UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

SiriusHardware · 21st Sep 2022, 12:19 am

Agree with everything above, but if BrackenFix feels confident enough to remove ICs in one piece then I think removal / socketing and swapping over of the display RAMs UF7 and UF8 would be the lowest cost next step as it wouldn't involve buying any new parts (apart from sockets, which are already on the way). If the fixed characters displayed then change to alternative fixed characters, at least one of the RAMs is faulty. If swapping them over does not change the initial display, then the video RAMs are ruled out and can stay in place.

Making the video RAMs removable would also allow us to impose test logic levels on the SD0-SD7 lines going from the display RAMs to the video latch and onwards to the character generator ROM, so we could test both of those out without having to unsolder the 74LS373 latch.

Before any of that though, and only after the ROM is repaired, I still think it is worth measuring the voltages on the display RAM SD0-SD7 lines in the hope that we may see a clearly stuck bit.

21st Sep 2022, 12:19 am	#44
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,560	Re: 1978 Commodore PET project Agree with everything above, but if BrackenFix feels confident enough to remove ICs in one piece then I think removal / socketing and swapping over of the display RAMs UF7 and UF8 would be the lowest cost next step as it wouldn't involve buying any new parts (apart from sockets, which are already on the way). If the fixed characters displayed then change to alternative fixed characters, at least one of the RAMs is faulty. If swapping them over does not change the initial display, then the video RAMs are ruled out and can stay in place. Making the video RAMs removable would also allow us to impose test logic levels on the SD0-SD7 lines going from the display RAMs to the video latch and onwards to the character generator ROM, so we could test both of those out without having to unsolder the 74LS373 latch. Before any of that though, and only after the ROM is repaired, I still think it is worth measuring the voltages on the display RAM SD0-SD7 lines in the hope that we may see a clearly stuck bit.