Editing Sinclair ZX Spectrum +2 Troubleshooting Guide (section)

== Supply and clock checks ==

The on-board supply derives everything from the external unregulated input:<ref name="sm128" />

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Rails and where they come from'''
! Rail !! Source !! Feeds
|-
| +5 V || 7805 regulator || All logic
|-
| +12 V || TR4/TR5 inverter, rectified and smoothed by D15/C44 || Video output and the RS232 drivers
|-
| −12 V || Charge pump C111/C112 with D28/D29 off the same square wave || RS232 drivers
|-
| −5 V || Derived from the −12 V rail via zener D19 || Expansion port
|}

The expansion connector carries +5 V (pin 3A), pulsed +12 V (23B), +12 V (23A), −5 V (20B), +9 V unregulated (4A) and ground (6A, 7A, 14A).<ref name="sm128" />

{| class="wikitable styled-table" style="width:95%; text-align:left;"
|+'''Clocks — expected oscilloscope readings'''
! Signal !! Test point !! Expected
|-
| Master crystal || IC37 pin 6 || 17.73447 MHz, no jitter
|-
| Z80 clock || IC2 pin 6 || 3.54689 MHz, no jitter
|-
| Colour encoder || IC1 pins 46, 47 || 8.8 MHz, no jitter
|-
| Sound carrier || IC38 pin 4 || Within 2 kHz of 6.0 MHz (UK) or 5.9 MHz (Europe)
|}

Source: 128 Service Manual.<ref name="sm128" /> If the sound-carrier frequency is out of tolerance it can be adjusted per the setting-up instructions.

The external supply should hold up under load: at minimum mains input (215 V AC) and 1.4 A output the '''voltage trough should not fall below 7.0 V''', and at maximum input (265 V AC) and 600 mA the '''peak should stay below 13 V'''.<ref name="sm128" />