Sinclair QL Voltage Regulator Replacement

Inside a QL: the finned heatsink above the Microdrives carries the +5 V regulator (photo: Cjp24, CC BY-SA 3.0)

The Sinclair QL makes its +5 V logic supply on the main board with a single 7805 regulator, IC35, bolted to the finned heatsink beside the Microdrives. It takes the unregulated 9 V DC from the external power pack. Tony Firshman of TF Services includes the regulator's output and its contact with the heatsink in his checklist for a crashing QL, and replaces the 7805 when its output falls below about 4.8 V.

Symptoms

• The +5 V rail is low. Sinclair's limit at the + side of C42 is 5.0 V ± 0.15 V with no discernible ripple; Firshman renews the regulator below about 4.8 V.
• Crashes or lock-ups, especially after the QL has warmed up. Firshman's checklist for a crashing QL includes the 5 V rail and the regulator's contact with its heatsink.
• Sinclair's fault table gives "TV output distorted" and "no TV output" with the remedy "renew IC35".

Faults that look similar:

• No power at all can be the ±12 V regulators IC37 and IC36, or the power pack, socket or plug. Firshman finds dry joints on the QL's power socket, especially on machines used with the case off. Check the 9 V input at C41 first: if it is outside 7 to 11 V the fault is before the regulator.
• A missing start-up screen with good rails is a logic fault; see the Sinclair QL Troubleshooting Guide.

Safety

There is no mains voltage inside the QL; the highest voltage on the board is the 44 V peak-to-peak AC feed from the power pack. The regulator and heatsink run hot: let them cool before handling. Use an earthed, low-power soldering iron; the QL's pads lift easily.

Tools and parts

• A 7805 regulator in a TO-220 package, the type in Sinclair's parts list. Firshman notes that a 78S05 (2 A) can be fitted but hardly seems worth it, and that it makes no difference to expansions.
• Thermal grease (Firshman uses silicone grease).
• A 10 nF capacitor for the regulator input.
• A cross-head screwdriver, a soldering iron, desoldering braid or pump, and a multimeter.

Firshman advises against the Qpower switch-mode regulator, which he says produces RF interference and does not help expansions, and against mounting the 7805 outside the case.

Procedure

Measuring the rail

Sinclair's Issue 6 layout around the power input: IC35 with its input (I) and output (O) marked, IC36, IC37, C38, C41 and C42

1. Open the case: remove the four 5/16-inch screws along the front edge and the four 1¼-inch screws along the rear edge, but not the two screws below the Microdrives, and lift the top half. It stays connected by the keyboard ribbons and LED leads.
2. Power the QL and measure the regulator input at the + side of C41 (9.0 V ± 2.0 V) and the output at the + side of C42 (5.0 V ± 0.15 V).
3. Firshman measures the 5 V line at the regulator itself, between the centre and left-hand pins viewed from the front of the QL.
4. Feel the heatsink after a few minutes. Firshman checks that the whole heatsink gets hot; if only the regulator is hot, its contact with the heatsink is poor.

Improving the heatsink contact

1. Switch off and unplug. Release the screw that holds IC35 to the heatsink and keep the M3 × 10 mm screw with its brass plain and crinkle washers.
2. Clean the old compound off the regulator and the heatsink, smear thermal grease on the back of the regulator, and refit the screw tightly.

Replacing the regulator

1. Sinclair's heatsink assembly lists a three-pin connector (Molex 4025) with the regulator; check how IC35 is connected on your board before desoldering anything.
2. To take the heatsink off, release the IC35 screw as above.
3. To work on the underside of the board, take it out: remove both Microdrives (three pan-head screws each, two at opposite corners of the drive chassis and one from under the lower case; do not confuse them with the countersunk motor screws), then the reset button, the loudspeaker leads from the wire post socket, the two pan-head screws near the expansion connector and the loudspeaker (¼-inch, with fibre washers) and the push-fit Microdrive extension bung. Ease the board off its locating dowels, lifting it at the expansion connector end.
4. Desolder the old 7805 with a desoldering technique that does not overheat the pads.
5. Fit the new 7805 the same way round: viewed from the front of the QL, Firshman's pin order is output on the left, ground in the centre and input on the right, and the layout marks the input I and the output O.
6. Solder a 10 nF capacitor directly across the input (right-hand) and centre pins.
7. Smear thermal grease on the back of the regulator, refit it to the heatsink with the M3 screw and its washers, and tighten the screw.
8. Inspect the board for solder splashes; Sinclair warns that hairline splatter can measure several ohms and give misleading symptoms.

Reassembly

1. Refit the board reset-button end first, then the screws, the Microdrive extension bung and the loudspeaker leads, and the reset button.
2. Refit the Microdrives. The drive in the left-hand (MDV1) position has an RF shield over its ULA; check that it is in place. Straighten the bare ends of each drive's ribbon cables and push them fully home.
3. Reconnect the keyboard ribbons and LED leads and close the case.

Testing

1. Measure 5.0 V ± 0.15 V at C42 with no discernible ripple.
2. Run the QL for a while and check that the whole heatsink gets hot.
3. Check that the start-up screen appears and the QL no longer crashes.

Expansions

For QLs running memory and disk expansions, Firshman also runs a thicker wire from the 5 V line at the regulator's left-hand pin to the RAM 5 V rail, the thick track running from just above R68 and under IC7.

Revision notes

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

Related pages

• Sinclair QL
• Sinclair QL Maintenance Guide
• Sinclair QL Troubleshooting Guide
• Sinclair QL Capacitor Replacement Guide
• Sinclair QL Service Manual
• Sinclair QL RGB Port Repair
• Sinclair QL Keyboard Membrane Replacement
• Sinclair QL Serial Port Repair
• Sinclair QL RAM Replacement