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.

Preventive maintenance on the Sinclair QL follows Section 2 (Disassembly/Assembly) and Section 4 (Fault Diagnosis and Repair) of the Sinclair QL Service Manual, supplemented by repair notes from Tony Firshman of TF Services, a QL repair and upgrade supplier.

The main maintenance items are the keyboard membrane, the +5 V regulator and its heatsink, the socketed ICs and edge connectors, and the Microdrives.

Safety

The QL is powered from an external supply and there is no mains voltage inside the computer. The highest voltage on the board is the 44 V peak-to-peak AC feed from the power pack.

The power pack contains the mains side. Unplug it before opening it. If it is replaced, the substitute must supply both outputs the QL needs: 9 V DC and the AC feed for the ±12 V regulators, which the Service Manual measures as 15.6 V AC ± 2.0 V at the board. Firshman also advises checking and cleaning the mains plug, its fuse holder and terminals, and the power pack connections.

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

Soldering on the QL board

The Service Manual says components should be renewed "using recognised desoldering/heatsinking techniques to prevent damage to the component or to the printed circuit board", with a suitably earthed, low-power soldering iron. It also warns that hairline solder splatter can measure several ohms and produce very misleading fault symptoms, so inspect the board after any rework.

Working rules:

• Use a temperature-controlled iron with a clean, fine tip and keep the time on each joint short.
• Add fresh leaded solder and flux to an old joint before removing a part.
• Support the board flat and do not flex it.
• Clear each hole with wick or a vacuum tool before lifting the part, and lever an IC out only when its solder is fully melted.

Firshman notes that the pins of the expansion connector J1 are very brittle and can be straightened only once, and that the power input socket often has dry joints, especially on machines used with the case off.

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 5/16-inch screws along the front edge under the overhang, and four 1¼-inch screws along the rear edge. Remove all eight.
3. Do not remove the two screws visible on the base immediately below the Microdrives.
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 leads from three LEDs.
5. To separate the halves fully, free the membrane tails from their sockets by pinching each ribbon next to the socket and pulling straight up. Release the six LED leads from the snap-action wire post socket next to the TV modulator can by lifting its black moulding until it resists.

To remove a Microdrive, first take off the heatsink by releasing the screw that holds the +5 V regulator IC35 (keep the M3 × 10 mm screw and its brass plain and crinkle washers). Then remove the three pan-head self-tapping screws holding the drive: two at opposite corners of the chassis and a third from the underside of the lower case. Do not confuse them with the countersunk motor screws.

When reassembling, the Microdrive in the left-hand (MDV1) position has an RF shield over its ULA; make sure it is in place.

Keyboard

The QL keyboard is a membrane under a rubber bubble mat, wired as an 8 × 8 matrix with partial diode isolation and connected to the board by two flex tails at J11 and J12. The Service Manual's fault-finding table gives these remedies:

Keyboard symptoms and remedies (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

A common fault is a break in the membrane. If the break is close to the connector, the ribbon can be removed from the connector, a short section sliced off above the break, and the shortened tail plugged back in; a break elsewhere needs a new membrane. Firshman's advice is to remove the ribbons, clean the tail ends and the sockets, and cut about 5 mm off a damaged tail. Dirty or damaged tails can cause a lock-up in which an on-screen clock keeps running, or a few dead keys.

To reach the membrane, separate the case halves and remove the six 1/4-inch screws holding the keyboard backing plate. Individual keys can be removed for cleaning by holding the key down and prising the retaining sleeve off its underside with a small screwdriver. When refitting, make sure both tails are fully inserted and not kinked.

The +5 V regulator and heatsink

The +5 V supply comes from IC35, a 7805 bolted to the heatsink beside the Microdrives (Table 5.2 of the Service Manual).

• The regulator body must make good contact with the heatsink. When IC35 is renewed, apply thermal grease to the back of the component body.
• Firshman measures +5 V at the centre and left-hand pins of the regulator (viewed from the front of the QL) and renews the 7805 if it reads below about 4.8 V. He also solders a 10 nF capacitor directly across the input (right-hand and centre) pins, makes sure the retaining screw is tight, and checks that the whole heatsink gets hot.
• A 78S05 (2 A) can be fitted in place of the 7805, though Firshman says it hardly seems worth it. He advises against the Qpower switch-mode replacement, which produces RF interference, and against moving the regulator outside the case.

Ventilation openings at the top rear of the case, and a new heatsink, appeared at build standard D04.

Socketed ICs and the ROM cartridge slot

Firshman's notes say dirty socket and connector contacts are very often the cause of crashes on his machines. Take out the socketed ICs, clean the legs and sockets with spray contact cleaner and push them firmly back in, taking anti-static precautions. Clean the ROM socket and the ROM cartridge edge connector as well, the latter with an eraser on the tracks; 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, Section 4, paras 2.2.3 and 2.4.2)

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	IC29 and IC30, pins 11, 7 and 9	+5 V DC ± 0.25 V, no discernible ripple
Microdrive motor supply	Pin 6 of each Microdrive ribbon connector	+9 V

Mandatory modifications

Appendix C of the Service Manual lists modifications to be checked for on every machine coming in for repair, and fitted if absent:

1. R104 (82 Ω) added in the TR1 collector circuit, fitted 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 and R103 (33 kΩ) added between pins 19 and 21 of IC23 and the −12 V rail, most easily 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.

Firshman uses the same two 33 kΩ pull-ups to −12 V when a serial port will not transmit, and identifies modified machines by build standard D14 or later, or by flying leads over IC23.

Cleaning

• Case: a damp cloth with a little mild detergent.
• Keyboard: clean the key surfaces; the Service Manual's remedy for sticky keys is cleaning the key surfaces or renewing the bubble mat.
• Board: a soft brush and low-pressure air. Clean socket and connector contacts with contact cleaner.
• Microdrives: see the Sinclair QL Microdrive Troubleshooting Guide for head cleaning and mechanical checks.

Recommended tools

• Screwdrivers for the self-tapping case screws, and an anti-static strap.
• Temperature-controlled soldering iron with a fine tip, solder wick or a vacuum desoldering tool, leaded solder and flux.
• IC extraction and insertion tools.
• Digital multimeter; an oscilloscope for clock, video and Microdrive signal work.
• Isopropyl alcohol, contact cleaner and an eraser for edge connectors.
• Thermal grease for the +5 V regulator.

See Recommended Tools for the general toolkit.

Maintenance checklist

1. Check the +5 V, +12 V and −12 V rails against the table above.
2. Check the regulator's contact with the heatsink and renew the thermal grease.
3. Clean and reseat every socketed IC and the ROM cartridge edge connector.
4. Inspect the keyboard tails at J11 and J12; clean the ends and sockets.
5. Check that the mandatory modifications are present.
6. Service the Microdrives (Sinclair QL Microdrive Troubleshooting Guide).
7. Inspect the power input socket and the rear connectors for dry or cracked joints.

Related pages

• Sinclair QL
• Sinclair QL Troubleshooting Guide
• Sinclair QL Capacitor Replacement Guide
• Sinclair QL Microdrive Troubleshooting Guide
• Sinclair ZX Spectrum Keyboard Membrane Repair
• Capacitor Failure Symptoms
• Recommended Tools