ZX Spectrum - Power & Transistor fault finding with my new Diag ROM

Описание: First, 9V is the nominal supply voltage under the normal load that
the Spectrum circuit imposes.

If this is not present it might be the most fortunate and easy fix:
Probably the voltage regulator (7805 RG) is damaged.
Consider replacing it with a modern DC to DC Converter (e.g., Traco Power),
as the most sophisticated solution without the need for a heatsink and all.
Second thing to look at, is the continuity of the COIL (in case it has failed)

Next fault includes all the rest of the values present in the ZX Spectrum
and involves the inverter circuits, the power transistors and the lower RAM.

The 4116 DRAM ICs need three power supplies: +12V (VDD),
+5V (VCC), and -5V (VBB), plus ground (VSS)

The 5V supply rail feeds the CPU, ULA, glue logic, ROM, and upper RAM.
Lower RAM memory, also uses -5V, so this must be generated as well.

Then, the video encoder chip, LM1889, uses a supply voltage of 12V.
Inverter circuit present: TR5, TR4 and the coil.
Extremely prone to failure...

If 12v. Rail faulty: TR4 & TR5 fried (?)
Lower RAM comes next...

-5v absent from 4116 (lower ram): ICs fried (?).

ZTX651 Transistor: Can replace ZTX650 (TR4)
Can replace C2236A on revision 4S boards, but you must fit the ZTX651
with other orientation.
ZTX751 Transistor: Replaces ZTX213 in TR5 position (and A733 on 4S boards