T86 Radar
It was on with the painting of the wheel arches and other bits and pieces under the chassis; a time consuming process of preparing, corrosion treatment, priming and top coat
Mitsubishi C-3920 Display
One of the display monitors developed a fault which gave the appearance of a free running display sync. Initial investigation soon exposed the problem, see the two accompanying oscilloscope screen shots of the H-Sync and V-Sync at the output of the video circuit (inputs to the deflection circuit). The first shows the defective monitor and the second a working one
Fortunately we do have the circuit diagrams for the monitor so it was down to some old fashioned fault finding.
The horizontal sync is stripped from the green video input from CHARGE, all RGB video channels were being processed correctly by the monitor so everything pointed to the creation of the vertical sync which is created by a few TTL 74 series devices including NAND gates and a couple of mono stable multivibrators with timing set by capacitor and resistor combinations. The fault was eventually fixed once the capacitors and TTL devices were changed, there was no single point of failure. Components were changed that improved the vertical sync but stability wasn’t achieved until all the TTL and capacitors in the vertical sync circuit were replaced. The display of the now serviceable and stable monitor is shown in an accompanying photo. The now serviceable monitor shows the ‘firmware display’ created by CHARGE before a video input is received from the Argus 700.
One of the challenges with the Mitsubishi monitors is the setting up of the video and deflection PCB’s. Following this latest repair a new BMPG version of the set up instructions is being created for the simple reason that set up instructions in the manual don’t do what you expect, that is balance the RGB video with sufficient contrast etc.
Pete H