I'll try and keep this brief. Already had a decent 520 STFM that I bought from the Cambridge Museum of Computing History over lockdown. Saw a 1040 STF and SM125 monitor on FB marketplace locally - non working - for £100, so just had to have it...obviously!!
Putting the SM125 aside for now (well, up in the loft actually)... the 1040 ST had a lot of water/liquid damage. I ended up replacing the reset switch, and many other components in the rear left of the machine - including the MIDI sockets, opto-coupler, 0R resistors and re-socketing all 32 DRAM chips. I had to replace 7 DRAMs - mainly those which showed signs of rust on the legs. After then repairing a couple of snags I'd made when removing the DRAMs, the machine worked perfectly in that it would always recognise the machine as a 520K using the original diag cart.
I traced the connections back to the MMU, and all was good, and also traced connectivity of the higher address lines back to the CPU - all good. Using a scope/LA, what I noticed was that RAS0, CAS0L & CAS0H were all functioning perfectly, as might be expected. RAS1 seemed to have activity, but nothing on CAS1L and CAS1H. I've tested connectivity throughout the DRAM area, and cannot find any shorts at all.
I decided to replace the MMU socket, as it was getting gradually looser. So having de-soldered both the MMU socket and the 68000 IC, soldered new sockets back into both and tried again. Exactly the same. At the same time as buying the MMU socket (from Exxos), I also bought a new 68000, and one of the 4MB upgrade kits, with correct cable and shifter PCB.
Having removed all original RAM (but leaving the connections to that area of the board intact), I tried the new 4MB RAM. It detected as 4M immediately, but I am getting lots of memory faults. As these upgrades are tested, I doubt that it is faulty DRAM on that new board (or at least I am putting that lower down on my list of suspicions - I will likely have to test it in the 520 ST). For now, I am basically wondering if anyone can think of anything else that might be causing me these issues on the upper (> 512K) RAM. I've not recapped anything on the motherboard, but the PSU was bought as overhauled and tested recently.
Something weird that I noticed when running the machine PRIOR to installing the 4MB upgrade was that the top four or five address lines seem like they are tied together. Is that normal? On the attached scope display (sadly I didn't take a later image when I'd connected more)...
- D0-D5 = RAS0, CAS0L, CAS0H, RAS1, CAS1L, CAS1H
- D6 = possibly write enable - can't remember.
D13-15 = top three address lines
- 0-512KB works OK, so the supporting DRAM circuitry (the LS244 and LS373) are most likely OK. In the same way, it's unlikely to be any capacitors failing.
- I have swapped the GLUE and SHIFTER chips from the 520 ST, with exactly the same issue.
- I have replaced the 1.00 2-chip TOS ROMs with a fresh 1.04 6-chip TOS ROM set - and changed the solder jumpers accordingly.
- I've not changed any pullups yet - but again, if 0-512K is working OK, would that really be the issue?
John