MK133 Quiz Table

I purchased and assembled two of these kits at the same time. Both seem to have the same problem: when a contestant button is pushed (to “ring in”), both the corresponding contestant LED and the quizmaster LED light up, but don’t stay lit - they go off as soon as I release the button. If I push the quizmaster button while holding one of the contestant buttons, both LEDs will go out momentarily but light up again as soon as I release the quizmaster button.

I’ve double checked the values and polarities of all components, and I can’t see any problems with the soldering job. Any suggestions??

Some basic things before we look any further.
Did you test this with a fresh 9V battery and with no external pushbuttons connected to the units ?

Hereby some voltages for you to check.
This will allow you to find the area where the problem might be:

Power supply : 9VDC
Unit idle (no buttons pushed, no leds lit)
All voltages referenced to GND
Tolerances of +/- 10% are normal

T1
E: 8V75
B: 8V
C: 8V75

R3/button1: 8V75

T6
E: 8V75
B: 8V75
C: 0V

T2, 3, 4, 5
E: 8V75
B: 8V75
C: 0V

T7,8,9,10
E: 0V
B: 0V
C: 8V3

I have built many of these kits and the only time I have had this problem is with a mistake in the assembly - often a diode the wrong way round, and once a transisto in backwards!