Editing Sinclair QL Maintenance Guide (section)

== Cooling and the +5 V regulator ==

The QL runs warm, and the 7805 at IC35 does most of the work. Maintenance points:<ref name="sm" /><ref name="tfs" />

* The regulator body must make '''good thermal contact with the heatsink'''. Apply fresh thermal grease when refitting, and make sure the retaining screw is tight.
* A healthy heatsink gets hot all over. If only part of it is warm, the thermal path is poor.
* Check the +5 V rail: measure at the regulator pins. If it is below about 4.8 V, renew the 7805.
* Ventilation slots were added to the case at build standard D04, along with a new heatsink — earlier machines have no top-rear ventilation and run hotter still.<ref name="cowo">König, Urs. [https://www.sinclairql.net/build_standards.html "Sinclair QL Build Standards"], sinclairql.net.</ref>

A 78S05 2 A regulator can be substituted, though it makes little practical difference. Fitting a switch-mode replacement is not recommended: it generates RF interference. Relocating the regulator outside the case is bad practice and causes more problems than it solves.<ref name="tfs" />