Hi everyone,

I am new on this forum and I am French so please excuse my English.

I have got a Williams System 11b pinball #Police Force.

The CPU board suffered from battery leaking when I purchased it. So I moved batteries off the board, cleaned board with vinegar to neutralize acid. I changed C24 C26 C29 capacitors, U42 U41 PIA, changed all chip supports, SRC2 SIL, U43 and Q50 for the blanking circuit and U25 ROM.
The board booted then fine for few weeks until I got the IRQ failure message.

I put off the CPU board from the cabinet, plugged it on an ATX PC power supply and changed U29 in the IRQ circuit. The board booted again. But when back in the cabinet, the IRQ failure came back.

Back again with my CPU board on my workbench and ATX power supply, I noticed a very low 5V voltage of only 3.9V! That's the problem.

I changed others PIA, U31 to U37 logic gates, SRC1 to SRC9 SIL and ZR1 D1 and D2 diodes... unsuccessfull. Still only 3.9V but with the CPU board booting (5v and blanking LEDs on, diagnostic LED flashing fast) on the workbench.

Moreover, not hot components and 1.3A current consumption on the 5V line with all disconnected connectors.

Does anybody already faced that trouble? Any idea?

Thanks in advance for your help.

20210326_183821 (resized).jpg

Thanks for your quick answer. I did not know that test method for 74xxx chips.
I will have a look on it and give you a feedback.

Hi,

Is your testing tip is also working for 74LSxx and 74HCxx séries?

Thanks