Oooh the 1902 monitor had an 8 pin DIN input for the 128's RGBI output. So that's two places you could plug the wrong cable to.

I know it's been 7 days since the last post on this thread, but I'm wondering what connector an RGB monitor cable would plug into. I wondered about that, I think the Amiga had a RGB cable that went from the Amiga's DB-23 to a DIN 6 connector on the monitor. https://www.ebay.com/itm/OEM-COMMODORE-Am...

It's not even "a" motherboard, it's a few separate boards that plug into each other, and sections of it are not even like the regular 64.
Why do you think troubleshooting it would be that hard?

update: I ordered some 74LS257 parts from Futurlec and they also fail in the same way, one out of 4 logic units fail, all the same.
guess I can't cheap out on these.

Well there we go. I received my device programmer, the TL866, and it has a logic IC test function. https://i.postimg.cc/XqWhZbCQ/failed257.jpg There's the problem with the 8708. Honestly I bought that programmer since it was recommended over at EEVBlog, I didn't check all its features. I was pretty ...

I tossed in a 157 instead of a 257 and now I have the full 60K free on power up. The 257 has three-state outputs but in the Plus 4 the OE is always low so the hi-z output is never even used so a 157 does the job. It's been a long time since I fooled around with discrete logic... But I got it to work...

I'll hook up the 8708 chips on a breadboard and take a look at how they work or rather don't work. My place is a mess, so later. In the meantime I noticed I was actually missing a lot of 74LS parts used in C= machines so I ordered a bunch from Futurlec. I don't know about buying 74LS parts on eBay. ...

Well it amuses me to no end that I can get TTL chips from behind the Iron Curtain! https://www.ebay.ca/itm/K555KP11-SN74LS257-SN74LS257N-IC-Microchip-USSR-Lot-of-2-pcs/132772129132?hash=item1ee9d5e96c:g:gpIAAOxydB1Sm~rh:rk:29:pf:0 LOL I'm going to give these a try. They'll probably be on 2.5mm cente...

Well all logic levels seemed good and since I could see real characters on screen during RESET, and I know that the KERNAL ROM also contains the character data, I figured it must be good. I started doubting the DRAM, they do go bad, usually they get quite hot but none were running hot. However the b...

Thanks, I'll look into it. I finally ordered a device programmer to handle old chips so as soon as I get it, I can try to read my Plus 4's BASIC and KERNAL.