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Macintosh IIsi – Weird data bus voltages

Posted by: robin-fo on 2022-08-07 05:24:46 Hi everybody I recently bought a non-working Macintosh IIsi logic board. I think it's a rev. A board and it lacks the on-board ROMs. I know that the first IIsi boards only had a ROM SIMM, but on my specimen the pads look a bit like somebody did some soldering on them... When I attempt to boot the board with a Rominator II inserted, I can hear BMOW's custom boot chime as expected, immediately followed by a death chime. The board is recapped, except for the two axial caps. I replaced all onboard RAM chips with those of a battery bombed IIsi – no change, but I cannot confirm that they are good. I checked some CPU signals and I found that the data bus has some strange voltage levels until the end of the death chime: Here's an arbitrary section of the signal recorded from D17 on the PDS connector while the system played the death chime. On some other data lines, there's a square wave going up only to around 1.6V. I can also constantly measure that voltage on the data lines beyond the RAM bus buffers (the ALS245's like UE5 etc..) As soon as the death chime is over, the data bus voltages immediately go up to a decent 5V square wave! WHAT is going on here???? 🤯

Posted by: rikerjoe on 2022-11-01 15:37:12 I got death chimes on my IIsi due to poor resoldering of the PSU connector that I removed to repair the +12 and -12 volt traces that were broken due to leaking cap goo from the PSU. Have you checked the PSU connector for broken traces or corroded solder joints?

Posted by: robin-fo on 2022-11-01 22:54:04 In the meantime, I solved the problem: the ALS245 bus buffers were somehow bad, after replacing them, the machine started as expected.

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