Took a brake from Kiss and went on to "easier" fixes, then came on this.

I have a Williams System 7 that is showing limited CPU activity. In both Leone and Pincoder test(01-a) ROMS I get no activity on CPU pins 16-33, so the entire Data bus. All of the other pins are showing appropriate activity on the CPU. Even when IC9 is removed I get no activity on the CPU legs. I can manually ground the various pins Data bus pins at the CPU and the 7 digit display reacts so it has good continuity throughout the data bus it appears.

Checked continuity between IC1(CPU) and IC9 and IC17
Socketed CPU, IC9, IC18, IC36, IC17, IC13 , IC19
Swapped out CPUs
Replaced IC9
Verified PIAs are good

Thoughts?

I had actually meant to delete the post, mainly due to my ignorance because I figured out what was wrong with this problem at any rate. Install a PIA backwards and funky things happen, fail.

I still can't get the board to boot, but at least that takes that problem out of the equation. Now as far as diagnosing what else is going on, for the LED test it uses CPU1, PIA18, IC17 (Test rom). The codes then says hey toggle some LEDs and 7 digit display which uses IC2, IC33, IC34 and IC 7. I mean technically i guess there are some transceiver/buffers thrown in the mix as well for good measure (IC 3, IC4, IC8, IC9)

Right now it appears that something is going on around IC2 as every pin is high except 7. The documentation says to check pin 15 on IC2, but not sure that's possible on a 14 pin chip. Also says that pin 1 on IC2 should be low, but pins 1, 4,10 and 13 are connected to the 5V rail through R20.

Ill start by checking the PIA outputs on pins 6-9 and work my way back.

Reset circuit is good, starts low goes high and stays High, no drop outs as tested on TP8.

CPU1 is responding to clock signal on pin 37. Pulsing is present on IC17 pin 15 so VMA looks to be good. IRQ at TP5/ IC1 Pin4 is pulsing. All of the other data lines have activity. Tested the clock with DMM and got 2.4 VDC but I am using an Elenco logic as well.

I've removed IC9, and still no pulsing on the Data bus from the CPU.

I've tried 5 different new CPUs but unfortunately don't have another machine to test them in at the moment but it would be odd for ALL of them to be bad. I broke down and bought a CPU tester when I first started messing with this problem which should be here Monday.

Just to verify that CR1 is doing it's thing, I hefted the old scope out.

PXL_20210328_181543754 (resized).jpg

I went ahead and put back in U9. Will continue playing with it.

I want back to basics and verified ROMS. 2732!=2532 No wonder nothing would work.

Yep, it's all good. Had to rejumper the U17 for 2732 EPROMS.

My understanding was that the CPU was the start point for the Data Bus signals however without U9 none of the Data bus lines were active. I suppose that is why it is called a transceiver