Erase function not working.
1. Check TR3, D28.
2. Renew IC23.
MDV1(2) does not run Check TR7(TR6), TR5(TR4)
2.5 Video
2.5.1 Video faults are either total, when there is absence of monitor and TV signal, or may be categorised as either monitor or TV faults.
2.5.2 If a TV is initially connected and is giving no picture it might be worth tuning the set slighly either side of the expected tuning point. (It is
possible the modulator has drifted slightly). If a picture is displayed but without colour, use the oscilloscope to check that crystal X3 is actually
oscillating. Look at the video output and check for colour modulation on top of the luminance signal. If it appears that chrominance and luminance
are present at the video output but the TV does not give colour, it probably means that the crystal frequency is wrong and the TV cannot lock on to it
properly. If no chroma is present then the chroma oscillator is not oscillating. If the chroma is present and there is no TV colour it is possible that
changing slightly the value of C31 may be sufficient to bring back the colour. Check, using a monitor or by looking at the signals that RGB inputs are
present. If they are, consider changing IC28, and checking the circuitry around it.
2.5.3 It is possible that the modulator is faulty. This is checked by removing IC28 and feeding the VIDEO signal (from TR9) in via R85. This gives a
rather low contrast signal in black and white only.
2.6 Fault-Finding Guide
2.6.1 The following table is not intended to be an exhaustive list of the faults that might occur on the Sinclair QL. It is intended as a guide only to
possible courses of action to follow when faults show up in particular areas of the circuit. These areas are listed in the table with sub-headings, in
no particular order of priority. It is envisaged that the system test has been loaded, or an attempt has been made to load it, in order to check for a
faulty condition.
AREA SYMPTOM ACTION
Screen TV/monitor screens black. Power light on. Fails to power-up.
Check voltages (para 2.2.3)
Renew IC22
Renew IC24
Check RAMs
Renew IC34
Renew IC18
Renew IC33
Power
PSU noisy. Change PSU
Loss of power. Check IC37, IC36
Display
Erratic display after warm-up period. Check RAMs.
No colour TV output.
Check C35, C36, R85, R86.
Check C29, R54 and components around IC28.
Renew modulator M1.
Renew IC28.
Dark TV screen. Renew modulator M1.
No B/W TV output. Renew TR9.
No colour monitor output. Renew IC22.
Fails colour test.
Colour loss/fades.
Renew crystal X3.
Check components around IC28.
Renew IC28.
TV output distorted.
No TV output.
Renew IC35.
Blurred vision. Renew IC22.
TV picture drifting continuously. Check C23, C24, C25.
TV/monitor picture drifting. Gradual loss of colour in use. Renew IC22.
Keyboard
Fails keyboard test:
Keyboard does not respond.
Check connectors J11, J12.
Check ribbon cable.
Multi-character printing.
Keyboard bounce.
Keys sticking down.
Key(s) does not register.
Renew membrane.
Key(s) does not always register. Renew bubble mat.
'Sticky' key(s).
Clean key surfaces.
Renew bubble mat.
Continues to fail keyboard test.
Renew IC24
Renew IC18
Renew IC23
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