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Sinclair QL Serial Port Repair

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Sinclair QL rear ports
Rear of a QL. The two serial sockets, SER1 and SER2, sit with the two joystick sockets between the UHF socket and the ROM slot (photo: Ewx, CC BY-SA 3.0)

The Sinclair QL has two RS-232 serial ports, SER1 and SER2, on BT-style sockets J5 and J6 at the rear of the main board. The two halves of the link are handled by different chips: the ZX8302 peripheral controller (IC23) sends data out through the 1488 line driver (IC25), and incoming data passes through the 1489A line receiver (IC26) to the 8049 intelligent peripheral controller, or IPC (IC24).[1][2] Tony Firshman of TF Services finds that when the ±12 V supplies are good the usual fault is that nothing comes out of the ports.[3]

The checks below run along the signal path, from the ZX8302 out to the sockets and back in to the IPC. Faults on the RGB monitor socket are covered in Sinclair QL RGB Port Repair.

Symptoms

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What the symptom points to
Symptom Where to look first
Nothing is sent from SER1 or SER2; the ±12 V supplies are good The 33 kΩ pull-ups on IC23, then IC25, then the ZX8302 itself[3]
The QL fails a loopback test between SER1 and SER2 J5 and J6, then IC25 and IC26, then IC23, then IC24[4]
The ports work, but a device that takes its power from the port (a serial-to-parallel converter, for example) does not Pin 1 (ground) or pin 6 (+12 V) of the socket not soldered to the board[5]
The printer prints about a page and then stops The printer lead: many printers need DTR on pin 20, not pin 4[5]
Data from SER1 is muddled after SER2 has been used A ROM bug, not a fault: once opened, SER2 cannot be closed properly. Unplug the SER2 lead when it is not in use[5]
Serial faults, and the centre contacts of the network sockets do not close when the plug is out Open network sockets J9 and J10[3][4]

How the ports work

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RS-232 signal path
Stage Part Notes
Transmit ZX8302, IC23 Sends the outgoing serial data.[1]
Line driver 1488, IC25 Turns the logic-level data into RS-232 levels; output on pin 6. Its supply pins are pin 14 (+12 V), pin 1 (−12 V) and pin 7 (ground).[1][6]
Sockets J5 and J6 BT-type sockets. Pin 1 is ground and pin 6 carries +12 V.[2][5]
Line receiver 1489A, IC26 Turns incoming RS-232 levels back to logic levels; output on pin 11.[1][7]
Buffer 74LS03 NAND gate IC27 (Issue 5) or HAL IC38 (Issue 6) IC27 was deleted on the Issue 6 board, and Firshman notes that the incoming data passes through the HAL in its place.[8][3]
Receive 8049 IPC, IC24 Takes the data in on pins 6 and 21, buffers it and passes it on to the main processor through the ZX8302.[1]
Supplies IC37 (78L12) and IC36 (79L12) +12 V at 80 mA and −12 V at 50 mA, made from the 44 V peak-to-peak AC feed from the power pack.[9]

Safety

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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.[9] Use an earthed, low-power soldering iron, and take care not to overheat the pads when desoldering.[10] If you remove a chip with hot air, keep the heat away from plastic parts and lever the chip out only when all of its solder has melted.[3]

Tools and parts

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  • A multimeter.
  • Two 33 kΩ resistors and a short length of insulated wire, if the pull-up modification is missing.[11]
  • A 1488 quad line driver and a 14-pin socket for IC25, and a 1489A quad line receiver for IC26.[2][3]
  • A soldering iron, desoldering braid or pump, or a hot-air station.[10][3]
  • A cross-head screwdriver.
  • A loopback cable for SER1 and SER2.[12]

Procedure

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Checking the build standard

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  1. Read the serial number on the label under the QL. Its prefix is the build standard: D6 to D13 is an Issue 5 board, and 14 onwards an Issue 6 board.[8]
  2. Look through the SER1 socket for R102 and R103. If they are there, Sinclair says you can assume the other mandatory modifications have been made; boards at build state D12 or higher had them from the factory.[8] Firshman's equivalent check is build standard D14 or later, or flying leads over IC23.[3]

Checking the supplies

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Sinclair QL Issue 6 layout, serial port area
Sinclair's Issue 6 layout around the serial port: the ZX8302 (IC23), the 1489A (IC26), the 8049 IPC (IC24) and socket J6, with the edge of the 1488 (IC25) at the left
  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.[13]
  2. Power the QL and measure the AC feed at the + side of D26 (15.6 V AC ± 2.0 V) and the +12 V output at the + side of C38 (12.0 V ± 0.25 V).[9]
  3. Measure the −12 V rail at pin 1 of IC25, the line driver's negative supply pin.[11][6]
  4. If either 12 V rail is missing, Sinclair's remedy for loss of power is to check IC37 and IC36.[4] Fix the supplies before going further; Firshman only looks at the serial chips once both 12 V lines are good.[3]
  5. Switch off. Connect a multimeter on its current range between pins 1 and 6 of each SER socket and check that 15 mA to 20 mA flows. If a socket gives nothing, inspect its pins 1 and 6 for missing or cracked solder joints.[5]

Fitting the pull-up resistors

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Firshman calls this "illogical" but says it "very often" cures a QL that sends nothing.[3]

  1. On a board without R102 and R103, solder one 33 kΩ resistor to pin 19 of IC23 and the other to pin 21, laying them on top of the chip.[11]
  2. Join the free ends of the two resistors and run an insulated flying lead from that joint to pin 1 of IC25, the −12 V rail.[11]
  3. Sinclair's parts list gives R102 and R103 as 33 kΩ on Issue 5 boards but as zero-ohm links on Issue 6 boards. Do not add the modification to an Issue 6 board until you have checked what is fitted in those positions.[2]

Checking the network sockets

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  1. With no plugs in the network sockets J9 and J10, check that their centre contacts close. Firshman has known open network sockets to upset the serial ports.[3][4]

Replacing the line driver

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  1. If the port still sends nothing, check the logic signal from the ZX8302 at pin 2 of IC25, the driver's first input. Firshman finds that some 8302s put less than 2 V on this pin, where he has been told to expect about 2.5 V.[3][6]
  2. To work on the underside of the board, take it out. Release the screw that holds the regulator IC35 to the heatsink and lift the heatsink away, keeping the screw and its washers. Remove both Microdrives (three pan-head screws each; do not confuse them with the countersunk motor screws), then the reset button, the loudspeaker leads, the two pan-head screws near the expansion connector and the loudspeaker, and the push-fit Microdrive extension bung, then ease the board off its locating dowels, lifting it at the expansion connector end.[13]
  3. Desolder IC25 without overheating the pads, or remove it with hot air and lever it out only once all the solder has melted.[10][3]
  4. Fit a 14-pin socket and a new 1488.[3] Check that the notch faces the same way as the original.
  5. Inspect the board for solder splashes before refitting it.[10]
  6. If the port still fails, Firshman tries different combinations of 8302 and 1488; in Sinclair's fault table the next parts after IC25 and IC26 are IC23 and then IC24.[3][4]

Issue 6 boards: the HAL

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  1. On Issue 6 boards Firshman's advice is to socket the HAL, IC38, and swap it as well, because incoming serial data passes through it. His note on the HAL ends: "YES - the official circuit diagram IS wrong!"[3]

Reassembly

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  1. Refit the board reset-button end first, then the screws, the Microdrive extension bung, the loudspeaker leads and the reset button.[13]
  2. Refit the heatsink and the IC35 screw with its washers.[13]
  3. 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.[13]
  4. Reconnect the keyboard ribbons and LED leads and close the case.[13]

Testing

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  1. Connect a loopback cable between SER1 and SER2 and send data from one port to the other. Sinclair notes that because received data passes through the IPC and the ZX8302 to the processor, a QL that passes this test is "close to a guarantee" of being fully functional.[12]
  2. Check 15 mA to 20 mA between pins 1 and 6 of each socket.[5]
  3. Print more than one page to a serial printer. If it stops after about a page, check that the lead's DTR wire goes to pin 20 at the printer end.[5]

Revision notes

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  • Issue 5 boards (build standards D6 to D13) buffer the received data through IC27, a 74LS03; Issue 6 boards (build standard 14 onwards) use the HAL IC38 instead.[8][2]
  • R102 and R103 are 33 kΩ on Issue 5 and zero-ohm links on Issue 6 in Sinclair's parts list, although the text says the resistors were added at Issue 6.[2][8]
  • The SER2 channel cannot be closed properly once it has been opened, on ROMs AH, JM and JS.[5]
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 Sinclair QL Service Manual (File:Sinclair QL Service Manual.pdf), Sinclair Research, prepared by Thorn (EMI) Datatech, issue 3, October 1985, Section 1 paras 4.6 to 4.11 "RS232 Link": IC24 handles the receive side and IC23 the transmit side; J5 and J6 are the two RS-232 connectors, J6 wired so the attached device acts as the DTE and J5 so the QL does; data is sent by IC23 via driver IC25 pin 6 and received by line receiver IC26 pin 11, then passes through NAND gate IC27 to IC24 pins 6 and 21; IC24 buffers up to about 20 bytes per channel.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Sinclair QL Service Manual, Section 5, parts lists: IC25 1488, IC26 1489A, IC27 74LS03 (Issue 5 only), IC36 79L12, IC37 78L12, IC38 HAL16L8 (Issue 6), J5 and J6 "603A RH", BICC, BT type; R102 and R103 33 kΩ on Issue 5 and "ZERO Ω" on Issue 6, R104 82 Ω on both, all marked as mandatory modifications.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 3.14 3.15 Tony Firshman, "QL Troubleshooting", TF Services, last updated 28 October 2003, "Serial ports": lack of outgoing RS-232 when the ±12 V lines are good; the two 33 kΩ pull-ups from the ZX8302's pins 19 and 21 to −12 V; open network sockets; replacing IC25 with a socket, by hot air or a carefully used paint stripper, levering the chip off only when the solder has fully melted; trying different 8302 and 1488 combinations, and 8302s that put less than 2 V on pin 2 of the 1488 where about 2.5 V is expected; socketing and swapping the HAL on Issue 6 boards. Also the build-standard check (D14 or later, or flying leads over IC23).
  4. ↑ 4.0 4.1 4.2 4.3 4.4 Sinclair QL Service Manual, Section 4, fault-finding guide, "RS232-C/Printer": fails RS232 loopback test, check J5, J6, renew IC25, IC26, renew IC23, renew IC24; "Network": check J9, J10; "Loss of power": check IC37, IC36.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 Sinclair QL Service Manual, Section 4, Appendix A, "Printing problems" and "SER outputs": printers that stop after about a page because the lead's DTR wire goes to pin 4 instead of pin 20; a SER socket whose pin 1 (ground) or pin 6 (+12 V) is not soldered still passes data but cannot power a device; test by connecting pins 1 and 6 and checking that 15 mA to 20 mA flows. Also the SER2 channel that cannot be closed properly once opened, on ROMs AH, JM and JS.
  6. ↑ 6.0 6.1 6.2 Texas Instruments, MC1488, SN55188, SN75188 Quadruple Line Drivers datasheet: pin 1 VCC−, pin 2 input 1A, pin 7 GND, pin 14 VCC+.
  7. ↑ Texas Instruments, MC1489, MC1489A, SN75189, SN75189A Quadruple Line Receivers datasheet: pin 7 GND, pin 14 VCC.
  8. ↑ 8.0 8.1 8.2 8.3 8.4 Sinclair QL Service Manual, Section 4 para 1.2.2: the Issue 6 board removed IC17 and IC27, added HAL IC38, added R102, R103 and R104, and replaced TC1 with a fixed 22 pF capacitor C53. Appendix C para 1.1: look for R102 and R103 through the SER1 socket to check the mandatory modifications; build state D12 or higher already has them. Section 5 para 1.1: the build standard is the prefix of the serial number on the label under the QL, for example D13; Issue 5 covers build standards D6 to D13 and Issue 6 build standard 14 onwards.
  9. ↑ 9.0 9.1 9.2 Sinclair QL Service Manual, Section 1 para 8 "Power Supplies": the AC input is applied to IC37 and IC36 to produce +12 V at 80 mA and −12 V at 50 mA; all three regulators have thermal and short-circuit shutdown. Section 4 para 2.2.3, supply test points: ±12 V regulator input at the + side of D26, 15.6 V AC ± 2.0 V; +12 V output at the + side of C38, +12.0 V ± 0.25 V.
  10. ↑ 10.0 10.1 10.2 10.3 Sinclair QL Service Manual, Section 4 para 3.1 repair notes: desoldering and heatsinking technique, an earthed low-power iron, and solder splashes that can measure several ohms and give misleading symptoms.
  11. ↑ 11.0 11.1 11.2 11.3 Sinclair QL Service Manual, Section 4 para 1.3 "Mandatory Modifications", item (g): add R102 and R103 (33 kΩ) between pins 19 and 21 of IC23 and VM12, the −12 V rail; lay the resistors on top of IC23 and run a flying lead from their common connection to pin 1 (VM12) of IC25.
  12. ↑ 12.0 12.1 Sinclair QL Service Manual, Section 1 para 9.1: a loopback cable between J5 and J6, with the two ports made to talk to each other, tests the path through the IPC and the ZX8302 to the processor; Section 3 para 3.5, the RS232C loopback test of Sinclair's System 2 test software, with the cable in SER1 and SER2.
  13. ↑ 13.0 13.1 13.2 13.3 13.4 13.5 Sinclair QL Service Manual, Section 2 "Disassembly/Assembly": paras 1.1 (case), 1.2 (heatsink and Microdrives), 1.3 (circuit board) and 2.3 to 2.4 (assembly).


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