Upon power up, the LED digit on the MPU board is designed to show zero and will stay that way until the CPU starts executing code. If it stays this way it means the CPU is not booting. This could be due to the reset circuit forever holding the RESET signal low.
In a working machine, the reset circuit holds the RESET line low for about 0.5 seconds, and then allows it to go high. When the CPU sees the RESET signal go high, it starts executing code.
If for some reason the reset signal goes low again, for even a microsecond, the CPU stops executing code and waits for the line to go high again. It then starts executing code from the beginning again.
There is another circuit on the MPU board (the blanking circuit) that watches to see if the CPU is continually updating the player displays. If it notices (or thinks) the CPU is not executing code, it cuts power to the solenoids, displays, and all lamps except the GI lamps. This is to prevent them from blowing.
If you are chewing through solenoid fuses (or any fuses for that matter) remove the fuse and leave it out until you find the problem. Sometimes transistors or PIAs get "stuck" and leave things energized. Leave the fuses out until you have replaced a part or made a change.
Since you've eliminated the power supply from being suspect, your symptoms are likely from something to do with the reset circuit (ie the 556 timer chip and/or related components).
It may also be your NVRAM chip (I see the 5101 chip has been replaced with it). Even if it's "working" it could have corrupt contents. Unfortunately the contents can only be reset by running another program to clear the contents). When the Williams software sees a totally empty 5101/NVRAM chip it will initialize it with default values -- but only if the coin door is open (and the coin door switch is connected and intact).
Your best bet at this point is to isolate problems from the smallest outward.. I have written diagnostic ROM images that will do exactly that. You can find them at:
http://pincoder.ca
They are free to use, and if you have a spare 2532 EPROM chip and a chip programmer, you can download and start running tests.
If you don't have a those items, you can also order a prebuilt, hand assembled, and individually tested adapter card.. complete with my diagnostic ROM images for all Williams system 3 through 7 machines. You can purchase one here:
https://pincoder.ca/index.php/2020/07/10/adapter-2020-06-23-0506/
The documentation on each of the tests, where to start etc, and the ROM images themselves are all available in the software download (See "Release Download" at the top of the page.
Essentially, you will start with the 01a-LEDs test, to verify that your CPU is booting, and is not being interrupted by a fluctuating RESET line.
Second test is "02-blanking" to verify that your blanking circuit is functioning properly.
From there you will test your 5101/NVRAM (referred to as "the CMOS chip" in the documentation), as well as your RAM and CPU chips.
The remainder of the tests gradually test more of your machine. When every test passes, you will have a working machine.
Hope they help!
Craig
ps: There is also a "clear_cmos" ROM image included in the pincoder software that will clear all of the values from your NVRAM, allowing the Williams code to reinitialize it.
Crofton, MD
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Leamington, ON
