Editing Sinclair ZX Spectrum+ Troubleshooting Guide (section)

== The DC-DC converter — the classic Spectrum fault ==

The 4116 lower RAM needs −5 V and +12 V, generated on-board by an inverter around '''TR4 (ZTX650)''' and '''TR5 (ZTX213)'''. When it fails, the lower RAM has no supply and the machine shows a white or patterned screen.<ref name="ian" /><ref name="sm48" />

'''If −5 V or +12 V is missing or low:'''<ref name="ian" />

* Replace '''TR4 (ZTX650)''' and '''TR5 (ZTX213)'''. Do not substitute other transistor types here — they will not work. (A ZTX651 is an acceptable TR4 substitute, being the same part with a higher voltage rating.)<ref name="tyne">Tynemouth Software, [http://blog.tynemouthsoftware.co.uk/2017/06/spectrum-128k-2-grey-repairs.html "Spectrum 128K +2 Grey Repairs"]. Source for the ZTX651 substitution, RAM IC mapping between 48K and +2 boards, typical +2 current draw and the use of diagnostic test ROMs.</ref>
* Check '''R79 (2k2, Issue 6)''', '''R62 (15 Ω, Issue 6)''' or '''R55 (56 Ω, Issue 4 and earlier)''' for discolouration. If a 56 Ω resistor is badly discoloured, change '''D15 (BA157)'''.
* Check the zener and signal diodes: on '''Issue 6/6A''' these are '''D19 (BZY88 5.1 V)''', '''D18''' and '''D11''' (1N4148); on '''Issue 3 and 2''', '''D16 (BZY88 5.1 V)''', '''D11''' and '''D12'''.
* There is roughly a '''20 % chance the coil has shorted turns'''. With the converter working you should be able to '''hear the coil buzzing'''.

The Service Manual's Repair Data adds an important warning: '''TR4, TR5 and the coil often fail together'''. If you replace only TR4 it can fail again immediately because the others are still faulty — check all three.<ref name="sm48" />

'''No coil buzz and the 7805 open circuit''' — before replacing the regulator, check the heatsink for damage or bad fitting, remedy any physical defect in the print or damage around the screws, and check TR4/TR5 and the coil.<ref name="sm48" />

'''Continual failure of power supply transistors''' — verify all modifications are present and the edge connector is sound and not loose or damaged; check the peripherals are properly fitted and establish whether the owner is hot-plugging add-ons. Check the plug, socket and cable, and check the mains for interference from other equipment. If all that is satisfied, confirm 0 V is correct on earth and monitor it while running to catch an intermittent, then work systematically through the capacitors and resistors to earth.<ref name="sm48" />