Working this Williams system 4 MPU using leon's test chip in IC17. Sockets have been replaced. the 6800 has been replaced. the pia has been replaced. and the 5101 has been replaced.

Board is on the test bench with 5.1VDC at the 6800.

All of the address and data lines are pulsing through the buffer chips and verified pulsing at IC17. The clock is ticking on the correct mpu pins.

MPU pin 4 IRQ pulsing
pin 5 VMA pulsing
pin 6 NM1 High
pin 7 BA Low

I pulled the 6800, powered up the board and verified the logic chips using a probe and logic pulser. I could find no problems.

One anomaly is with A0 from the MPU to the 8T97 chip. With the 6800 pulled the input pin 2 of IC4 for A0 is H along with the output pin 3. If pulsed low on the input, the output follow suit. With the 6800 in place the address lines pulses.

I question the chip selection circuit. It is high on all of the outputs of the IC15 decoder which is normal for a "bad" input. I don't have a spare to put into my Bug Trapper to test in circuit, but have put one on order.

Just a bit frustrated at this point.

After fully verifying the I/O chips associated with the 6800 and manually checking the simple logic gates I brought out the Bugtrap tester. This device uses a good working chip and compares it working in-circuit with a like chip and reports any differences.

I used it with with IC11, which is a triple input nand - 7410 and the Bugtrap reported a fault as can be seen in the picture with pin "14" which equates to pin 8 of the chip under test. The fault was repeatable. The pin numbering is skewed as the Bugtrap test socket can handle up to 20 pin chips.

I was thinking IC15 7442 the decoder was faulty and the Bugtrap would have found it good if I had a good 7442 to test with the tester.

I replaced IC11 and the problem is resolved. For these types of problems the Bugtrap is handy test tool to use - as long as you have a matching "good/new" chip for the chip under test.

P1000594 (resized).JPG

And the RAM 5101 was bad. All is now well with the MPU