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SE/30 - help with UH7 pin continuity

Posted by: desertrout on 2020-08-22 09:37:19 I'm trying to degub simasimac on my SE/30, and noticed the R–W line (which is connected to, like, everything important - RAM, CPU, GLU, SWIM, VIAs, etc...) was buzzing to both ground as well as to WR (which connects to J8 - 18 / J6 - 19... all having party together. It would seem the 'short' (if it's that) is inside the 16L8D PAL at UH7 (Apple # 341S0689-A or -C on some)... I pulled that chip and verified that pins 10 (GND), 16 (WR), and 17 (R–W) have continuity - enough for my DMM to buzz. Those lines on the board don't connect when the chip is removed. I can't imagine this is normal, is it? Does someone know? I don't have another SE/30 board to test, or another board with this chip... is someone able to verify?

Posted by: Daniël on 2020-08-23 04:35:29 They are not connected on one of my SE/30 boards (had it on hand for a quick test). I too suspect the IC in question is internally shorted.

Posted by: desertrout on 2020-08-23 07:26:16 They are not connected on one of my SE/30 boards (had it on hand for a quick test). I too suspect the IC in question is internally shorted. Thank you! I had suspected as much, but it's so helpful to have it confirmed. I'll be getting my hands on another local SE/30 shortly to verify as well. This of course leads me to the next obvious question for anyone who might have some insight... what may have caused this? This seems to coincide with my recapping of the PSU and AB, though the voltages on coming from the AB and on the MB are solid and on spec...

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