It's at this point that I would be contacting Rottendog.

I don't know if you have looked at this but it's probably the best site for what your trying to troubleshoot and he works with Leon's chip. The recommended test is to use an led vice logic probe or oscilloscope as you will get false indications with noise on the line.

http://pinball.flippers.info/system6resources.asp

From the sounds of it your CPU is running which rules out the CPU and crystal as being the problem. PIA's should flip flop with the chip with the exception of a couple gates (see reference).

First ensure your getting clean power. The original power supply can be rebuilt to provide satisfactory power or you can go Rottendog route.

Beyond the PIA's there are IC's on the data and address busses that can also give you trouble. Again following the advanced testing techniques on the posted site will get you working through the problems systematically.

Thanks. I've been going through Mark's site. It was suggested from Leon's site as a more English friendly version hehe so I've been following that along with pinrepair.com.

I'm also running on a benchtop power supply as it is pulled from the machine, so I'm sure I have a clean source of power.

I am currently at this section:
3.4.8  The R/W line
The "R/W" line is the Read/Write line.  This the signal that the CPU generates to tell the chip it has selected whether is wants to read or write data from the chip.  This line is high for a read and low for a write.  When the test chip is running (or attempting to run), this line should be high.  It passes through the data buffer chip IC8 (pins 2 to 3).

The signal should be identical on either side of the buffer (pins 2 & 3) or constantly high. It is high on the input side but fluctuates between high and low on the output side. Chip/socket as been replaced so I am trying to figure out what else could cause this signal to be pulled low on the output side of the device. The only chip still in that receives this signal is IC11. Looks like I will replace this and then move from here.

Thanks again for the time/help.


Steven

Rottendog is an option... but I'm not quite there yet!

Replacing IC11 was no help, except it did bring something to my attention. When I was checking for continuity between pins I noticed 1.2 ohms between pins 1 and 2 ( BUS02 and R/W respectively). I double and triple checked the socket and it does not seem to be occurring here. I followed the traces and noticed they ran side by side through the 6810 RAMs and thought maybe during the socket replacement something happened here, but again double and triple checking I came up with nothing.

After completely looking through the schematic I see no reason to have such a low resistance between these 2 lines. I am hoping whatever I turn up here will be what is causing the R/W line to go low intermittently after the buffer.


Steven.

What TTL IC is data buffer IC8? I would check the logic on it. Leon's chip puts the output high on the CPU but the oscillation may be a function of the data buffer.

Pin assignment for the 6800/6808 has R/W on pin 34.

You've done the basics i.e. separated the MPU from the driver board and are testing it independently right? 40 Pin interconnect is notorious for creating issues on these systems.

Thanks again for your help.

I am using a 74LS367 as the replacement of IC8. I removed the chip and tried it only on a breadboard and I do not observe the oscillation I was noticing on board. It is very severe, down to about 60% duty cycle from 100%.

And yes, I have done all the basics, removed from driver board, replaced ALL scanbe sockets, reset (unreg 12V) modification as well as the crystal upgrade.

To me, I just see no reason for the low impedance between the R/W and BUS02 lines, and it is too much of a coincidence that the one pin on the buffer that I am having problems with is also the suspect low impedance pin (R/W). I'll gather up some more energy and get back at it tomorrow.

Cheers.


Steven