Editing Sinclair QL Troubleshooting Guide (section)

== TV and composite video faults ==

The RGB and sync signals also feed IC28, which produces composite PAL for the internal UHF modulator M1; TR9 mixes the same signals for a composite monochrome output.<ref name="sm" />

{| class="wikitable styled-table" style="width:100%; text-align:left;"
! Symptom !! Action
|-
| No TV picture || Try tuning slightly either side of the expected point — the modulator may have drifted. Renew IC35 if the output is distorted or absent (Service Manual "TV output distorted / No TV output")
|-
| Picture but no colour on TV || Check crystal X3 is oscillating; look for colour modulation on the luminance signal. If chroma and luminance are both present but the TV will not lock, the crystal frequency is wrong; if no chroma at all, the chroma oscillator is not running
|-
| Chroma present, still no TV colour || Changing the value of C31 slightly may restore it; check RGB inputs to IC28, then consider renewing IC28
|-
| No colour TV output || Check C35, C36, R85, R86; check C29, R54 and the components around IC28; renew modulator M1; renew IC28
|-
| Dark TV screen || Renew modulator M1
|-
| No black-and-white TV output || Renew TR9
|-
| Fails colour test / colour fades || Renew crystal X3; check components around IC28; renew IC28
|-
| TV picture drifting continuously || Check C23, C24, C25
|}

To test whether the modulator itself is faulty, remove IC28 and feed the VIDEO signal from TR9 in via R85 — this gives a low-contrast black-and-white picture.<ref name="sm" />