Editing Sinclair QL Troubleshooting Guide (section)

== Serial, network, sound, clock and joystick ==

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Service Manual fault-finding guide (remaining areas)'''
! Area !! Symptom !! Action
|-
| RS232-C / Printer || Fails the RS232 loopback test || Check J5, J6; renew IC25, IC26; renew IC23; renew IC24
|-
| Network || Fails the network test || Check J9, 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, C53
|-
| Joystick || Fails the joystick test || Check J3, J4, J2
|-
| Reset || Does not reset || Check the reset switch and its spring
|-
| Display || Erratic display after warm-up || Check the RAMs
|-
| General || Locks up or crashes at random after prolonged use || Renew crystal X2; renew IC23; renew IC34; renew IC24
|}

Source: Service Manual.<ref name="sm" />

'''Serial ports that will not transmit.''' If the +12 V and −12 V rails are good but outgoing RS-232 does not work, fit the two 33 kΩ pull-ups from ZX8302 pins 19 and 21 to the −12 V rail (mandatory modification (g)); this very often cures it. An open network socket — centre pin not making contact with no plug inserted — has also been known to affect the serial ports. Failing that, reseat or replace IC25 (1488), and try different ZX8302/1488 combinations.<ref name="tfs" /><ref name="sm" />

A loopback cable between J5 and J6 makes a good overall system test: RS-232 reception passes through the IPC, the ZX8302 and back onto the processor data bus, so if that works the whole system is close to proven.<ref name="sm" />