502 board with No VFD and says NO Display

[shebaatkins]

I have several 502 boards with vfd blank and NO DISP showing. I am getting 0 olhms readings on U67 pins 15 and 17 on the first one I am diagnosing.  Does this mean that chip needs replaced? I have replaced plenty batteries and just trying to learn more.  I am pretty sure a bad machine shorted my boards. I am starting with U67.

Thanks
Sheba

[lookes]

You have a NETPLEX problem. U67 is the SENET driver. If it was bad, you would have nothing or random characters on the LED displays. You will gain nothing by replacing it. If you decide to do it anyway, make sure you get a 74C240. NOT a 74HC240. It is not the same and does not have a high enough voltage rating on the high side.

[shebaatkins]

Thank you very much. Would you mind telling where you would start looking?

Thanks
Sheba

[shebaatkins]

Here are my notes on my diode measurements on chips and picture of the board.

Thanks
Sheba

[lookes]

Those readings don't mean much. I use a logic probe and oscilloscope to track the signals in and out.
If you really want to shotgun it, replace Q4 and Q10. They are the most likely components to fail. After that, it's U63, U81, U91, or corroded traces to or under U69.
I do repair these boards every day. That's another option lol...

[shebaatkins]

Yes I am aware of your expertise and I am very grateful. I will get the hardware and tools I need to move forward and let you know how it goes.

Thank you,
Sheba