Editing Sinclair ZX Spectrum Troubleshooting Guide (section)

== Self-inflicted faults ==

Things that break a working machine if done in the wrong order, or with the wrong part:

* '''TR4 replacement type KTC2236A-Y has a different pinout from every other approved replacement''' — pin 1 is the emitter, pin 2 the collector and pin 3 the base, so base and collector are reversed. Fitting one as if it were a ZTX650 puts the inverter straight into the fault it was replaced for.<ref name="sm9" />
* '''If TR3 is a KSC839, R24 must be 15 K''', not the 1 K value used with other types.<ref name="sm9" />
* '''IC3, IC4, IC25 and IC26 must not be National Semiconductor 74LS157s.''' Sinclair marks this explicitly in every parts list.<ref name="sm4" />
* '''All eight expansion RAM devices must be the same type''', and the board links must match. On an Issue 2 board, link 3 is fitted for 4532-3 parts and link 4 for 4532-4; on Issue 3 two links in the grid between the edge connector and the MIC socket select manufacturer and enable polarity.<ref name="sm4" />
* '''From Issue 4A the ULA must be 6C001-7 or later''', because two spare gates of IC24 were brought into the RASL path to improve timing. An earlier ULA in that board will not behave.<ref name="sm9" />
* '''If you change the ROM to a different manufacturer, move the H/N links.''' They are between the ROM and the heatsink on Issue 1, under the raised part of the heatsink on Issue 2, and next to the loudspeaker on Issue 3.<ref name="sm4" />
* '''Refitting the heatsink carelessly.''' On some Issue 3 boards the heatsink hole allows enough play to foul the edge connector. Fit it away from the connector, and use thermal grease behind the regulator body.<ref name="sm7" />
* '''Cutting the track to IC25 pin 16''' to isolate the expansion RAM is Sinclair's own suggestion, but it is destructive — plan the repair before you make the cut.<ref name="sm5" />