Sinclair QL Maintenance Guide

Sinclair QL opened. The main board runs the width of the case, with the +5 V regulator heatsink at centre and both Microdrive mechanisms at the right.

This guide covers preventive maintenance for the Sinclair QL. Procedures follow Section 2 (Disassembly/Assembly) and Section 4 (Fault Diagnosis and Repair) of the Sinclair QL Service Manual.

The QL's maintenance priorities are the keyboard membrane, the +5 V regulator and its heatsink, the Microdrives, and — above everything else when a soldering iron comes out — the board itself, which is easily damaged.

⚠️ Safety

The QL is powered from an external supply and there are no mains voltages inside the computer; the highest voltage on the board is the 44 V peak-to-peak AC feed from the power pack. The QL is therefore far safer to open than a mains-powered machine such as the Acorn Archimedes A3000.

The power pack itself is a sealed mains unit. If it is faulty, it can be opened and serviced by someone competent with mains equipment — unplug it, discharge the reservoir capacitor and confirm 0 V before touching the primary side — or replaced with a supply matching the original's 9 V DC / 44 V pk-pk AC output.

Observe normal anti-static precautions for the 68008, ZX8301, ZX8302 and IPC.

⚠️ The QL board is fragile — read before soldering

This is the single most important thing to know before working on a QL. The board is densely populated, the tracks are fine, and pads lift easily. The Service Manual is explicit that renewal of components "should be carried out using recognised desoldering/heatsinking techniques to prevent damage to the component or to the printed circuit board", and that only a suitably earthed, low-power soldering iron should be used.

Working rules:

• Use a temperature-controlled iron at a modest setting with a clean, well-tinned fine tip. Heat that is too low forces you to dwell on the joint, which is what actually lifts pads.
• Keep dwell time short — a few seconds per joint, then let the area cool before returning to it.
• Add fresh leaded solder and flux to every old joint before removing anything. The original joints are decades old; reflowing them with new solder lowers the melting point and makes removal far cleaner.
• Never pull a component while the solder is still solid. Lever a part out only when the joint is visibly molten — this is the advice given for removing soldered ICs, and it applies to every part on the board.
• Support the board. Do not flex it. Flexing is what cracks tracks and breaks the solder joints of through-hole connectors.
• Desolder with wick or a vacuum tool, clearing each hole fully before trying to move the part.
• Check for solder splatter afterwards. The Service Manual warns that hairline solder bridges can measure several ohms and produce very misleading fault symptoms, so inspect the board carefully after any rework.

The areas most at risk are the rear connectors — the RGB monitor socket, the serial, joystick and network sockets and the power input — because they take mechanical stress in use and their pads are the first to lift. Treat any connector work as a strain-relief job as well as a soldering job; see the Sinclair QL Troubleshooting Guide for the RGB socket in particular.

Opening the case

Follow the Service Manual sequence:

1. Unplug every lead.
2. Turn the QL upside down. There are eight self-tapping screws: four short ones along the front edge under the overhang, and four longer ones along the rear edge. Remove all eight.
3. Do not remove the two screws visible on the base immediately below the Microdrives — these hold the Microdrive assemblies, not the case.
4. Holding the halves together, turn the machine right side up and lift the top half. It stays connected by two flexible keyboard ribbons and the LED leads.
5. To separate fully, free the membrane tails from their edge-connector sockets by pinching the ribbon next to the socket and pulling straight up. Release the LED and speaker leads from the snap-action wire post socket next to the TV modulator can.

To remove a Microdrive, first take off the heatsink by releasing the +5 V regulator's screw (keep the M3 × 10 mm screw and its brass plain and crinkle washers), then remove the three self-tapping screws holding the drive — two at opposite corners of the chassis and one from the underside — taking care not to confuse them with the countersunk motor screws.

Keyboard

The QL keyboard is a membrane under a rubber bubble mat, in an 8 × 8 matrix with partial diode isolation, connected to the board by two flex tails at J11 and J12. It is a wear item, and the usual failures map cleanly onto symptoms:

Keyboard symptom to remedy (Service Manual)

Symptom	Remedy
Keyboard does not respond at all	Check connectors J11 and J12 and the ribbon cable
Multi-character printing, key bounce, keys sticking down, a key does not register	Renew the membrane
A key does not always register	Renew the bubble mat
"Sticky" keys	Clean the key surfaces; renew the bubble mat
Continues to fail the keyboard test	Renew IC24 (IPC), then IC18 (CPU)
Keyboard locks up after prolonged use	Renew IC23 (ZX8302), then IC24, then IC18

Source: Service Manual fault-finding guide.

Membrane tail repair. A common fault is a break in the membrane close to the connector. If the break is near the tail end, the ribbon can be removed from the connector, a short section sliced off above the break, and the shortened tail plugged back in. Cleaning the tail ends and the sockets is worth doing routinely — dirty or damaged tails cause lock-ups and dead keys.

Individual keycaps can be pulled for cleaning. When refitting a membrane, make sure both tails are fully inserted and not kinked.

Cooling and the +5 V regulator

The QL runs warm, and the 7805 at IC35 does most of the work. Maintenance points:

• The regulator body must make good thermal contact with the heatsink. Apply fresh thermal grease when refitting, and make sure the retaining screw is tight.
• A healthy heatsink gets hot all over. If only part of it is warm, the thermal path is poor.
• Check the +5 V rail: measure at the regulator pins. If it is below about 4.8 V, renew the 7805.
• Ventilation slots were added to the case at build standard D04, along with a new heatsink — earlier machines have no top-rear ventilation and run hotter still.

A 78S05 2 A regulator can be substituted, though it makes little practical difference. Fitting a switch-mode replacement is not recommended: it generates RF interference. Relocating the regulator outside the case is bad practice and causes more problems than it solves.

Socketed ICs and the ROM cartridge slot

The QL's main ICs are socketed, and dirty socket contacts are a genuine and common cause of random crashes. Periodically remove each socketed IC, clean the legs and the socket with contact cleaner, and refit firmly, observing anti-static precautions. Clean the ROM cartridge edge connector too — an eraser on the solder-side tracks works well, as the contacts are not gold plated.

Use proper IC removal and insertion tools, and avoid handling the pins.

Supply rail checks

QL supply test points (Service Manual, para 2.2.3)

Measurement	Test point	Expected
±12 V regulator input	+ side of D26	15.6 V AC ± 2.0 V
+5 V regulator input	+ side of C41	+9.0 V ± 2.0 V
+12 V regulator output	+ side of C38	+12.0 V DC ± 0.25 V, no discernible ripple
+5 V regulator output	+ side of C42	+5.0 V ± 0.15 V, no discernible ripple
Microdrive logic supply	IC30 (IC29) pins 11, 7, 9	+5 V DC ± 0.25 V, no discernible ripple
Microdrive motor supply	Pin 6 of the ribbon connector	+9 V

Values are quoted from the Service Manual; MDV2 references are in brackets.

Mandatory modifications

Sinclair specified a set of modifications to be checked for, and fitted if absent, on every machine coming in for repair. On an unrestored QL these are worth verifying:

1. R104 (82 Ω) added in the TR1 collector circuit — fitted non-standard, in the collector leg between the transistor and the board.
2. R92 changed from 220 Ω to 390 Ω.
3. R105 (1 kΩ) added across capacitor C19.
4. R106 (1 kΩ) added across diode D17.
5. D22 (1N4148) added in series with R100.
6. D23 (1N4148) added in series with R101.
7. R102, R103 (33 kΩ) added between pins 19 and 21 of IC23 and the −12 V rail; conveniently done by laying the resistors on top of IC23 with a flying lead to pin 1 of IC25.
8. EPROM-fitted machines upgraded to the D11 firmware standard.
9. The MDV2 stand-off spacer fitted.

The R102/R103 pull-ups are the same fix the repair community reaches for when the serial ports will not transmit.

Cleaning

• Case: mild detergent on a damp cloth. The QL's black finish scratches easily.
• Keyboard: clean key surfaces; the bubble mat can be washed and thoroughly dried.
• Board: brush and low-pressure air. Clean socket and connector contacts with contact cleaner on a foam swab.
• Microdrives: see the Sinclair QL Microdrive Troubleshooting Guide for head cleaning and mechanical checks.

Recommended tools

• Pozidriv/Philips screwdrivers and an anti-static strap.
• Temperature-controlled soldering iron with a fine tip, solder wick and/or a vacuum desoldering tool, fresh leaded solder and flux.
• IC extraction and insertion tools.
• Digital multimeter; a two-channel oscilloscope for clock, video and Microdrive signal work.
• Isopropyl alcohol, foam swabs, contact cleaner, an eraser for edge connectors.
• Thermal grease for the +5 V regulator.
• Replacement keyboard membrane and bubble mat.

See Recommended Tools for the general toolkit.

Preventive maintenance checklist

1. Verify the +5 V, +12 V and −12 V rails against the table above.
2. Check the regulator's thermal contact and re-grease it.
3. Clean and reseat every socketed IC and the ROM cartridge edge connector.
4. Inspect the keyboard tails at J11/J12; clean the ends and sockets.
5. Check the mandatory modifications are present.
6. Service the Microdrives (Sinclair QL Microdrive Troubleshooting Guide).
7. Inspect the rear connectors — especially the RGB socket — for cracked joints and lifted pads.

Related pages

• Sinclair QL
• Sinclair QL Troubleshooting Guide
• Sinclair QL Capacitor Replacement Guide
• Sinclair QL Microdrive Troubleshooting Guide
• Sinclair ZX Spectrum Keyboard Membrane Repair — comparable membrane work on the Spectrum
• Capacitor Failure Symptoms · Recommended Tools