Editing Sinclair QL Troubleshooting Guide (section)

== ⚠️ Before you start ==

* '''There are no mains voltages inside the QL.''' The highest voltage on the board is the 44 V peak-to-peak AC feed from the external power pack.<ref name="sm" />
* '''The board is fragile.''' Use a low-power, earthed, temperature-controlled iron and short dwell times; see the soldering rules in the [[Sinclair QL Maintenance Guide]].<ref name="sm" />
* '''Never plug or unplug the RGB monitor lead with the machine powered on.''' See the RGB section below — this has destroyed ZX8301 chips.<ref name="junker">[https://junkerhq.net/xrgb/index.php?title=Sinclair_QL "Sinclair QL"], Classic Console Upscaler Wiki. Source for the ZX8301 fragility warning, the 8-pin DIN pinout, the required series-resistor values and the video timing figures.</ref>
* Hairline solder splatter can measure several ohms and produce very misleading symptoms — inspect the board visually before chasing anything complicated.<ref name="sm" />
* When substituting a suspect component, fit the '''suspect part into a known-good machine''', not the other way round, so a faulty part cannot damage a working one.<ref name="sm" />

[[File:Sinclair QL circuit diagram (Issue 5).png|center|thumb|760px|Sinclair QL main board circuit diagram, Issue 5 (Figure 1.4 of the Service Manual). The Issue 6 diagram and both PCB component-layout drawings are also on the wiki.]]