Editing Sinclair QL Troubleshooting Guide (section)

== Dead machine: supply checks first ==

A missing copyright screen means a fundamental failure. Check the supplies before anything else:<ref name="sm" />

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''QL supply test points (Service Manual 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 into J8; it is unregulated apart from a thyristor limiting the DC peaks, and has significant ripple by design.<ref name="sm" /> On the board, IC37 (78L12) and IC36 (79L12) derive +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.<ref name="sm" />

* '''Loss of power''' — check IC37 and IC36.<ref name="sm" />
* '''+5 V low''' — if below about 4.8 V, renew the 7805. Re-grease it and make sure the heatsink screw is tight and the whole heatsink gets hot.<ref name="tfs" />
* '''Noisy PSU''' — change the power pack.<ref name="sm" />
* Clean the mains plug, its fuse holder and the QL's power socket. The power socket often develops dry joints, particularly on machines used with the case off.<ref name="tfs" />