Hi all,
I'm neck deep into a 5440A repair that's very reluctant to start working. There's reasons to believe the cause of the issues reside on module A2 (Front Panel Control). It's what seems to be a Z80 CPU headed 6402-based UART interface to the main controller design.
I've been investigating around the UART and the CPU to no avail. Some of the things I've found are below (more of a data dump, I'm not entirely sure where to head from here). Also, note the A2 schematic enclosed below, which is not widely available, but I'm putting it here as I'm asking for help.
I am looking at this under the following circumstances:
- A16 is pulled to allow A2 to try things out undisturbed
- I've replaced the Z80, IM6402, the RAMs (TMS4016-20NL), and recapped the board. Not a fan of scattershot troubleshooting, but some things being on sockets, why not eliminate some of the possibilities?
- The troubleshooting switches on A2 during the captures below are 0100
Some observations:
- U16/pin16 seems stuck low (goes high for one pulse, then seems to stay low permanently).
- forcing reset of the CPU by shorting C38 (actually, grounding U3/pin1) doesn't lift U16/pin16 high
I also enclose a read of A0 through A15 on the CPU upon starting the unit. Most images following in a second post.
What could cause the issues I'm seeing with A2, and what else should I be looking into?