Sinclair ZX80 Troubleshooting Guide

Sinclair's ZX80 circuit diagram, the source for the designators and pin numbers below.

Fault finding on the Sinclair ZX80 at board level. Designators, pin numbers and values are from Sinclair's circuit diagram and the component list in the kit assembly instructions; the description of how the circuit works is Grant Searle's.

The ZX80 has no ULA: its logic is 74LS TTL, and every IC except the regulator sits in a socket in the kit. The RAM is static (two 2114s), so the only regulated rail is +5 V. The keyboard contacts are on the main board under the membrane overlay, so there are no keyboard ribbons or connectors. Advice written for the ZX81 or the Spectrum about ULAs, DRAM rails and keyboard tails does not apply.

Quick reference

ZX80 symptoms

Symptom	Probable cause	Action
Dead, regulator cold	No supply, plug in the wrong socket, open C8 or regulator	Check the adaptor off load and on load, the socket, then IC22 input and output
Works, then goes off; regulator very hot	Regulator not bolted to the heatsink	Tighten the 4BA bolt; check the card insulator is in place
Regulator warm, no picture	Tuning, lead, modulator or video path	Tune to channel 36, then 33 to 39; then see No picture
Cursor shown, keyboard does not enter	D3 to D10 reversed or IC10 badly seated	Check diode polarity and IC10
Screen clears, no cursor	Reset network, or a solder bridge	Power off for a few seconds and retry; check C10 and R21; remove the ICs and check the sockets for bridges
Horizontal lines through the picture	Ripple on the supply	Check the adaptor and C8
Picture jitters and the television's controls do not cure it	C15 in the X1 oscillator	The earlier printing of Sinclair's assembly instructions says to experiment with the value of C15, within 0 to 330 pF
Some keys dead	Overlay contact or one matrix line	See Keyboard

Power

• The adaptor is rated 9 V DC unregulated at 600 mA, with a 3.5 mm jack plug, tip positive. The Sinclair ZX Archive measured about 14 V off load.
• Sinclair allows 7 to 12 V from another supply and gives the machine's current as 350 to 500 mA, typically 400 mA. The earlier printing of the instructions allows 7 to 11 V, and the circuit diagram printed with it marks the input "9V-11V". Television magazine's ZX81 article treats an excessive current reading as a fault on the board.
• The input goes straight to C8 (22 µF 16 V) and IC22 (7805 or LM340T5) with no polarity protection. On the ZX81, which has the same arrangement, Television found that a reversed plug often blows the regulator and spares the rest of the board.
• A plug in the EAR or MIC socket does no harm; the machine just does not start.

ZX80 checkpoints (circuit diagram)

Point	Expected
Positive lead of C8	Adaptor voltage, unregulated
IC22 output	+5 V
IC1 (Z80) pin 11, pin 29	+5 V, 0 V
IC2 (ROM) pin 24, pin 12	+5 V, 0 V
IC3, IC4 (2114) pin 18, pin 9	+5 V, 0 V
IC1 pin 6	3.25 MHz clock (labelled 3.2 MHz on the diagram)
IC1 pin 26 (/RESET)	Low for a moment at switch-on through R21 (220 kΩ) and C10 (1 µF), then high

Clock and reset

X1 is a 6.5 MHz three-lead ceramic filter (CDA6.5MC) in an oscillator built from IC20 gates, with C15 (220 pF) and R27 (1 kΩ). IC18 divides the 6.5 MHz by two to give the 3.25 MHz CPU clock, buffered through IC15; the 6.5 MHz itself clocks the shift register IC9.

• No clock at IC1 pin 6: check the oscillator output at IC20, then IC18 and the IC15 buffers.
• Clock present but the CPU does nothing: check /RESET at pin 26 goes high after switch-on. Sinclair's check for "screen clears but no cursor" is C10 and R21.
• Clock and reset correct but the address lines are static: remove any add-on and check nothing holds /WAIT, /BUSRQ or /NMI low before suspecting IC1.

No picture

The display is produced by the CPU. It executes the display file in RAM while IC14 and IC15 force NOP instructions onto the data bus; IC5 latches each character code, the 74LS157 multiplexers IC6 to IC8 pass it to the ROM as a character-pattern address together with the line count from IC21, IC9 shifts the pattern out, and IC20 inverts it for inverse characters. IC18 and IC19 make the sync pulses.

1. Tune the television. Sinclair gives channel 36 and suggests trying 33 to 39.
2. Check +5 V at the modulator and at IC1 pin 11.
3. Check the clock and /RESET as above.
4. Check the video signals into the modulator. VIDEO comes through R30 (1 kΩ) and SYNC through R32 (330 Ω); R31, R33 and TR1 are not fitted.
5. With sync but no characters, follow the path above: IC5, IC6 to IC8, IC2, IC9, IC20.
6. The modulator is a UM1233, as on the ZX81. Television notes that on the ZX81 the tuning screw normally sits about 3 mm down inside the former.

Memory

RAM

IC3 and IC4 are 2114 static RAMs, 1K × 4 each. They do not split the byte into nibbles: IC3 holds data bits 0, 1, 2 and 7 and IC4 holds bits 3, 4, 5 and 6. In the display file, bit 7 of each byte selects inverse video and bit 6 decides whether the NOP circuit acts on it.

RAM is selected when /MREQ is low and A14 is high, through IC12 and R19 to the RAM CS line. The circuit diagram printed with the earlier assembly instructions does not show /WR reaching pin 10 of either 2114, which the diagram hosted here does ("WR 10" on both). Trace the write line from the later diagram. Both RAMs are socketed in the kit; substitution is the practical test. Sinclair allows IC3 to be the same -1 type as IC4.

ROM

IC2 is a 24-pin 4 KB ROM: an 8332, or a TMS 4732 on some boards. It is selected when A12, A14 and /MREQ are low. Searle's own board has a 2532 EPROM in this position, with one pin cut and wired to +5 V because the 2532 lacks the mask ROM's second chip select. A 2732 needs two pins swapped instead. His page shows the wiring for both.

Data bus resistors

R4 to R11 (1 kΩ) sit between the CPU data lines and the memory side of the bus (D0' to D7'). Television describes the same arrangement on the ZX81 as a fault-finding aid: a dead device at one end of a resistor does not pull down the other end, so comparing the signals either side shows which device is at fault.

Grant Searle lists two drawing errors in this area of Sinclair's diagram: R7 is drawn from D4' where it should be D6', and R28 is drawn to D6 where it should be D6'.

Keyboard

The keyboard is a membrane overlay over contacts printed on the main board. Address lines A8 to A15 drive the matrix through diodes D3 to D10; the five return lines are pulled up by R13 to R17 (47 kΩ) and read through IC10 (74LS367 or 74LS365), which also reads the cassette input on D7.

• Keyboard does not enter at all: Sinclair's checks are the orientation of D3 to D10 and the seating of IC10.
• One group of keys dead: check the diode on that address line and the pull-up on that return line.
• Single keys dead: the overlay contact over that key.

CPU

IC1 is an NEC µPD780C-1, a Z80 equivalent. Check the supply, clock and /RESET first and make sure no add-on is holding a bus control line low. The CPU is socketed in the kit, so substitution is the test.

Edge connector

The rear connector carries the Z80 bus with +5 V, +9 V and 0 V. The /ROMCS input that the ZX81 has is not connected on the ZX80, so add-ons that rely on it misbehave. Remove all add-ons before fault finding, and clean the connector fingers.

Things that are not faults

• The picture blanks while a program runs and flickers when a key is pressed.
• The regulator heatsink gets hot and the large ICs get warm.
• Grey screen and horizontal streaks during SAVE, and a grey, agitated screen during LOAD.

Related pages

• Sinclair ZX80
• Sinclair ZX80 General Maintenance
• Sinclair ZX80 Capacitor Replacement Guide
• Sinclair ZX80 Assembly Instructions
• Sinclair ZX81 Troubleshooting Guide
• Recommended Tools