Sinclair ZX Spectrum ULA Replacement

A Ferranti 6C001E-7, the last 48K Spectrum ULA, date code 8543

The Sinclair ZX Spectrum ULA (IC1) is the 40-pin Ferranti gate array that generates the picture, scans the keyboard and drives the tape and sound pin on the 16K and 48K Sinclair ZX Spectrum and on the Sinclair ZX Spectrum+, which uses the same boards. Sinclair fitted it in a socket: the parts list includes a DIL socket for IC1, and Sinclair's fault-finding sequence says to replace IC1 "if IC1 is the plug-in type" once the supplies and clock have been checked. Swapping it is therefore the quickest test on the board, but the ULA types are not all interchangeable between board issues, and some boards need resistor changes when the type changes.

Symptoms

Sinclair's and the community's fault lists name the ULA for:

• no video at all;
• a dead keyboard, a wrong keyboard response or a keyboard response that is too fast;
• a red, blue or green "Sinclair" logo, or the paper area too low on the screen;
• character break-up or random crashes once the machine is warm; the T.I.S. notes check the heatsink first for faults that appear only when warm, and then IC1;
• no sound, or the border changing colour when sound plays;
• no loading stripes from a tape that is known to be good, or loading that improves with another ULA ("gone a bit deaf", in Retroleum's words).

The ULA runs quite warm, while every other chip on the board runs cold; that alone is not a fault. A whole column of keys dead, or wrong characters from one column, can come from the ULA's keyboard inputs as well as from the membrane; see the troubleshooting guide.

Before you start

• Rule out the supply. Sinclair's checks: +5 V with no discernible ripple at IC1 pin 14, ground at pin 40 and about +3 V at pin 13 (the second supply, fed through R34). In 1984 Ferranti found 6C001 ULAs that worked at 5 V and above and failed below, so measure pin 14 at the chip itself as well as at the regulator.
• Check the clock. Sinclair's checks are 14 MHz at IC1 pin 39 (crystal X1), the CPU clock at IC1 pin 32, and the clock reaching IC2 pin 6 through TR3; low amplitude points at R25.
• Take normal anti-static precautions and use the correct removal and insertion tools for plug-in ICs, without handling the pins.
• Find out which board issue you have before choosing a replacement (see below). The issue is printed on the board's silkscreen, for example "© 1983 ISSUE 4B" along the bottom edge of an Issue 4B board.

Which ULA fits which board

48K ULA types and the boards they suit

ULA	Boards	Notes
5C102E	Issue 1 and a few Issue 2	Needs Sinclair's 74LS00 "cockroach" patch on a small board with flying leads; fitted to about 40,000 units
5C112E, -2, -3	Issue 2; also Issue 1 with the cockroach patch removed	Needs the diode-and-resistor or transistor (TR6, ZTX313) modification, which became TR6 on the board from Issue 3
6C001E-5, -6	Issues 1, 2 and 3	The -5 is rare and found only in late Issue 2 machines; the -6 is the usual Issue 3 and 3B part
6C001E-7	Every 48K board, Issue 1 to 6A	Required from Issue 4A, where two spare gates of IC24 are put in the RASL path to improve ULA timing
SAGA	Issue 6A	Issue 6A takes either a Ferranti or a SAGA ULA

Spectrum for Everyone gives this list as the combinations Sinclair documented and supported, and has seen a 5C112E work in an Issue 6A. Retroleum's advice is that any type can go in an Issue 2 board (preferably the type already fitted), but that Issue 3 and later boards should have only a 6C001E-6 or 6C001E-7. Worsley treats the 5C112 and 6C001 as interchangeable. The 7K010E belongs to the Spectrum 128 and grey +2 and is not a 48K part.

The 6C001 shifts the whole picture one character to the left, by a change Sinclair made to the timing of the colour burst to suit certain television sets. Expect this after fitting a 6C001 in place of a 5C ULA; it is not a fault.

Modern replacements

• Charlie Ingley's vLA82 is a plug-in replacement for the 6C001E-7 and earlier types, with the 6C001-7's functions, in a low 40-pin DIL footprint designed to fit a rubber-key case without changes. Most existing sockets take it, but some accept only very small pins; a turned-pin socket is offered for those.
• The Nebula, designed by Phil Ruston and Alessandro Dorigatti and sold by Retroleum, is a daughterboard for UK boards Issue 2 to 6 that emulates a 6C ULA, with the same YUV output as the original. On rubber-key machines there is very little room between the ULA socket and the top cover, and if IC3, IC4, IC24 or IC25 are in sockets they can put the case under strain; the designers strongly advise soldering those chips directly to the board. On Issue 2 boards VR1 and VR2 need adjusting afterwards, as with any ULA change.

Tools and parts

• A replacement ULA of a type that suits the board (above), or a modern replacement.
• A DIL IC extractor, or a flat blade to lever the chip out evenly.
• A multimeter; an oscilloscope is useful for the clock checks.
• For an Issue 2 board changing from a 5C112 to a 6C001: resistors for R47 (1 kΩ), R49 (10 kΩ), R56 and R63 (470 Ω).

Procedure

Open the machine

1. Unplug every lead, turn the computer upside down and remove the five self-tapping screws.
2. Turn it keys-up and lift the top half. It stays connected by the two keyboard ribbons; note how they lie.
3. To lift the board out, remove the single screw next to the IC22 socket.
4. On a Sinclair ZX Spectrum+ the base has eight screws, seven 8 mm pan-head screws round the overhang and one 13 mm countersunk screw at the back next to the expansion slot, and the board is held by two 6.5 mm pan-head screws at its front corners.

Remove the old ULA

A Ferranti ULA 6C001E-7 (IC1) in its socket on an Issue 4B board, with IC25 (74HCT157), IC24 (74HCT00) and IC23 (74LS32) to its right and the upper RAM chips IC22 and IC21 below them

1. Note the type and suffix printed on the chip (for example 6C001E-7) and the board issue.
2. Note which way round the chip sits: its notch end must go the same way when the new chip goes in.
3. Lever the ULA out of its socket evenly from both ends, using the correct removal tool.
4. Look at the socket and the pads. Sinclair's first checks after a ULA change are a visual check of IC1 and its socket, and a short between C65 and R53. The T.I.S. notes replace a faulty holder completely by cutting it out.

Fit the new ULA

1. Check that every pin is straight, and insert the chip with its notch the same way as the original, without folding any pin under or outside the socket.
2. On an Issue 2 board going from a 5C112 to a 6C001, change the four video resistors: R47 from 220 Ω to 1 kΩ, R49 from 8.2 kΩ to 10 kΩ, and R56 and R63 from 220 Ω to 470 Ω. On an Issue 3 board with a 6C001 the values are 1 kΩ, 10 kΩ, 1 kΩ and 1 kΩ. On Sinclair's Issue 3 diagram R47 runs from the ULA's luminance output, pin 17, to 0 V, and R56 and R63 from its colour-difference outputs, pins 15 and 16, to 0 V. The background is in General Maintenance.
3. On an Issue 1 board, a 5C112 goes in with the cockroach patch removed; a 5C102 needs it fitted.
4. On Issue 1 and Issue 2 boards, readjust the colour presets VR1 and VR2 after any ULA change: see Sinclair ZX Spectrum Colour Adjustment.
5. The T.I.S. notes suggest an extra heatsink on IC1 if it has already been replaced once.

Close the machine

1. Check that neither keyboard ribbon is trapped between components as the case goes together.
2. Refit the five case screws without overtightening them.

Testing

1. Power on: the machine should clear the screen and show the copyright message at the lower left.
2. Check the picture, keyboard, sound and tape. Ferranti's own 1983 functional test of the 6C001 in a Spectrum checked colour bars at normal and BRIGHT with FLASH working, the border stepping through all eight colours on keys 0 to 7, six BEEPs of rising pitch, and red and pale blue stripes in the border during a SAVE; a short program for the same checks is in the troubleshooting guide.
3. If there is no picture after the change, check IC1 and its socket, the C65 to R53 short and the power supply.
4. If the paper area is corrupt after the change, check the IC1 socket, check that the power socket is not intermittent, and check IC1 pin 14 for +5 V with no ripple.
5. A machine whose RAM tests good but which crashes only from cold, or only once warm, may have a ULA-to-RAM timing problem: try a ULA of a different mark and note which work on that board. See the troubleshooting guide and Sinclair ZX Spectrum Memory Timing.

Related pages

• Sinclair ZX Spectrum ULA
• Sinclair ZX Spectrum
• Sinclair ZX Spectrum+
• Sinclair ZX Spectrum Troubleshooting Guide
• Sinclair ZX Spectrum General Maintenance
• Sinclair ZX Spectrum Colour Adjustment
• Sinclair ZX Spectrum Memory Timing
• Ferranti Archives 2026