Hi!
I've purchased one and will post schematics as soon as I can draw them, but in the meantime:-
1) Check the +5V and -5V supplies to the 7107 chip on the front display PCB, pin 1 +ve and Pin 26 -ve;
2) You'll need to check the four ICs clustered round the AT89S52, 5U2 (74HC00), 5U3 (74HC00), 5U4 (74LS08), 5U5 (74HC4040) and 5U6 (74LS132), I recommend you obtain one of these:-
https://www.ebay.co.uk/itm/IC-Transistor-Tester-Detect-Meter-Maintenance-Digital-led-Tester-MOS-PNP-NPN/292443757028?hash=item44170199e4:g:8fIAAOSwT5tWMIfJMyself and several colleagues use these little testers and they will identify the "family" (LS, HC, HCT, etc) as well as the type, & they test linear and opto as well, but you'll need to remove each device for test!
3) Have a look at the three '9011 transistors in the top LH corner near the AT89S52 - these could be digit-select transistors!
Watch this space, schematics, test waveforms and other data to follow in due course when I get mine!
Chris Williams
PS!
It's now supplied in a white, dark blue and grey format front panel & case assembly with white knobbery, but sadly nobody thought of using s spellchecker when they made the revised front panel design! Also it's gone up £10 or so in the week or so since I ordered mine!