Sinclair ZX Spectrum Colour Adjustment

The colour circuit of an Issue 1 board: the LM1889N colour encoder (IC14) at the left, the 4.433619 MHz and 14.000 MHz crystals, two trimmer capacitors beside the crystals, and the two 2.2 kΩ skeleton presets below them. On Issue 1 the upper preset is VR2.

Issue 1 and Issue 2 boards of the Sinclair ZX Spectrum set their colour with two presets, VR1 and VR2, and two trimmer capacitors, TC1 and TC2. VR1 and VR2 null the offsets on the colour-difference inputs of the LM1889 encoder (IC14), TC2 sets the 4.43 MHz colour sub-carrier, and TC1 trims the 14 MHz master oscillator. When they drift, or after the ULA has been changed, the picture takes on a yellow or blue tint, loses its colour, or shows bands of distortion. Issue 3 introduced colour circuitry that needs no adjustment and has none of these parts, so on Issue 3 and later boards (including every Sinclair ZX Spectrum+) a colour fault is a component fault.

Symptoms

• A yellow tinge over the whole picture on an Issue 1 or 2 machine. Spectrum for Everyone says this most likely needs only the adjustment below.
• A blue picture after adjusting: the television has decided the signal is SECAM. Reverse the adjustment a little at a time.
• A black-and-white picture on an Issue 2 board: adjust TC2.
• Waves of distortion flowing across the screen with some colour combinations, such as red INK on green PAPER ("dot crawl"). TC1 can reduce them but not remove them; Retroleum treats some dot crawl as normal on every Spectrum.

A red, blue or green "Sinclair" logo is a ULA fault, not an adjustment. Other colour faults are listed in the troubleshooting guide.

Tools and parts

• A multimeter with a millivolt range, and two grabber probes or a second pair of hands.
• A small screwdriver, non-conductive if possible, for the presets and trimmers.
• For TC2: a frequency meter, a 4.7 pF capacitor and a 10 kΩ resistor; Sinclair suggests building the probe on an IC test clip. Alternatively a colour television whose locked sub-carrier can be measured.
• Optionally an oscilloscope, which Spectrum for Everyone calls by far the quickest method.
• A bench supply set to 9 V, if you want to watch the current: Sinclair expects about 500 mA to 700 mA on a 16K machine and 700 mA to 900 mA on a 48K.

Procedure

Prepare

1. Remove the board from the case: unplug every lead, remove the five self-tapping screws from the base, lift the top half, then remove the single board screw next to the IC22 socket. The keyboard may be disconnected for convenience.
2. 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.
3. Check the supplies first. At IC6, pin 9 should be +5 V ±0.25 V, pin 1 between −4 V and −5.5 V and pin 8 +12 V ±1.2 V, and the LT side of C52 +12 V ±1.2 V.
4. Find the parts. VR1 and VR2 are the two skeleton presets just to the left of the ULA; on Issue 2 VR1 is the upper one, and on Issue 1 the order is reversed, with VR2 at the top. IC14, the LM1889, is just below the RF modulator.
5. Note where TC2 is set before touching it, so that it can be returned to that position.

Set VR1 and VR2 with a meter

Sinclair's figures, measured with respect to IC14 pin 3:

VR1 and VR2 settings (Sinclair Service Manual, section 3.1.3)

Preset	IC14 pin	Factory setting	Optimum	Overall range
VR1	4	130 mV ±20 mV	+50 mV	+45 mV to +150 mV
VR2	2	75 mV ±20 mV	−50 mV	−45 mV to −100 mV

Sinclair set the presets to non-zero voltages to allow for thermal drift, and says the factory settings were satisfactory but not optimal; the optimum values "may be used to advantage in servicing". The T.I.S. service data gives the factory figures as 130 mV on pin 4 and −75 mV on pin 2, within 20 mV.

1. Set the meter to its millivolt range. Clip the negative probe to IC14 pin 3, taking care not to short it to the neighbouring pins.
2. Clip the positive probe to IC14 pin 4 and adjust VR1 until the meter reads as close to +50 mV as possible.
3. Switch off, move the positive probe to IC14 pin 2, power up again and adjust VR2 until the meter reads as close to −50 mV as possible. Switch the power off each time you move a clip.
4. Look at the picture. If it is not right, readjust VR1 and then VR2.

Set VR1 and VR2 with an oscilloscope

1. Connect the oscilloscope to the composite video wire going into the modulator.
2. With the copyright message on screen, trigger on the drop between the white signal and the back porch.
3. Adjust VR1 and VR2 until the video waveform is as flat as possible, with no oscillation riding on it. A badly set board shows the oscillation clearly along with the yellow tint.

Set the sub-carrier with TC2

The sub-carrier should be 4.433619 MHz ±50 Hz.

1. Either feed the modulator output into a colour television and measure the frequency of its locked chroma sub-carrier,
2. or connect IC14 pin 17 to the frequency meter through a 4.7 pF capacitor, with a 10 kΩ resistor across the meter's input terminals. Sinclair recommends building this on an IC test clip, and says pin 18 "must be removed to minimise stray capacitance".
3. Adjust TC2 only if the frequency is outside tolerance.

Reduce dot crawl with TC1

TC1 cannot be set to a figure; Sinclair warns that the 14 MHz frequency drifts visibly with temperature, so the aim is to reduce the effect, not remove it.

1. Set green PAPER with red INK.
2. Type three or four lines of characters and watch the screen.
3. Adjust TC1 to minimise the waves of distortion.

If it will not adjust

Spectrum for Everyone's list, for a tint that cannot be removed:

• Replace the electrolytic capacitors; faulty capacitors can put noise on the video output. See Sinclair ZX Spectrum Capacitor Replacement Guide.
• Set TC2 to 4.433619 MHz as above, measuring at IC14 pin 17.
• Fit a 4.7 kΩ resistor between IC14 pin 14 and ground.
• Replace the LM1889.

Retroleum adds that grubby presets should be replaced, and that if no amount of adjustment gives a stable white background the ULA has probably degraded. Sinclair's own modification list for Issue 1 boards includes a 47 kΩ resistor from LM1889 pin 13 to ground, "required only if the difference between white and yellow colours is inadequate".

Sinclair's fault list for colour tuning on Issue 1 and Issue 2 boards:

Colour tuning faults (Sinclair Service Manual, section 6.5)

Symptom	Action
Any colour tuning fault	Visually check the IC14 circuits
VR1 or VR2 will not adjust the IC14 pin 13 waveform	Monitor IC14 pin 2 while trimming VR2 and pin 4 while trimming VR1; change IC14
VR1 or VR2 shifts the DC level of the IC14 pin 13 waveform	Change IC14
Alternate dark lines on the screen	Change IC14
No colour after trimming TC2, and IC14 pin 17 not oscillating	Check crystal X2; change IC14; change X2
No colour after trimming TC2, IC14 pin 17 oscillating but will not tune	Check for a broken track; change TR2; change IC14
IC14 pin 17 output correct but still no colour	Check the pulses on IC14 pins 2 and 4 with the logo on screen; if they are correct, change the modulator

Worsley's rule for no colour on an Issue 2 machine is to adjust TC2, VR1 and VR2, and to change the LM1889 if that fails.

Testing

1. Run a colour-bar program and check that all eight colours appear correctly, from black to white. The T.I.S. notes use:

10 FOR A=0 TO 7
20 FOR B=0 TO 3
30 PAPER A: PRINT "     ";
40 NEXT B
50 NEXT A
60 GOTO 10

1. Check the white background of the copyright screen for any remaining tint.
2. Refit the board and close the case, checking that neither keyboard ribbon is trapped and without overtightening the screws.

After a ULA change

Changing the ULA on an Issue 2 board means readjusting VR1 and VR2, and a change between ULA types also changes four resistor values in this circuit; see Sinclair ZX Spectrum ULA Replacement.

Related pages

• Sinclair ZX Spectrum
• Sinclair ZX Spectrum ULA Replacement
• Sinclair ZX Spectrum Troubleshooting Guide
• Sinclair ZX Spectrum General Maintenance
• Sinclair ZX Spectrum Service Manual