Sinclair ZX81 Troubleshooting Guide

Issue 3 ZX81 board. The ULA (IC1) is next to the modulator, the CPU (IC3) in the centre. Photo: Carlos Pérez Ruiz, CC BY-SA 2.0, Wikimedia Commons.

Board-level fault finding for the Sinclair ZX81. The sequence below follows the 1986 Television magazine article on servicing the ZX81, with Sinclair's own fault list from the kit instructions and designators from the circuit diagram. Component values and the supply specification are on Sinclair ZX81 General Maintenance.

Quick reference

ZX81 symptoms

Symptom	Probable cause	Action
Dead, regulator cold after a couple of minutes	No supply, or the plug in EAR or MIC	Check the adaptor and use the socket nearest the keyboard
Works, then goes off; regulator very hot	Regulator not bolted to the heatsink	Tighten the 4BA bolt
Regulator warm, blank screen	Tuning, or no video from the ULA	Tune between channels 33 and 39; then No picture
Bright screen, no cursor	A fault other than the ULA	See ULA and Checking the ICs
Screen clears, no cursor	Reset not completing	Disconnect, wait a few seconds, retry; check R15 and C5
Cursor shown, keyboard dead	Tails, diodes, or a short between keyboard lines	See Keyboard
One row or bank of keys dead	Crack across a keyboard tail	See Keyboard
Horizontal black and white stripes	Supply poorly smoothed or too low	Check the adaptor and C3
Crashes with a RAM pack fitted	The pack or its edge connector contacts	Remove the pack and test on the internal 1 KB first; see RAM pack and edge connector
Works cold, fails after warming up	One possible cause is a marginal ULA	See ULA; Ferranti's 1983 returns include ULAs that failed only when warm
New ULA dead soon after fitting	The fault that killed the first ULA, such as overvoltage	Check the adaptor, the 7805 and the +5 V rail before fitting another; see Before replacing the ULA

Before opening the case

1. Remove any RAM pack or other add-on. A RAM pack disables the internal 1 KB, and the checks below assume the internal RAM is in use.
2. Check the adaptor: about 14 V off load, tip positive. With a solder-on plug fitted, check the on-load voltage, which should be about 11 V; this shows whether the machine is overloading the supply or drawing nothing.
3. A machine drawing nothing may have had the plug connected the wrong way round at some point. That often blows the 5 V regulator and spares the rest.
4. Tune the television. The modulator is usually set accurately to channel 36, but sweep the band. With no video from the ULA the modulator sends carrier only and the screen shows almost nothing.

Fault-finding sequence

With the board out of the case and the TV and power leads reconnected, the ZX81 starts to the inverse K cursor without the keyboard, so leave it unplugged until the fault is found.

Power

• The supply should give about 11 V at 400 mA on load. Less than 7 V is not enough for the regulator. A high current means a fault on the board.
• The regulator should deliver 5 V to every IC. Its heatsink normally runs hot to the touch, but not unbearably so.
• Sinclair's own figures for a supply are 8 to 12 V on load and at least 600 mA. The circuit diagram on the same sheet marks the input "+7V to 11V DC", so Sinclair's two figures disagree at both ends.

No picture

• ULA pin 16 feeds the modulator. A logic probe should show pulses with the high LED lit; an oscilloscope should show about 2 V from the peak to the bottom of the sync pulses. The inverse K appears as a very faint signal near the end of the field.
• The factory figures for the three levels at the ULA pin are larger than the Television figure: sync 0 V, black 2.8 V and white 4 V in a Ferranti sketch, and white 4.0 V, black 3.0 V and sync 0.1 to 0.4 V in Timex's TS1000 specification. None of the sources says what load its figure assumes. On the factory figures, sync takes the pin to within 0.4 V of ground, so a pin 16 that never falls near 0 V points at the ULA's sync or at a fault loading the line. The output is an emitter follower feeding the pin through an internal 200 Ω resistor.
• If that signal is present but there is no TV output, check the modulator's supply voltage and its tuning screw, which normally sits about 3 mm down inside the former.
• The ULA generates the 3.25 MHz CPU clock from the 6.5 MHz ceramic filter X1 at pin 35. The clock leaves ULA pin 14 and reaches the Z80 at pin 6 through the TR2 buffer.
• /RESET at IC3 pin 26 is held low at switch-on by R15 (220 kΩ) and C5 (1 µF), then goes high.

Keyboard

• With the cursor showing, bridge one contact of the five-way socket KB1 to one of the eight-way socket KB2 and watch for a character or keyword. Bridging more than one contact does no harm, but the ROM ignores the result, because it only accepts one key at a time apart from shift.
• Open circuits in the matrix, whether a line or a diode, affect only the keys on that line. A short between lines holds one key on and so disables the whole keyboard. Check for shorts where the tracks run at an angle under KB1; the resistance between the KBD tracks should be a few thousand ohms.
• Sinclair's check for a keyboard that will not enter is that the tails are fully in their connectors and not twisted, and that the diodes D1 to D8 and the connectors are the right way round.
• Before refitting the keyboard, bridge each KB1 contact to at least two KB2 contacts to confirm the board, then check the tails for splits and the end contacts for wear.
• A whole row or bank of keys dead is usually a crack across a tail. Near the end of the tail, a clean square cut may remove the crack if the tail still reaches. Do not clean the contacts with liquid solvent cleaners; some destroy the plastic. A tail that cannot be repaired means a new keyboard.

The diodes D1 to D8 sit on address lines A8 to A15; the five return lines go to ULA pins 33, 31, 29, 27 and 25 (KBD0 to KBD4), pulled up by RP3 (5 × 10 kΩ, individual resistors R43 to R47 on Issue 3). Each of those ULA inputs also has a 16K pull-up inside the chip.

The order of the lines on the two board connectors is given in Maplin's 1982 add-on keyboard article, whose cables plug into them in place of the membrane's tails. On the 5-way connector the line at the striped end of the cable is KBD4. On the 8-way connector the order from the striped end is A11, A10, A9, A8, A15, A14, A13, A12. Maplin's amendment sheet for the kit says to fit both cables with the black stripe to KBD4 and to A11 respectively.

ROM

Run this check, which sums all 8192 bytes and takes just 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 is correct. 854885 is the early ROM, which PRINT SQR .25 confirms by giving 1.359 instead of .5. Any other answer is a ROM fault. IC2 is a 24-pin 2364 8 KB mask ROM.

For work with a logic analyser or an in-circuit emulator, these ROM addresses mark the routines that run at switch-on and while the display is generated:

ZX81 ROM landmarks (The ZX81 Companion)

Address (hex)	Routine
0000	Start of execution at reset
0038	RST 38: interrupt routine that handles the lines of the TV display
0066	NMI routine that forms the display in SLOW mode
0207 to 02BA	Routines that produce the TV display
02BB to 02E6	Keyboard scan; returns FFFF in HL when no key is pressed
02F6 to 033F	SAVE
0340 to 03A7	LOAD
03CB to 03E4	RAM check, run at initialisation and after NEW
03E5 onwards	Initialisation
1E00 to 1FFF	Character set, 64 characters of 8 × 8 dots

Cassette

Save a short program, switch off, switch on and load it back. The EAR input is C10 (10 nF) and R33 (4.7 kΩ) to ULA pin 20, with R34 (220 Ω) across the socket; the MIC output comes from the ULA's TV/tape line through R29 (1 MΩ) and C12 (47 pF), with R27 (1 kΩ) and C11 (47 nF) across the socket. Inside the ULA, pin 20 has clamp diodes to +5 V and ground and a 400 Ω series resistor. Timex's TS1000 specification originally asked for a tape input with hysteresis; Ferranti struck that out and noted that its input has none, with thresholds of 0.2 V maximum low and 1.0 V minimum high. In 1982 shops sold signal boosters to go between the recorder and the computer, one advertised as "usually cures loading difficulties".

RAM pack and edge connector

A RAM pack switches off the internal RAM through the RAM CS line on edge contact 2A; ROM CS is on 23B. Sinclair's manual prints the full contact list, which is reproduced on the Sinclair ZX81 page. A pack that rocks on the connector can make the machine crash; in 1982 dk'tronics sold a flexible connector described as "useful for getting rid of RAM Pack wobble". Clean the edge connector fingers before suspecting the pack or the board.

Checking the ICs

If the fault survives the checks above, it is in an IC or on the board. First look for signs of earlier repair, solder splashes and shorted tracks. Sinclair fitted sockets either to every IC or to none, so a board with only some ICs socketed has probably been modified.

The table gives the Television article's logic probe readings at every IC pin, taken with the machine at the inverse K cursor and the probe powered from the board's 5 V rail, on a board with two 2114 RAMs. P is pulsing (both LEDs lit), PH pulsing and high, PL pulsing and low, H high, L low, and OC no LED lit (open circuit).

ZX81 logic probe readings at the inverse K cursor (Television, Table 2)

Pin	IC1 (ULA)	IC2 (ROM)	IC3 (CPU)	IC4a/b (2114)
1	P	P	P	P
2	P	P	P	P
3	P	P	PL	P
4	P	P	P	P
5	P	P	P	P
6	P	P	P	P
7	PH	P	P	P
8	PH	P	P	P
9	P	P	P	L
10	P	P	P	P
11	P	P	H	P
12	P	L	P	P
13	P	P	P	P
14	P	P	P	P
15	PH	P	P	P
16	PH	P	P	P
17	P	P	PH	P
18	P	P	P	H
19	P	P	P
20	L	P	PH
21	P	P	P
22	OC	P	PH
23	P	P	H
24	P	H	PH
25	H		H
26	P		H
27	H		P
28	P		P
29	H		L
30	P		P
31	H		P
32	P		P
33	H		P
34	L		P
35	H		P
36	P		P
37	P		P
38	P		P
39	P		P
40	H		P

The article's order for quick checks is the 5 V and 0 V pins of every IC first, then the CPU's reset and memory request pins, then the chip select and read pins of the ROM and RAM.

Buffer resistors

The data lines between the CPU and the ROM and RAM pass through 470 Ω resistors (R7 to R14), and ROM address lines A0 to A8 through 1 kΩ resistors (R18 to R26). Lines on the far side carry a prime mark on the diagram (A1', D0'). A dead IC at one end of a resistor does not affect the ICs at the other end, so comparing the two sides shows which device is at fault.

Isolating a fault without removing a 40-pin IC

• Cut a track to trace a short (a missing logic signal), and place the cut where a socket will not later cover it.
• Piggy-back a good IC on a suspect soldered-in one, with its legs sprung in so every pin makes contact. This works best against open-circuit chips.
• When removing a soldered IC, free every pin before lifting. A pin still held in its hole can lift and crack the track on the top of the board, and a socket fitted over the crack then hides it. Check continuity from each IC pin to the next component.

ULA

IC1 is the ULA, called "Sinclair Computer Logic" on the circuit diagram; Sinclair's component list identifies it by "158" printed underneath. It handles the video, the tape port, the keyboard reading and the ROM and RAM selects, and produces the CPU clock.

Television notes that the screen is bright if the ULA works, even when every other chip is faulty, so a bright screen with no cursor points away from the ULA. The Sinclair Shop, which sold replacement ULAs, listed these symptoms of a failed one: a very dark screen with a dark, blurred cursor at the bottom left; a black screen with no cursor (which can also be a power fault); and a white screen with black zig-zag lines moving up or down.

ULA types

Ferranti made the ZX81 ULA in at least three types with the same pin allocation: the 2C158 (drawings of 1980), the 2C184 (1981) and the 2C210 (1982). The kit list's "158 printed underside" matches the first; the Issue 3 board in the photograph at the top has a 2C210E. Timex approved the 2C158 for the TS1000 with the 2C184E as the alternate. The types are pin-compatible on paper, but no document says that every board issue accepts every type. More on Sinclair ZX81 ULA.

ULA pin checks

ZX81 ULA pins to check first (Ferranti drawings)

Pin	Signal	What to look for
40, 34	+5 V, 0 V	Supply at the chip
35	Ceramic resonator	6.5 MHz oscillation; the ULA derives the CPU clock from it
14	CPU clock	3.25 MHz, totem-pole output, to the Z80 through TR2
16	TV/TAPE	Three levels with sync near 0 V; see No picture
15	/NMI	Open-collector output with a 5K pull-up inside the ULA
12, 13	/RAM CS, /ROM CS	Totem-pole outputs; both pulse at the cursor in the Television table
20	TAPE IN	Low at the cursor in the Television table

Pin functions and circuit types are from Ferranti's drawings.

Before replacing the ULA

In February 1983 Ferranti tested ZX81 and TS1000 ULAs (2C184 and 2C210) returned by Timex in the United States and Portugal. Of 17 US returns, three had been blown up by overvoltage, four passed production test but did not work in ZX81s, three failed in a ZX81 when warm (two of them also marginal on the testers), and four passed everything. Of nine Portugal devices, two had been blown up, two more possibly so (one with its tape input, pin 20, working poorly and one with pin 24 stuck high), one was a temperature failure and one was good after a broken pin was repaired. Read as history, not as a failure rate, these suggest three checks:

• Find out why the old ULA died. Measure the adaptor, the 7805 and the +5 V rail before fitting a replacement, because overvoltage killed several of the returned chips.
• A ZX81 that fails after warming up can have a marginal ULA. Several returns passed every factory tester and failed only in a ZX81, or only when warm, so test a suspect ULA in the machine that shows the fault.
• A damaged ULA can fail on a single pin, such as the tape input, while the rest of the chip works.

Things that are not faults

• In FAST mode the picture blanks while a program runs and flickers on a key press.
• PRINT SQR .25 returning 1.359 on an early ROM.
• A hot regulator heatsink, provided it can still be touched.
• The internal 1 KB disappearing when a RAM pack is fitted.

Related pages

• Sinclair ZX81
• Sinclair ZX81 General Maintenance
• Sinclair ZX81 Capacitor Replacement Guide
• Sinclair ZX81 Assembly Instructions
• Sinclair ZX81 ULA
• Sinclair ZX80 Troubleshooting Guide
• Ferranti Archives 2026
• Recommended Tools