Acorn damaged displays a dash

[Danto1]

For me it looked like it was working fine at first, but after checking the floppy drive something went wrong. Now only a dash is displayed and no sound when turned on. when entering mode 7, only a line appears, the rest of the modes are ok. I checked everything you wrote, there are ic15 signals, I didn't find anything damaged, I replaced all systems and cleaned the contacts, except the video hive because I have a second BBC, but there is vc2023. can replace the socket in the 6845. I found a damaged 17.7345 MHz quartz, I have already ordered it, but I think it is only for RF. I have an RGB cable.

[Danto1]

I copied my answers from another post "GAL16V8 as stopgap for dead video ULA". All the chips in the sockets have been checked beforehand, except for the videoula and the removed floppy disk controller, so I don't have any replacements, so IC27, IC36, IC39 and IC40. Maybe someone had a similar problem.

[maniacminer]

Have you checked IC32 (74LS259)? Its failure would kill the sound and cause other issues (scrolling, DRAM CAS etc.)

[Danto1]

Yes, checked on the tester, ok, any other hints. I have an oscilloscope but no experience with BBC. I've repaired tons of other retro computers.

[maniacminer]

Yes, checked on the tester, ok, any other hints. I have an oscilloscope but no experience with BBC. I've repaired tons of other retro computers.

Yeah, tester... definitely worth checking it with the scope in situ. I've had several ICs that tested OK logically in a chip tester, but in circuit have some strange effects, one of those was this IC... Have you checked that NMI is high and IRQ is bouncing around?

[Danto1]

Ok, thanks. There is a clue, NMI is not ok. IRQ is ok.

[Danto1]

ic27, ic 32, I replaced them with others, and still the same, unfortunately

[Danto1]

I removed the entire controller FDD system. I didn't have P8271 to replace. s9 compact. Nothing the same either. Videoula is also ok, I already checked it because I replaced it with Gal. There's a TTL lying around somewhere.

[Danto1]

Fixed, found fault IC 43, 74s04. And since I didn't have one, I put the 74F04 on and it started. Well, I will replace it with S, because it does not always start and the image flickers slightly.

[maniacminer]

Fixed, found fault IC 43, 74s04. And since I didn't have one, I put the 74F04 on and it started. Well, I will replace it with S, because it does not always start and the image flickers slightly.

Which gates are damaged? The 74F04 is resonating with a higher frequency overtone from the crystal. I'm guessing that C37 has decreased in value, it should be 33pF it's possible the value has diminished over 42 years?

[Danto1]

I installed 74s04, everything is OK, it is possible that the c37 value has dropped, I will check it again. Damaged gate pin 10,11.