Sinclair QL RGB Port Repair

Rear of a QL: the power socket, the 8-pin DIN RGB socket J7 with its locating notch at the top, and the UHF socket for the TV lead.

The RGB monitor socket of the Sinclair QL is J7, an 8-pin DIN socket on the rear edge of the main board. Its red, green, blue, composite sync and vertical sync pins are wired straight to the ZX8301 master chip (IC22), with no buffer, transistor or resistor in between, and the same five lines run the length of the board to the expansion connector J1 and to the TV circuits. When a QL gives no picture, or the wrong colours, on an RGB monitor, the fault lies in the lead, the socket's joints, one of those tracks or the ZX8301 output, or in the monitor itself.

The Issue 5 and Issue 6 main boards are the ones Sinclair's Service Manual documents, and the video circuit and its designators are the same on both. Samsung-built boards add diodes piggyback on the ZX8301's RGB pins and are not described in the manual.

Symptoms

What the symptom points to

Symptom	Where to look first
No picture on the RGB monitor; the TV picture is normal	The RGB-only part of the path: the cable, J7 and its joints, the tracks from J7 back to the branch points. See the steps below.
No picture on monitor or TV, power light on	Not an RGB port fault. A completely blank screen is often a faulty ZX8301 (IC22); start with the Sinclair QL Troubleshooting Guide.
One or two colours wrong or missing on the monitor	The red, green or blue line for the missing colour; use the colour test below to tell which.
Picture rolls, tears or will not lock	The composite sync line (J7 pin 4) and the lead's sync wiring. The monitor takes all its sync from the computer.
Picture cuts in and out when the lead is moved	An intermittent connection in the lead, its plug or the J7 solder joints; see the inspection steps below.
No colour monitor output; blurred picture; picture drifting with gradual loss of colour	Sinclair's remedy for each is to renew IC22.
Picture too bright on a SCART television	Series resistors in the lead missing or too small.

A plain black screen on both the monitor and the TV, a white or green screen, or stripes and dots are start-up faults; the Sinclair QL Troubleshooting Guide covers them.

Safety

• There is no mains voltage inside the QL. The external power pack supplies 9 V DC and 44 V peak-to-peak AC to the board.
• Switch the QL and the monitor off before connecting or disconnecting the video lead. Unplugging the RGB connection while the machine is running has been known to damage the ZX8301. Tony Firshman reports trouble with some monitors, Microvitec in particular, when leads are removed, possibly from floating earths blowing the 8301, and switches all equipment on from one switch where possible.
• Take normal anti-static precautions when handling IC22 and IC28, and use an earthed, low-power soldering iron.
• If the monitor itself is suspect, its tube holds a charge after switch-off; follow the CRT Discharge Procedure before opening it.

How the RGB port is wired

The socket pins

Sinclair's diagram of the monitor socket as seen from the rear of the QL, from the 12/84 User Guide. The Service Manual's Appendix A prints the same numbering.

The User Guide (issue 12/84) and Appendix A of the Service Manual give the same pin numbers, functions and levels, and both draw the socket as seen from the rear of the QL. Appendix A says its table replaces the wiring instructions on page 31 of the User Guide's Concepts section; in the 12/84 User Guide the monitor page is page 33 and already carries the same table.

J7 monitor socket, as numbered by the User Guide and Appendix A

Pin	Signal	Level	Comes from	Also on
1	Composite PAL	1 V peak-to-peak into 75 Ω	IC28 (MC1377P) pin 9	UHF modulator M1, through R85
2	Ground		0 V
3	Composite monochrome video	1 V peak-to-peak into 75 Ω	TR9 (ZTX313) emitter, through R88 (75 Ω)
4	Composite sync (CSYNC)	0 to 5 V TTL, active low	IC22 pin 12 (CSYNCL)	J1 pin 13b; IC28 pin 2; R56
5	Vertical sync (VSYNC)	0 to 5 V TTL, active high	IC22 pin 11 (VSYNCH)	J1 pin 15b; IC23 (ZX8302) pin 32
6	Green	0 to 5 V TTL, active high	IC22 pin 31	J1 pin 17b; R49; R58
7	Red	0 to 5 V TTL, active high	IC22 pin 30	J1 pin 12a; R48; R57
8	Blue (centre pin)	0 to 5 V TTL, active high	IC22 pin 32	J1 pin 18b; R50; R59

Pin functions and levels are from the User Guide and Appendix A; the sources of each signal are from the Issue 5 and Issue 6 circuit diagrams and the User Guide's expansion connector table.

The circuit diagrams (Figures 1.4 and 1.5) number J7 differently: they label pin 1 VIDEO, 3 PAL, 4 VSYNCH, 5 CSYNCL, 6 RED and 7 GREEN. That is the mirror image of the User Guide's drawing, with pins 1 and 3, 4 and 5, and 6 and 7 swapped. Richard Cooke, building a QL-to-SCART lead, found the circuit diagram's labels for pins 5, 6 and 7 to be wrong: he found red on pin 7 and green on pin 6, and took composite sync from pin 4, as the User Guide gives them. The Classic Console Upscaler Wiki's SCART table uses a third numbering (composite sync on 3, vertical sync on 4, red 6, blue 7, green 8) and itself warns that several conflicting QL diagrams exist. Follow the User Guide diagram, and confirm each pin with a meter or oscilloscope against the IC22 pins in the table before wiring or condemning anything. A cable plug seen from its pin side is the mirror image of the drawing.

The QL supplies composite sync and a separate vertical sync, but no horizontal-only sync signal. Sinclair's own Vision QL monitor took separate red, green and blue TTL inputs with negative-going composite sync.

The signal path from the ZX8301

The video output section of the Issue 5 circuit diagram (Figure 1.4): J7 at the top, IC28 and TR9 with their resistors in the middle, and the ZX8301 (IC22) with the J1 expansion connector at the bottom. The J7 pin numbers on this drawing are mirrored; use the User Guide numbering.

The ZX8301 produces red, green, blue, CSYNCL and VSYNCH on pins 30, 31, 32, 12 and 11, and they run to J7 to drive a colour monitor. On the circuit diagrams each line goes from its IC22 pin to J7 and to the expansion connector J1 with nothing in series. Nasta, writing on the QL Forum about the 8301, points out that this leaves the chip open to static discharge and short circuits through the monitor connection and to bent and shorted pins on the expansion bus, and that the signals travel the whole length of the main board. The monochrome and composite outputs are buffered and much less exposed.

The TV and composite outputs branch off the same lines:

• Red, green and blue go through the level-setting network R48 to R53 (R48, R49 and R50 are 8.2 kΩ, R51 to R53 are 1 kΩ) and the 22 µF coupling capacitors C23, C24 and C25 to IC28, the MC1377P RGB-to-PAL converter, on pins 3, 4 and 5. Composite sync goes to IC28 pin 2.
• IC28's composite output on pin 9 goes to J7 (composite PAL) and, through R85 (100 Ω), to the UHF modulator M1, with R86 (270 Ω) to ground dividing it down.
• Composite sync, red, green and blue are also mixed through R56 (6.8 kΩ), R57 (11 kΩ), R58 (5.6 kΩ) and R59 (22 kΩ) into the base of TR9 (ZTX313 or MPS2369), with R87 (1 kΩ) to the +5 V supply. TR9's emitter drives J7's composite monochrome pin through R88 (75 Ω).
• VSYNCH also goes to the ZX8302 (IC23), which uses it for the frame interrupt.

Table 5.5 lists C23 to C25 as 22 µF 16 V axial electrolytics, while both circuit diagrams print them as 22 µF 10 V.

This gives the main test for localising the fault. The TV picture is built from the same red, green, blue and sync outputs of IC22, taken off at R48 to R50 and R56 to R59. If the TV shows every colour correctly, IC22 is producing those signals and the copper from IC22 to those resistors is intact; the break is in the copper between that branch and J7, or at J7 itself, or in the cable. The Issue 6 diagram (Figure 1.5) has the same connections and designators in this section.

The same section of the Issue 6 circuit diagram (Figure 1.5), with the same connections and designators. Its J7 pin numbers are mirrored in the same way.

Where the parts are on the board

• 
  Issue 5, J7 area (Figure 5.1): J7 at the rear edge between the modulator M1 and the power socket J8, with R87, R88 and R56 to R59 in a row beside it, TR9 below them, and IC28 with R48 to R55 to the left.
• 
  Issue 6, J7 area (Figure 5.2): the same parts in the same places.
• 
  Issue 5, IC22 area (Figure 5.1): the ZX8301 (IC22) beside the 68008 (IC18), with the expansion connector J1 along the end of the board.
• 
  Issue 6, IC22 area (Figure 5.2).

On both board issues J7 sits at the rear edge, to the right of the UHF modulator M1 and to the left of the power socket J8 as the component layouts are drawn. The resistor row R87, R88, R56, R57, R58 and R59 stands beside it, TR9 is just in front of that row, and IC28 with R48 to R55 is beside the modulator. IC22 is at the other end of the board, next to the 68008 (IC18) and the expansion connector J1. The issue number is printed on the component side near J5 (ISS.5 or ISS.6 on the layout drawings). Urs König records that the issue number is also printed on the solder side under the TV modulator, and that Samsung boards carry their US issue number only there.

The test points are the same pins on both issues: the J7 pins, IC22 pins 30, 31, 32, 12 and 11, and J1 pins 12a, 17b, 18b, 13b and 15b.

Things that look like a dead port

Rule these out before opening the machine.

• The F1 and F2 keys at start-up choose monitor mode (Mode 4) or television mode (Mode 8); they do not switch outputs. Sinclair's own system test runs with a colour monitor and a TV connected together, and the start-up message appears on both screens before either key is pressed.
• A lead wired from the circuit diagram's J7 numbers has red and green swapped and the two sync signals swapped. Check any home-made or second-hand lead pin by pin against the table above.
• A SCART television or a scan converter with 75 Ω inputs needs series resistors in the lead: 680 Ω in each of red, green and blue, and 1.2 kΩ in composite sync to SCART pin 20. Cooke worked these values out to bring the QL's 5 V outputs down to the levels a 75 Ω input expects; with 470 Ω the picture was still slightly too bright, and with 680 Ω the monitor's controls needed no adjustment. Without them the QL may be damaged, and Nasta says that driving the outputs as analogue signals without series resistors severely overloads the ZX8301's video pins.
• A SCART set shows RGB only when pin 16 is held at 1 to 3 V; at 0 to 0.4 V it takes the composite input. The Classic Console Upscaler Wiki feeds pin 16 from the QL's vertical sync through 10 Ω for this. Some older televisions also need a voltage on pin 8 before they switch to the SCART input at all. Cooke used his television's remote control to switch it to the QL's input, and earthed SCART pin 17.
• A monitor that expects separate horizontal and vertical sync has no horizontal-only signal to use, because the QL provides composite sync and vertical sync.
• A composite or monochrome monitor needs only pin 2 and pin 1 (composite PAL) or pin 3 (composite monochrome), not the RGB pins.
• RED, GREEN, BLUE, CSYNCL and VSYNCH are also on the expansion connector J1, so a bent J1 pin or a faulty expansion card is on the same lines. Firshman's first checks are for shorting pins on J1 and for faults that go away when all expansions and ROMs are removed. With a Gold Card fitted and a complete screen failure, he replaces the 8301.
• A flicker near the top left of the screen when windows are redrawn is common to all QLs and all ROM versions.

Tools and parts

• Multimeter with a continuity range, and an oscilloscope with a ×10 probe. Sinclair lists both as minimum test equipment, with a rise time of 0.02 µs/cm for the oscilloscope.
• A television on the UHF output, or a composite monitor, to compare with the RGB monitor.
• Temperature-controlled, earthed soldering iron, solder wick or a vacuum desoldering tool, flux and leaded solder; see the soldering notes in the Sinclair QL Maintenance Guide.
• 30 AWG Kynar wire-wrap wire for track repairs (see broken traces).
• IC extraction and insertion tools for IC22 and IC28, which are plug-in parts.
• Parts, from Table 5.5:
  • J7: 8-pin DIN socket. The manual gives no maker or part number; compare a replacement's pin and fixing-lug layout with the old socket before removing it.
  • IC22: ZX8301 (Plessey/Sinclair). Urs König records two versions, CLA2310 and the later CLA2345.
  • IC28: MC1377P (Motorola). TR9: ZTX313 or MPS2369.
  • R48 to R50: 8.2 kΩ; R51 to R53: 1 kΩ; R56: 6.8 kΩ; R57: 11 kΩ; R58: 5.6 kΩ; R59: 22 kΩ; R85: 100 Ω; R86: 270 Ω; R87: 1 kΩ; R88: 75 Ω; all 5 %, ¼ W. C23 to C25: 22 µF 16 V axial electrolytic.
• For a monitor lead to a 75 Ω RGB input: three 680 Ω resistors and one 1.2 kΩ resistor.

Procedure

Find where the picture is lost

1. Switch off the QL and the monitor. Connect a television to the UHF socket and the monitor to the RGB socket.
2. Switch on. The power light should come on, a coloured memory-test pattern should appear and then the start-up screen with F1 and F2 should appear on both screens.
3. If neither screen shows anything, the fault is not in the RGB port. Follow the Sinclair QL Troubleshooting Guide, starting with the supply checks and IC22.
4. If the TV picture is correct and the monitor is blank or wrong, the fault is in the RGB-only part: the lead, J7, or the copper between J7 and the branch points.
5. Try a composite monitor on pin 1 or pin 3 with pin 2. A picture there shows that IC22 is running and that IC28 or TR9 is reaching J7, so the socket and its ground pin are at least partly sound.
6. Type MODE 8 (a QL started with F2 is already in Mode 8), then fill the screen with each colour in turn: PAPER 0 : CLS, PAPER 1 : CLS and so on up to PAPER 7 : CLS. Compare the monitor with the table below. Sinclair's own colour test checks that the colours on the monitor are the same as those on the TV.
7. Check the lead: continuity from each plug pin to the far end, and no continuity between pins. Try a different lead or monitor if one is available.

Mode 8 colour codes and the lines that make them (User Guide, Concepts, "colour")

Code	Colour in Mode 8	Green	Red	Blue
0	black
1	blue			yes
2	red		yes
3	magenta		yes	yes
4	green	yes
5	cyan	yes		yes
6	yellow	yes	yes
7	white	yes	yes	yes

A missing line removes its component from every colour that uses it. With the blue line open, for example, blue shows as black, magenta as red, cyan as green and white as yellow. If the TV shows all eight colours and the monitor does not, IC22 is driving that colour into the TV circuits and the fault is between the branch point and J7.

Open the QL and lift the board

The J7 joints and the tracks under the board can be seen only with the board out of the case. Sinclair's procedure:

1. Unplug every lead. Turn the QL upside down and remove the 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. Do not remove the two screws on the base immediately below the Microdrives.
2. Hold the case halves together, turn the QL the right way up and lift off the top half. Free the two keyboard membrane tails from their sockets by pinching each tail next to the socket and pulling up, and release the six LED leads from the wire-post socket next to the TV modulator.
3. Remove 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.
4. Remove each Microdrive: two pan-head screws at opposite corners of its chassis and a third from the underside of the lower case. Do not confuse them with the countersunk motor screws. Lift the drive and free its two ribbon cables.
5. Remove the reset button. Release the loudspeaker leads from the wire-post socket and remove the two pan-head screws, with their fibre washers, near the expansion connector and the loudspeaker.
6. Remove the Microdrive extension bung from the lower case and work the board free of its locating dowels by lifting it at the expansion connector end.

A QL with the top case removed: the main board with the UHF modulator can, the black heatsink and the two Microdrives at the right. J7 is out of sight at the rear edge, to the right of the modulator and under the heatsink, so the heatsink and Microdrives have to come off first.

Inspect the socket and the tracks

Sinclair advises a careful look at the board before any measurement: burnt parts and overheated tracks show at once, and hairline solder splatter can measure several ohms and give very misleading symptoms.

1. Under a magnifier, look at the eight J7 pins and its fixing lugs on the solder side for cracked rings, dull or grainy joints and lifted pads. The socket is on the rear edge, where the cable puts its strain on it. Firshman finds the power socket beside it, on the same edge, often has dry joints, especially on machines used with the case off.
2. Follow the five tracks from J7 towards the resistor row R56 to R59, towards R48 to R50 at IC28, and on towards IC22. Look for cracks, scratches, corrosion and solder splashes between them.
3. Check that IC22 is fully seated in its socket. Firshman removes, cleans and firmly reseats the socketed ICs as a routine step.
4. Check the J1 pins for bends and shorts. Firshman warns that they are very brittle and can be straightened only once: ease a bent pin roughly straight with a thin screwdriver, then finish with long-nose pliers.
5. On a Samsung board, look at the diodes fitted piggyback on the ZX8301's RGB pins.

Continuity checks, power off

Measure with the board out and the power pack disconnected. Measure from the J7 pin on the solder side to each point in the row; every point should read as a direct connection.

Continuity from J7 (User Guide numbering) back along each line

Signal	J7 pin	IC22 pin	J1 pin	Branch parts on the same line
Composite sync	4	12	13b	R56; IC28 pin 2
Vertical sync	5	11	15b	IC23 pin 32
Green	6	31	17b	R49; R58
Red	7	30	12a	R48; R57
Blue	8	32	18b	R50; R59
Ground	2	0 V

1. Measure from each J7 signal pin to its IC22 pin. A reading that is open, or that comes and goes when the socket is pressed, is the fault.
2. Where a line is open, move the second probe to the points between: the J1 pin, then the leads of that line's resistors, until continuity stops. The break lies between the last good point and the first bad one.
3. Measure from each J7 pin to its neighbours and to ground. A reading near zero ohms is a short: a solder bridge, a crushed socket, a failed piggyback diode on a Samsung board, or a short inside the ZX8301 (lift IC22 from its socket and measure again to tell which).
4. Repeat for J7 pin 2 to the 0 V plane, and pins 1 and 3 to IC28 pin 9 and to R88.

Signal checks, power on

Sinclair's method is to compare waveforms and levels with the same points on a known-good board where one is available. For powered checks, refit the board in the lower case and run it with the top case off; Sinclair's service centres used extension leads for the keyboard tails J11 and J12 and the front-panel LEDs so that the machine could run with the cover off. The J7 pins cannot be reached from above, so plug in a lead that has passed its own continuity check and probe its free end, or probe the J1 pins and the leads of R56 to R59.

1. Check the +5 V supply at C42: 5.0 V ± 0.15 V with no discernible ripple.
2. Put the oscilloscope on IC22 pins 30, 31, 32, 12 and 11 in turn, then on the matching pins at the end of the lead. Expect 0 to 5 V TTL levels.
3. Composite sync (IC22 pin 12, J7 pin 4) is active low, with a pulse each line; QL lines are 64 µs long, about 15.6 kHz.
4. Vertical sync (IC22 pin 11, J7 pin 5) is active high, with a pulse each frame of 312 lines, about 50 Hz.
5. Red, green and blue change with the picture; fill the screen with a colour that uses the line under test (white uses all three).
6. The composite outputs on pins 1 and 3 should be 1 V peak-to-peak into a 75 Ω load.
7. A signal present at IC22 but absent at J7 confirms a break between them. A signal missing at the IC22 pin itself points to IC22, or to a short on that line pulling it down; see the ZX8301 section below.

Reflow the J7 joints

1. With the board out, add a little fresh leaded solder and flux to each J7 pin and fixing lug, and reheat each joint just long enough to flow. Use an earthed, low-power iron and keep heat and force low so that the pads do not lift.
2. Clean off the flux and inspect the area for solder splatter between the pins and the nearby tracks.
3. Repeat the continuity and short checks.

Replace the J7 socket

1. Compare the new socket with the old one: eight pins in the same pattern as the User Guide diagram, the locating notch at the top, and fixing lugs that fit the board's holes.
2. Desolder each pin and lug with wick or a vacuum tool, using recognised desoldering technique, and lift the socket out only when all the solder has melted.
3. Inspect the pads and the tracks leading from them. Repair any lifted pad or broken track as in the next section before fitting the new socket.
4. Fit the new socket in the old one's position, solder all pins and lugs, and inspect for splatter.
5. Repeat the continuity table from J7 to IC22 and the short checks between pins.

Bridge a broken track

1. Find the open section with the continuity checks above, so that both ends of the break are known.
2. Choose two sound points on the same line either side of the break: the J7 pin, the IC22 socket pin, the J1 pin, or a resistor lead on that line from the table. A link from the J7 pin straight to the IC22 socket pin of the same signal joins the two ends of the line.
3. Where the wire must be soldered to a track, scrape the solder mask back to bright copper. Use 30 AWG Kynar wire-wrap wire, the usual choice for logic-level signals such as these.
4. Run the wire on the solder side of the board and solder it at both ends. Sinclair's own Appendix C modification cuts a track and adds two wire links on Issue 5 boards, with the Issue 6 track cuts on the solder side.
5. Check continuity along the new path, and check that it does not short to the tracks it passes.

Composite outputs: TR9 and IC28

These parts matter to the RGB port only through J7 pins 1 and 3. If the monitor works and those pins do not, follow Sinclair's fault-finding guide: no black-and-white output, renew TR9; no colour TV output, check C35, C36, R85, R86, C29, R54 and the parts around IC28, then renew the modulator M1 or IC28. IC28 is a plug-in part.

When the ZX8301 output itself is missing

1. Remove all expansions and the ROM cartridge. A high level on the DSMCL line, which also reaches J1, disables the 8301.
2. Check for a short on the line before blaming the chip; a short to ground or to another line stops the output reaching the socket.
3. IC22 is socketed. Substitute a known-good ZX8301 with an extraction tool, taking anti-static precautions. To test a suspect chip, fit it in a known-good QL, not a good chip in the faulty one.
4. Sinclair's fault-finding guide renews IC22 for no colour monitor output, a blurred picture, and a picture that drifts with gradual loss of colour in use.
5. Protect the replacement: switch everything off before connecting a lead, and use series resistors on any 75 Ω input.

Reassembly

Reassembly is the reverse of the steps above.

1. Locate the board reset-button end first, then refit the Microdrive extension bung and the loudspeaker leads, and refit the reset button.
2. Make sure the RF shield over the ULA of the left-hand (MDV1) Microdrive is in place. Straighten the bare ends of the Microdrive ribbon cables and push them fully home.
3. Refit the heatsink with good contact between IC35 and the heatsink.
4. Reconnect the keyboard tails, fully inserted and not kinked, and the LED leads, then screw the case halves together.

Testing

1. With the board still out, repeat the continuity table and check that no J7 pin is shorted to its neighbours or to ground.
2. Power up with both the TV and the monitor connected. The start-up screen should appear on both.
3. In Mode 8, fill the screen with each colour code from 0 to 7 and check that the monitor shows the same colours as the TV.
4. Move the lead at the socket while watching the picture; it should not flicker or drop out.
5. If an oscilloscope is available, confirm composite sync on J7 pin 4 and vertical sync on J7 pin 5 at the levels and rates given above.

Revision notes

• Issue 5 and Issue 6: the video circuit, the IC22, J7 and J1 pin numbers and the designators R48 to R59, R85 to R88, C23 to C25, TR9 and IC28 are the same on both circuit diagrams, and Table 5.5 gives the same values for both. The parts sit in the same places on both component layouts. Issue 6 was introduced at build standard D14.
• Samsung boards (US3 to US6) carry diodes piggyback on the ZX8301's RGB pins and are not covered by the Service Manual. Earlier UK issues (ISS.2 and ISS.4) and ISS.7 are not covered either.

Related pages

• Sinclair QL
• Sinclair QL Troubleshooting Guide
• Sinclair QL Maintenance Guide
• Sinclair QL Capacitor Replacement Guide
• Sinclair QL Service Manual
• Battery Explosion, Capacitor or Corrosion Damage