On initial testing, all video outputs appeared blank with no vertical sync. There was no CPU activity even though /HALT and /RESET were not asserted. So, I replaced the CPU. After doing this, I could see activity on the address and data lines, but the video output remained the same.
I found that the GLUE was generating a horizontal sync pulse, but no vertical sync, /BLANK, or DE. After fitting a replacement GLUE, all the video timing signals were present, but the display remained blank (though sync was now present). Once again there was no CPU activity, but this time /HALT was asserted.
The reset circuit seems fine (both inverter inputs to the /HALT line are low). The system ROMs are also probably okay (my EPROM programmer doesn't have a definition for that type of IC, but I did manage to get the first 64K off each chip and it looks valid).
So, the likely explanation seems to be that there is another device that is failing to respond properly to the CPU, causing it to halt with a bus error.
The main suspects seem to be:
- DMA controller
- 68901 MFP
- GLUE (unlikely, but I suppose it's not guaranteed that some random replacement chip from eBay is definitely good)
Clock signals and supply voltages all look okay.
I have an analogue scope with single-sweep mode, but frustratingly it does not have a storage function. Using this, when I switch the machine on:
- /RAM and /DTACK on the MMU have at least one falling edge
- /CS and /DTACK on the 68901 have at least one falling edge
- /CS on the DMA controller has at least one falling edge, and RDY has at least one rising edge
- /DTACK on the GLUE has at least one falling edge
It's very hard to see what the signals are actually doing in the fraction of a second they're visible, but it seems none of the chips are completely dead.
Does anyone know a way of narrowing down which device (or devices) might be causing trouble, or figuring out exactly what the system is trying and failing to do? I don't want to start replacing out-of-production custom chips without being sure they're faulty.
CPU and GLUE are now socketed, so can be removed if needed for troubleshooting. System is a stock 520STFM, motherboard revision C103414 rev 1.1, with 2-chip TOS 1.04.
Thanks in advance for any ideas.