Sinclair ZX81 Troubleshooting Guide

A ZX81 motherboard. Five ICs, one regulator and twelve capacitors — the whole machine.

Component-level fault-finding for the Sinclair ZX81. For routine care, the component list and the supply specification see Sinclair ZX81 General Maintenance; for capacitor values see Sinclair ZX81 Capacitor Replacement Guide.

The designators, pin numbers and part types on this page are Sinclair's own, from the ZX81 Service and Assembly Manual and circuit diagram SRC 049, both hosted here.

External peripherals — the ZX Printer, cassette recorders and the RAM pack itself — are not covered beyond their effect on the computer.

The board at a glance

What you are working on

Ref	Part	Notes
IC1	Sinclair Logic IC (ULA), 40-pin	Video, keyboard scan, tape I/O, chip selects and the CPU clock
IC2	2364, 24-pin	8 KB mask ROM. Not a 2716 — that is a 2 KB part and cannot hold this ROM
IC3	Z80A or D780C-1, 40-pin	CPU
IC4	MK4118, 24-pin	1 KB × 8 static RAM
IC4a, IC4b	µPD2114LC, 18-pin	The alternative fit — two 1K × 4 static RAMs in place of IC4
REG	7805	5 V regulator on an aluminium heatsink
TR1, TR2	ZTX313	Video output stage; master oscillator
X1	CDA 6.5MC	6.5 MHz ceramic filter at IC1 pin 35
MOD	UM1233	UHF modulator, channel 36

Board issues. At least three PCB versions exist and the component reference numbers are the same on all of them; the notable Issue 3 change is that resistor packs RP1 and RP3 are replaced by individual resistors R35–R42 and R43–R47.

Power and voltage checks

The ZX81's supply is unregulated by design. Sinclair specifies an external unregulated DC adaptor; the 7805 on the board does the regulating. A high off-load reading on an original adaptor is normal and is not a reason to replace it. What the machine needs is a supply able to deliver not less than 600 mA, or 1.2 A if a ZX Printer runs from the same adaptor.

Test points

Where	Expect	Note
Power jack	Unregulated DC, well above 9 V off load	3.5 mm mono jack — the same size as EAR and MIC beside it
Across C3	The unregulated rail	22 µF, the board's only reservoir, ahead of the regulator
7805 output	+5 V	And at the Vcc pin of each IC
IC3 pin 26 (/RESET)	Low at switch-on, then released high	Held low by R15 (220 k) charging C5 (1 µF)
IC3 pin 6 (CLK)	3.25 MHz	Derived by the ULA from the 6.5 MHz filter at X1
IC1 pin 35	6.5 MHz	The ceramic filter connection
IC1 pin 16 to the modulator	About 2 V peak, picture peak to the bottom of sync

3.25 MHz is the clock on every ZX81 board issue. 3.5 MHz is the ZX Spectrum figure and does not occur on a ZX81 — a chart quoting it for an Issue 2 board is describing the wrong machine.

Get the polarity right. A supply re-terminated the wrong way round is a recognised way to destroy the regulator. It usually dies and spares the rest of the board, which is luck, not protection.

Supply faults

• 5 V missing — dead machine or white screen. 5 V high — chip damage.
• No input at the jack — completely dead. Check the lead: Sinclair power leads are a documented failure point.
• Intermittent drop-outs — cracked joints at the power jack, which takes all the mechanical strain.
• Regulator overheating — look for a short or an excessive load on the board, not for a fault in the supply. Excessive current drawn from the adaptor means a board fault.

No display or white screen

1. Confirm the unregulated input and the 7805 output.
2. Check /RESET at IC3 pin 26 actually releases. If the screen is clear with no inverse-K cursor, Sinclair's own first response is to disconnect the supply, wait a few seconds and try again — that gives C5 time to discharge.
3. Check the 6.5 MHz at IC1 pin 35 and the 3.25 MHz at IC3 pin 6.
4. Check the video at IC1 pin 16 into the modulator — about 2 V peak.
5. Remove the RAM pack and every peripheral and retest.
6. Swap the ULA (IC1), then the CPU (IC3), then the ROM (IC2), then the RAM.
7. Inspect for broken traces, especially in the video section around TR1, C1, R1 and R17.

ULA faults

IC1 is the most common failure on this machine and the hardest to replace. Symptoms:

• No video at all, white screen or black screen
• No clock at IC3 pin 6 — the ULA generates it
• Keyboard dead even with a known-good membrane and tails
• The ULA noticeably hotter than everything else

Sinclair fitted more than one ULA type across the production run. Read the part number off your own chip rather than assuming a mapping from the board issue; this wiki has not been able to establish a reliable issue-to-ULA correspondence from a primary source. Modern FPGA-based replacements exist.

CPU faults

The Z80A rarely fails, but over-voltage and ESD will do it.

• No display with a known-good ULA and ROM
• Address or data lines static under a logic probe
• Lock-ups or random characters

Any Z80A in a 40-pin DIP will do as a replacement.

RAM

Internal RAM is either IC4, an MK4118 in a 24-pin package, or IC4a and IC4b, two µPD2114LC parts in 18-pin packages. The two fits are alternatives and the board takes one or the other.

• "RAM pack wobble" — crashes and resets with an external pack fitted. This is mechanical, at the edge connector. Clean the contacts and stop the pack moving before suspecting anything electronic.
• Wrong free-memory figure reported at boot, or random characters — test with the internal RAM only.

ROM

IC2 is a 2364, a 24-pin 8 KB mask ROM. Replacements are made with a 2764-class 8 KB EPROM on an adapter; a 2716 is 2 KB and cannot hold this ROM at all.

A BASIC checksum program that sums all 8192 bytes is on Sinclair ZX81 General Maintenance. Note that 854885 is an early ROM version with known bugs rather than a fault — confirm with PRINT SQR .25, which returns 1.359 on the early ROM instead of .5.

Keyboard

The flat membrane cracks and its tails break. Symptoms are dead keys, whole dead rows, or phantom presses.

• Inspect the membrane and both tails for cracks; check continuity of every row and column.
• Check the D1–D8 matrix diodes are the right way round.
• A short across the keyboard tracks holds one key permanently on and disables the whole keyboard — that presents as a completely dead keyboard, not as one stuck key.
• 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.

Video and modulator

Output is through a UM1233 modulator on UHF channel 36.

• Confirm about 2 V peak at IC1 pin 16, measured from picture peak to the bottom of the sync pulses.
• The video output stage is TR1 (ZTX313) with C1 (47 pF), R1 (10 k) and R17 (1 k) — that is where to look for a video fault on the board, not at C5 or C6. C5 is the reset capacitor and C6 is part of the master oscillator.
• A failed modulator can be bypassed for direct composite output.

Cassette

• C10 with R33 (4k7) and R34 (220 R) form the EAR input network; C11 with R27 (1 k) and R29 (1 M), and C12, form the MIC output network. C8 and C9 are 47 nF decoupling ceramics on the 5 V rail and have nothing to do with tape.
• Clean the EAR and MIC jacks and check for broken traces from the sockets.
• Check the lead and the recorder's output level before suspecting the board.

Things that are not faults

• The display blanks while a program runs and flickers on a keypress in FAST mode. The Z80 cannot compute and generate video at once. SLOW mode is the answer.
• PRINT SQR .25 returning 1.359. An early ROM, not a failing one.
• A hot 7805. It is dropping the whole difference between the unregulated input and 5 V.
• No reset button. There is none. The only reset is R15 charging C5 at switch-on, which is why unplugging and waiting is the remedy for a hung machine.

Method

• Start at the supply and work forward.
• Test in minimal configuration — no RAM pack, no peripherals.
• Power off before removing or inserting any chip, and watch pin 1.
• Thermal: after 30 to 60 seconds, a device markedly hotter than its neighbours is usually shorted. Freeze spray is the complementary test.
• Fit sockets for replacement chips where you can.

Related pages

• Sinclair ZX81
• Sinclair ZX81 General Maintenance
• Sinclair ZX81 Capacitor Replacement Guide
• Sinclair ZX80
• Recommended Tools