
|-
|-
| Issue 2 || "ZX81 ISSUE TWO" || Improved ULA, revised RAM, better video circuit || ULA socket failures, keyboard connector wear
| KB1 / KB2 || 5-way / 8-way || Keyboard tail connectors
|}
|}


== Power Supply & Regulator Care ==
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.<ref name="zx81man" />
The ZX81 uses a linear power supply:
* '''PSU Part:''' Sinclair 9V DC Adapter (center negative)
* '''Specs:''' 9V DC, 700mA minimum
* '''On-board Regulator:''' 7805 (IC3)
* '''On-board Electrolytic Capacitors:''' C5, C6, C7


'''Failure Modes:'''
'''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.<ref name="tv2">"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.</ref> Work from the issue number silkscreened on your own board.
* PSU: Overvoltage, ripple, plug wear
* 7805: Overheating, output drift
* Capacitors: Leakage, bulging, ESR rise


'''Capacitor Designators:'''
== Power supply ==
* '''C5''' – Input smoothing
* '''C6''' – Output smoothing
* '''C7''' – Video circuit decoupling


== Capacitor Replacement Table ==
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.<ref name="kio81" />
{| class="wikitable styledtable"
 
! Position !! Value (µF) !! Voltage (V) !! Notes !! Series/Type
{| class="wikitable styled-table" style="width:95%; text-align:left;"
|+'''ZX81 external supply, as specified by Sinclair'''
! Property !! Value !! Source
|-
| Polarity || '''Positive to the tip''' of the jack; shaft ground || <ref name="zx81man" /><ref name="kio81">''Sinclair ZX81 Connectors'', The Sinclair ZX Computers Archive, k1.spdns.de. Secondary source: connector types, supply polarity and open-circuit voltage.</ref>
|-
|-
| C5 || 1000 || 16 || PSU input filter || Low ESR, radial
| Type || DC only. Need not be regulated, but should be well smoothed || <ref name="zx81man" />
|-
|-
| C6 || 22 || 16 || 7805 output || Low ESR, radial
| 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 || <ref name="zx81man" /><ref name="tv2" /><ref name="kio81" />
|-
|-
| C7 || 1 || 25 || Video smoothing || General purpose
| Current || Not less than 600 mA, or 1.2 A if the ZX Printer is to run from the same supply || <ref name="zx81man" />
|}
|}


== Voltage & Clock Test Points ==
'''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.<ref name="tv2" />
{| class="wikitable styledtable"
 
! Test Point !! Pin/Location !! Expected Value !! Tolerance
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.<ref name="clinic">"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.</ref>
 
== Supply, clock and video checkpoints ==
 
{| class="wikitable styled-table" style="width:95%; text-align:left;"
! Point !! Expect !! Source
|-
|-
| 5V Rail || IC3 output pin || 5.0V DC || ±0.2V
| Positive end of C3 || The incoming unregulated supply || <ref name="zx81sch">''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.</ref>
|-
|-
| ULA Clock In || IC1 pin 35 || 3.25 MHz || ±0.05 MHz
| REG (7805) output || +5 V, reaching every IC on the board || <ref name="zx81man" /><ref name="zx81sch" />
|-
|-
| RAM Vcc || IC5 pin 8 || 5.0V DC || ±0.2V
| IC3 (Z80A) pin 11 Vcc / pin 29 ground || +5 V / ground || <ref name="zx81sch" />
|-
|-
| Video Out || Edge connector pin 16 || 1.0V p-p || ±0.2V
| IC4 (4118 RAM) pin 24 Vcc / pin 12 ground || +5 V / ground || <ref name="zx81sch" />
|-
| IC4a, IC4b (2114 RAM) pin 18 Vcc / pin 9 ground || +5 V / ground || <ref name="zx81sch" />
|-
| IC1 (ULA) pin 35 || Connection to the 6.5 MHz ceramic filter X1; the ULA derives the 3.25 MHz CPU clock from it || <ref name="tv2" /><ref name="zx81sch" />
|-
| IC3 (Z80A) pin 26 || /RESET, held low briefly at switch-on by R15 (220 k) and C5 (1 µF), then high || <ref name="zx81sch" />
|-
| IC1 (ULA) pin 16 to the modulator || About 2 V peak, measured from picture peak to the bottom of the sync pulses || <ref name="tv2" />
|-
| Keyboard tracks between the KBD lines at KB1 || A few thousand ohms || <ref name="tv2" />
|}
|}


== Essential Tools ==
'''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.<ref name="zx81sch" /><ref name="zx81man" />
* ESD wrist strap & mat
* Phillips and flat screwdrivers
* Soldering iron (fine tip)
* Desoldering pump or braid
* Multimeter (with continuity & voltage)
* Oscilloscope (for clock/video)
* Small side cutters
* Isopropyl alcohol & brush


== Preventive Maintenance Checklist ==
== Capacitors ==
# Power off and unplug the ZX81.
# Remove case and visually inspect PCB for corrosion or damage.
# Check all electrolytic capacitors for bulging or leakage.
# Clean dust from PCB using a soft brush.
# Inspect and clean edge connector contacts.
# Verify tightness of ULA and RAM IC sockets.
# Check keyboard membrane for cracks or poor contact.
# Test output voltages from PSU and on-board regulator.
# Reassemble and test basic operation.


== Common Faults & Quick Checks ==
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.<ref name="zx81man" />
{| class="wikitable styledtable"
 
! Symptom !! Likely Cause !! Quick Check
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.<ref name="tv2" />
 
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 <code>PRINT SQR .25</code>: the early ROM returns 1.359 instead of .5.
* '''Anything else''' — a ROM error.<ref name="tv2" />
 
== Preventive maintenance ==
 
# 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.<ref name="tv2" />
# 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.<ref name="tv2" />
# 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.<ref name="tv2" />
# 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.<ref name="tv2" />
# 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.<ref name="tv2" />
 
'''Do not use liquid solvent cleaners on the keyboard tails.''' Some of them do not merely soften the plastic, they disintegrate it.<ref name="tv2" />
 
== 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.<ref name="zx81man" /><ref name="tv2" />
* '''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.<ref name="kiodiff">''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.</ref>
* '''Fitting a RAM pack disables the internal 1 KB.''' That is by design, not a fault.<ref name="tv2" />
* '''<code>PRINT SQR .25</code> returning 1.359.''' An early ROM, not a failing one.<ref name="tv2" />
 
== Common faults and first checks ==
 
{| class="wikitable styled-table" style="width:100%; text-align:left;"
! 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<ref name="zx81man" />
|-
| Works then cuts out, regulator very hot || Regulator not properly bolted to the heatsink<ref name="zx81man" />
|-
| Excessive current drawn from the supply || A fault on the board, not in the supply<ref name="tv2" />
|-
|-
| No power || PSU failure, 7805 fault || Measure 9V at input, 5V at IC3 output
| 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)<ref name="zx81man" /><ref name="tv2" />
|-
|-
| No video || ULA, C7, TV cable || Check ULA temp, replace C7, test cable
| Screen clears but no cursor || Unplug, wait a few seconds, try again; then the R15/C5 reset network at IC3 pin 26<ref name="zx81man" /><ref name="zx81sch" />
|-
|-
| Random resets || C5/C6, RAM socket || Replace caps, reseat RAM
| 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<ref name="zx81man" /><ref name="tv2" />
|-
|-
| Keyboard unresponsive || Membrane, connector || Inspect/replace membrane, clean connector
| One row or bank of keys dead || A cracked tail. If the crack is near the end, a clean square cut may recover it<ref name="tv2" />
|-
|-
| Garbage screen || ULA, RAM || Swap ULA, test RAM chips
| Horizontal black-and-white stripes through the picture || The supply: inadequate smoothing, or output too low on load<ref name="zx81man" />
|-
| 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
|}
|}


== Troubleshooting Flowcharts ==
For a symptom-by-symptom walk-through, see the [[Sinclair ZX81 Troubleshooting Guide]].
# '''No Power'''
 
## → Check PSU output → If OK, check 7805 output → If 5V missing, replace 7805 or C5/C6.
== Tools ==
# '''No Video Output'''
 
## → Confirm 5V present → Check ULA temperature → Replace C7 → Test with alternate TV/cable.
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.
# '''Random Crashes/Resets'''
 
## → Inspect/replace C5/C6 → Reseat RAM/ULA → Check for dry solder joints.
== Related pages ==
# '''Keyboard Issues'''
 
## → Inspect membrane for cracks → Clean edge connector → Replace membrane if needed.
* [[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 ==
 
<references />


[[Category: Sinclair Computers]]
[[Category:Sinclair Computers]]
[[Category:Maintenance Guides]]

Latest revision as of 17:36, 22 September 2026

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

[edit | edit source]

Sinclair's own component list.[1]

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[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]

ZX81 external supply, as specified by Sinclair
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]
  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.[2]
  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.[2]
  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.[2]
  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.[2]
  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.[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 .25 returning 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.

[edit | edit source]

References

[edit | edit source]
  1. ↑ 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. ↑ 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. ↑ 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.
  4. ↑ "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. ↑ 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.
  6. ↑ 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.
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