Editing Sinclair ZX Spectrum Troubleshooting Guide (section)

== Diagnostic technique ==

Sinclair's own advice is worth repeating: in a machine like this, where everything depends on everything else, the most reliable method is to '''compare waveforms and levels against the same point on a known-good board'''. A careful visual examination first often finds the fault outright — burnt discretes and overheated tracks show up immediately, "as do the attentions of an enthusiastic amateur".<ref name="sm5" />

'''Look for solder splatter, not just dry joints.''' A hairline short from splatter can measure several ohms and produce convincingly misleading symptoms.<ref name="sm5" />

=== Thermal checks ===

After a minute or two, feel the ICs. One package noticeably hotter than its neighbours is a strong suspect. Freeze spray is useful in reverse: if behaviour changes as a chip cools, change that chip. Sinclair's own servicing addendum suggests '''testing at elevated temperature''' to bring out marginal picture quality and RAM faults.<ref name="sm9">''ZX Spectrum Service Manual'', Sinclair Research Ltd, Supplement No. 1: section 9.1 (introduction and the recommendation for the DC-DC converter modification), 9.2 (the modification itself — see the warning below), 9.5 to 9.9 (Issue 4A, 4B, 5 and 6A board descriptions), 9.10 (fault finding servicing addenda), 9.11 (warranty seal) and 9.12.1 (parts-list notes, including the TR3 and TR4 substitution notes).</ref>

=== Substitution and the test clip ===

IC1 is socketed on every board, so the ULA is easy to swap. For the soldered-in devices Sinclair describes making a '''test IC''': clip a known-good part of the right type (4116 or 4532) onto an IC test clip and bridge it across each suspect device in turn. It is not guaranteed to work, but it saves a great deal of unnecessary desoldering.<ref name="sm5" />