Adrian,
There is a jumper on the A28 board for 525/625 operation of the composite video but it should affect the 7000 plugin readout (CPU generated) and the read gun scanning consistently. There are multiple jumpers for normal or fast scan and these need to be consistent. Pages 4-9 onwards of service manual 2 give all the jumper settings. If my memory is correct the digital display does not show the readout from the plugins.
Regards,
Roger
