Sinclair QL Troubleshooting Guide

Fault diagnosis on the Sinclair QL is set out in Section 4 (Fault Diagnosis and Repair) and Appendices A to D of the Sinclair QL Service Manual, which is written at component level. Repair notes from Tony Firshman of TF Services supplement it.

Microdrive faults are covered on the Sinclair QL Microdrive Troubleshooting Guide.

Before you start

• 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 external power pack.
• Use an earthed, low-power soldering iron and recognised desoldering technique; see the soldering notes in the Sinclair QL Maintenance Guide.
• Switch the QL and the monitor off before connecting or disconnecting the video lead. Unplugging the RGB connection while the machine is in use has been known to damage the ZX8301 (IC22).
• Hairline solder splatter can measure several ohms and produce very misleading symptoms. Inspect the board before chasing anything complicated.
• When testing a suspect component by substitution, fit the suspect part into a known-good machine, not the other way round, so a faulty part cannot damage a working unit.

Sinclair QL main board circuit diagram, Issue 5 (Figure 1.4 of the Service Manual). The Issue 6 diagram and both PCB component-layout drawings are also on the wiki.

Normal power-on behaviour

At switch-on the yellow power LED lights and the machine should clear the screen and show, at bottom centre:

F1...MONITOR
F2...TELEVISION
(c) 1983 Sinclair Research Ltd

Reaching this screen means most of the system is working. If it does not appear, switch off and try two or three more times: the QL can lock up on start-up and appear lifeless.

If only 32 KB of the 48 KB ROM is in place, the start-up screen appears with its windows but without the F1, F2 and copyright text.

Reading the start-up screen

The Service Manual reads the power-up screen to locate the fault:

Start-up screen and fault area (Section 4, paras 2.2.5 to 2.2.14)

What you see	What it means	Where to look
Completely blank screen	Often a faulty ZX8301	IC22 is socketed, so substitute it. If there is no spare, check that the CPU clock and the RGB outputs are present
"Tweed" pattern: fine green, white, red and black dots moving quickly down the screen	The RAM test has started, so the ZX8301 works and the ROM is being read and executed	Start at the RAMs
Geometric pattern (for example red and green vertical bars), going no further	The ZX8301 works but the ROM code is not running and RAM is not being overwritten	Missing ZX8301 clock to the CPU, faulty RAM, or an absent CPU
RAM test completes, geometric pattern, then blank	ROM, IPC communication or interrupt problem	ROM fault; ZX8302 to IPC communication (check crystal X4 is oscillating and look for activity on IPC pins 29 and 35); or a continuous interrupt to the CPU
Random flickering patterns with "excited" beeping	Faulty ROM	The ROM; also check the ROM enable signal on ZX8301 pin 33
Plain white screen	First part of the RAM test failed (cannot store data)	RAM, or an address or data line short
Plain green screen	Second part of the RAM test failed (data not retained)	Refresh failure, or a short on an address or data line
Nearly white or green screen with narrow vertical stripes or a pattern of dots	One bit is faulty	One RAM chip or a shorted data or address line; use the probe method below
Tweed pattern after RESET that then reverts to white	A failure in the second bank of RAM	Second RAM bank

The manual notes that a white or green screen shows the RAMs are generally working and the machine is very close to being fully operational.

Pinpointing a faulty RAM chip

Sinclair's method when the screen shows stripes or dots:

1. Connect one end of a probe to ground.
2. Touch the probe on the data pin of each RAM in turn and watch the screen.
3. Shorting a data line puts a vertical black line or lines on the screen. When the black line coincides with the existing dots or stripes, you have found the chip.
4. Because the two RAM banks share the data bus, the fault could be in either bank's device (for example IC1 or IC9). Change the one in the memory-mapped area first: IC1 in that example.

A long vertical stripe suggests a data line short; dots suggest a RAM fault.

For devices that are not socketed, the manual suggests a "test IC": an IC test clip with a known-good device of the same type (a 4164 for RAM) attached, bridged across each suspect chip in turn.

Firshman recommends the Minerva ROM for RAM faults: its memory test flags the first faulty address, identifying the chip. The standard QL memory test does not detect RAM errors where null bits are written.

Dead machine: supply checks first

A missing copyright screen means a fundamental failure. Check the supplies first:

QL supply test points (Section 4, para 2.2.3)

Function	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

The external pack supplies 9 V DC at 2 A and 44 V peak-to-peak AC. It is unregulated apart from a thyristor that limits the DC peaks, and has significant ripple. On the board, IC37 (78L12) and IC36 (79L12) give +12 V at 80 mA and −12 V at 50 mA from the AC feed, and IC35 (7805) regulates the DC feed to +5 V.

• Loss of power: check IC37 and IC36.
• Noisy power supply: change the power pack.
• +5 V low: Firshman renews the 7805 if it reads below about 4.8 V, re-greases it, and checks that the heatsink screw is tight and the whole heatsink gets hot.
• Clean the mains plug, its fuse holder and terminals, and the power pack connections. The QL's power socket often has dry joints, especially on machines used with the case off.

Signals to check on a dead board

If the supplies are good, compare these signals with a known-good board (Section 4, para 2.2.16):

Signals to compare with a known-good board

Device	Pins and signals
ZX8302 (IC23)	32 VSYNCH; 28 RESETOUTL; 10 PCENL; 30 and 31 XTAL; 8 DCSML; 25 CLOCK
68008 (IC18)	31 DTACK; 30 R/W; 29 DS; D0–D7 data lines; A0–A17 address lines
ZX8301 (IC22)	38 VDA; 36 TXOEL; 37 ROWL; 4 RDWL; 33 ROMOEH; 12 CSYNCL; 9 CAS0L; 10 CAS1L; 8 RASL; 40 WEL; 30 RED; 31 GREEN; 32 BLUE; 7 CLOCK

The para 2.2.16 list prints DTACK as pin 21 of IC18; paragraph 2.2.15 and the Figure 1.4 circuit diagram both give pin 31.

If the machine has not started the RAM test at all, check for a regular DTACK signal on CPU pin 31; without it the RAM test can never start. Check for a short on the DTACK line, and check the ZX8301's DSMCL signal: a high level disables the ZX8301.

If the fault persists, change the socketed ICs in the order IC18, IC22, IC23, IC24 and IC28, powering up after each change.

RGB monitor output

The ZX8301 generates red, green, blue, composite sync (CSYNCL) and vertical sync (VSYNCH), which run to the 8-pin DIN socket J7. Certain issues of the ZX8301 are fragile, and unplugging the RGB lead while the machine is in use has been known to damage it. Firshman reports that some monitors, Microvitec in particular, can cause trouble if leads are removed, perhaps through floating earths blowing the ZX8301; he switches all equipment on from one switch where possible. Samsung-built QLs have diodes fitted piggyback on the ZX8301's RGB output pins.

The Service Manual's fault-finding guide gives these display remedies for the monitor path:

Symptom	Remedy
No colour monitor output	Renew IC22
Blurred picture	Renew IC22
TV or monitor picture drifting, with gradual loss of colour in use	Renew IC22
Erratic display after a warm-up period	Check the RAMs

If the picture cuts in and out when the lead is moved, reflow the J7 joints and check the pads and tracks from J7 back towards IC22 before replacing the chip.

The monitor socket pinout

Appendix A of the Service Manual gives Sinclair's revised wiring for the monitor socket, which it says replaces the instructions on page 31 of the Concepts section of the QL User Guide:

QL monitor socket (Service Manual, Appendix A)

Pin	Signal	Level
1	Composite PAL	1 V peak-to-peak into 75 Ω
2	Ground
3	Composite monochrome video	1 V peak-to-peak into 75 Ω
4	Composite sync	0 to 5 V TTL, active low
5	Vertical sync	0 to 5 V TTL, active high
6	Green	0 to 5 V TTL, active high
7	Red	0 to 5 V TTL, active high
8	Blue	0 to 5 V TTL, active high

The Classic Console Upscaler Wiki's QL-to-SCART table numbers the pins from the User Guide's diagram instead (composite sync on 3, vertical sync on 4, red 6, blue 7, green 8) and warns that several conflicting QL diagrams exist. Identify each signal with an oscilloscope before wiring a lead. The same source gives the series resistors needed to drive a 75 Ω RGB input: 680 Ω in each of red, green and blue, 1.2 kΩ in composite sync and 10 Ω in vertical sync. Driving a 75 Ω input directly from the TTL outputs, without them, can damage the QL.

TV and composite video faults

The RGB and sync signals also feed IC28 (MC1377P), which produces composite PAL for the internal UHF modulator M1, and are mixed in TR9 for a composite monochrome output.

TV faults (Section 4, para 2.5 and fault-finding guide)

Symptom	Action
No TV picture	Tune the set slightly either side of the expected point; the modulator may have drifted
TV output distorted, or no TV output	Renew IC35
Picture but no colour on the TV	Check that crystal X3 is oscillating and look for colour modulation on the luminance signal. If chroma and luminance are both present but the TV will not lock, the crystal frequency is wrong; if there is no chroma, the chroma oscillator is not running
Chroma present, still no TV colour	Changing the value of C31 slightly may restore it; check the RGB inputs to IC28, then consider renewing IC28
No colour TV output	Check C35, C36, R85 and R86; check C29, R54 and the components around IC28; renew modulator M1; renew IC28
Dark TV screen	Renew modulator M1
No black-and-white TV output	Renew TR9
Fails colour test	Renew crystal X3
Colour loss or fading	Check the components around IC28; renew IC28
TV picture drifting continuously	Check C23, C24 and C25

To test the modulator, remove IC28 and feed the VIDEO signal from TR9 in via R85. This gives a low-contrast black-and-white picture.

Keyboard faults

The keyboard is an 8 × 8 matrix with partial diode isolation, on a membrane connected at J11 and J12.

The Service Manual says apparent major keyboard problems can come from the ZX8302 or the IPC, but are more commonly on the membrane or the connectors. If a whole row or column is dead, suspect a connector. If a pair of keys misbehaves, suspect a short between them; two dead rows also suggest a short. Check with an ohmmeter.

To watch the scan, trigger a two-channel oscilloscope from one keyboard output (for example KB01) and check that the other outputs go high at successive intervals. A short shows clearly as disturbed scan timing.

A common fault is a break in the membrane. A break near the connector can be cured by trimming the tail; elsewhere the membrane must be replaced. If the QL locks up while an on-screen clock keeps running, or a few keys do not work, Firshman suspects the keyboard ribbons: clean the tail ends and the sockets, and cut about 5 mm off a damaged tail. The symptom-to-remedy table for membrane, bubble mat and ICs is in the Sinclair QL Maintenance Guide.

Serial, network, sound, clock and joystick

Service Manual fault-finding guide (other areas)

Area	Symptom	Action
RS232-C / printer	Fails the RS232 loopback test	Check J5 and J6; renew IC25 and IC26; renew IC23; renew IC24
Network	Fails the network test	Check J9 and J10; check TR2 and its associated components
Sound	No sound	Check TR1; check R104 (post-Issue 6); renew IC24
Real-time clock	Fails the real-time clock test	Check R91, R80, C1 and C53
Joystick	Fails the joystick test	Check J3, J4 and J2
Reset	Does not reset	Check the reset switch and its spring
General	System locks up after prolonged use	Renew crystal X2
General	Crashes at random intervals or after prolonged use	Renew IC23; renew IC34; renew IC24

If the +12 V and −12 V rails are good but the serial ports will not transmit, Firshman fits the two 33 kΩ pull-ups from ZX8302 pins 19 and 21 to the −12 V rail (mandatory modification (g) in Appendix C), which he says very often cures it. He has also known an open network socket, its centre pin not making contact with no plug inserted, to affect the serial ports. Failing that, replace IC25 (1488) and try different ZX8302 and 1488 combinations.

A loopback cable between J5 and J6 is a good overall system test: received RS-232 data passes through the IPC and the ZX8302 back onto the processor data bus, so if it works the whole system is very likely to be working.

The IPC and the Hermes replacement

The 8049 IPC (IC24) handles sound, serial input and keyboard input, and carries custom Sinclair firmware. TF Services makes Hermes, a replacement IC that plugs into the 8049's socket next to Microdrive 1. It corrects faults in the original IPC code, including poor serial handshaking that can lose characters or cause a "serial overrun" curable only by switching off, key bounce, and the inability to receive at 19200 baud.

Random crashes and lock-ups

Firshman's checklist for a crashing QL:

1. Remove all expansions, external ROMs and upgrades and see whether the fault persists.
2. Remove, clean and firmly reseat every socketed IC, and clean the ROM socket and the ROM cartridge edge connector (the latter with an eraser on the tracks). He calls this very often the cause of crashing on his machines.
3. Check the +5 V rail and the regulator's contact with its heatsink.
4. Check the machine is to the latest build standard (D14 or later, or flying leads over IC23).
5. If expansions are marginal, run a thicker wire from the 5 V line at the regulator to the RAM 5 V rail (the thick track running from just above R68 and under IC7).
6. Make sure the expansion connector pins are straight and the socket is clean.
7. Switch all equipment on from one switch where possible.

Where a second working QL is available, swap all the socketed ICs across and then replace them one at a time.

Repair notes

The Service Manual's repair notes:

• Use recognised desoldering and heatsinking technique and a suitably earthed, low-power iron.
• Make good contact between the IC35 regulator body and its heatsink, and apply thermal grease when renewing it.
• Renew the modulator as a complete unit.
• Use proper IC removal and insertion tools, and avoid handling the pins.
• When replacing a keyboard membrane, make sure the ribbons are fully inserted and not kinked.

Quick reference

Symptom	First check
Power light on, no display	Supply rails, then IC22, IC24, the RAMs, IC34, IC18, IC33 (the manual's order for a black screen)
Blank screen	ZX8301 (IC22)
White or green screen	RAM fault or an address or data line short; use the probe method
Stripes or dots on a white or green screen	A single RAM bit; probe each RAM data pin
Flickering patterns with beeping	ROM
Start-up screen with empty windows	Only 32 KB of ROM present
No colour monitor output	ZX8301 (IC22); the J7 joints
Lock-ups with an on-screen clock still running, or dead keys	Keyboard ribbons and membrane
No outgoing RS-232	33 kΩ pull-ups to −12 V on ZX8302 pins 19 and 21
Random crashes	IC sockets and the ROM edge connector; the +5 V rail
Microdrive problems	See the Sinclair QL Microdrive Troubleshooting Guide

Related pages

• Sinclair QL
• Sinclair QL Maintenance Guide
• Sinclair QL Capacitor Replacement Guide
• Sinclair QL Microdrive Troubleshooting Guide
• Sinclair ZX Spectrum Troubleshooting Guide