Tuner Boards - Sinclair Microvision MTV1 Service Manual

Tuner board
The VHF and UHF tuner units are not easily repairable, although this manual includes their circuit
diagrams. In order to cure any faults in this part of the receiver it is suggested that the complete
board be replaced.
Other faults that can occur on this board include:
1. Fuse from batteries – open circuit
2. Batteries faulty – open circuit
3. Loudspeaker – open circuit
The tuner board is now very often impacted by chemical damage from battery leakage, even if the
batteries are now basically dry. Best advice here to perform a visual inspection, then check
continuity across any suspect-looking tracks, making repairs as required.
Odd tuning results can be expected if the -33V rail isn't correct. This is derived from the -45V rail by
a single 120k resistor on the video IF board. If the voltage is low (ie numerically less than 33) and
you've checked for and corrected any and all chemical nasties, it's quite possible that one of the
varicaps in the tuner section has gone leaky. The types originally used aren't known for certain but
most VHF/UHF varicap diodes of the period go between 2pf and about 15pf or 2pf and about 35pf.
If the set appears OK but won't tune (ie you get different-looking noise when switching between
VHF I and III, and UHF, but nothing happens when attempting to tune in a known-good analogue
signal), check that there is 33V across the tuning pot (note that the pin furthest from the yellow
knob is the wiper), then check that adjusting the tuning position alters the voltage on the wiper. If
that's happy, check the second Berg pin in from the tuning knob ('AFC Out' on circuit diagram).
This should have a tuning voltage of 0 to -33V that varies with the position of the tuning pot. If it
doesn't have a variable voltage it's possible that IC201 on the video IF panel is suspect.
Temporarily isolate the Berg pin, eg by cutting the track on the Tuner board, and check to see if
tuning now works. If so then no further action is required – IC201 is Unobtanium and all that you've
done by cutting the track is disable the AFC and possibly upset the tuning dial calibration.
If the tuner board is beyond repair then it may pay to convert the set into a monitor. Details on
doing this are in a separate section.
Sinclair MTV1 service manual V2.1
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