Editing Sinclair ZX Spectrum+ Troubleshooting Guide (section)

== Supply checks ==

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Basic checks, in order of priority (Service Manual)'''
! Function !! Test point !! Expected
|-
| Voltage regulator input || +ve side of C50 || +9 V DC ± 2.0 V. Below +7 V the regulator may not operate correctly
|-
| Voltage regulator output || +ve side of C34 || +5 V DC ± 0.25 V, no discernible ripple
|-
| On-board +5 V || IC6 pin 9 || +5 V DC ± 0.25 V, no discernible ripple
|-
| On-board −5 V || IC6 pin 1 || −5 V DC
|-
| On-board +12 V || IC6 pin 8 || +12 V DC
|-
| +12 VA (to IC14 only) || Left end of C52 || +12 V DC
|-
| Clock || IC1 pin 32, TR3 base, IC2 pin 6 || 14 MHz square wave at +5 V amplitude
|-
| Address and data lines || Right side of R17–R23 and R1–R8 || 3.5 V amplitude
|}

Source: Service Manual.<ref name="sm48" /> If these pass, check IC1 pin 14, IC2 pin 11 and IC5 pin 28 for +5 V, and ground at IC1 pin 40, IC2 pin 29 and IC5 pin 14. If all is well and the ULA is socketed, replace IC1.<ref name="sm48" />

=== The community multimeter-only sequence ===

Ian Worsley's field guide gives a diagnostic path needing only a multimeter on the 20 V DC and 20 kΩ ranges, with the negative lead on the modulator case:<ref name="ian" />

# '''Power up briefly and measure the 7805 output''' (right-hand leg, tab away from you). '''4.85–5.2 V''' is normal.
#* Higher than that — change the regulator, and hope not too many chips have been taken out.
#* Lower, but not below 4.3 V — there is a short somewhere. Feel the chips: '''the ULA runs quite warm, everything else should run cold.'''
#* Missing altogether — replace the regulator, but do not run the machine yet.
# Switch to 20 kΩ. Measure '''pin 1 of IC6''': it should read '''above 12 kΩ'''. If lower, nick the track to the right of C4 (above IC6).
# Measure '''pin 8 of IC6''': if below 700 Ω, nick the track to the left of C44 (100 µF 16 V, bottom right of IC6).
# Leave the tracks cut, power up, and measure on 20 V DC to the right of each cut: the first should read '''−4.5 to −5.2 V''', the second '''+11.5 to +12.3 V'''.

This isolates the DC-DC converter from the RAM it feeds, which is the crux of most Spectrum faults.