BBC Model B Issue 4 - continuous tone. A note from 1990 inside the machine stated "stopped working after thunderstorm" (I have the PSU repair separately from the BBC mainboard) I wasn't expecting much.
Checked through the usual clocks and everything seemed OK. Tricky's Test ROM didn't do anything, so that got me looking further into the memory and all the outputs were garbage. I thought I had all dead RAM but I checked the addresses and the RAS/CAS signals and found RAS continually asserted. I checked the output of IC44 (74LS74) pin #6 and it was dead, however, the inputs to it were working. I replaced IC44 and the machine booted with the bongs but the screen was garbled. I then checked the RGB output and found that IC48 (74LS86) was dead. I replaced that and yay! I can see the Frogger screen, but it looks weird, checked what's going into IC5 and find that IC37 has also failed (74LS04) I replaced that and MODE7 problem has gone. I put the OS back in and all seems good. Now to put Econet into it![:D]( "Very Happy")
Checked through the usual clocks and everything seemed OK. Tricky's Test ROM didn't do anything, so that got me looking further into the memory and all the outputs were garbage. I thought I had all dead RAM but I checked the addresses and the RAS/CAS signals and found RAS continually asserted. I checked the output of IC44 (74LS74) pin #6 and it was dead, however, the inputs to it were working. I replaced IC44 and the machine booted with the bongs but the screen was garbled. I then checked the RGB output and found that IC48 (74LS86) was dead. I replaced that and yay! I can see the Frogger screen, but it looks weird, checked what's going into IC5 and find that IC37 has also failed (74LS04) I replaced that and MODE7 problem has gone. I put the OS back in and all seems good. Now to put Econet into it
Statistics: Posted by maniacminer — Sun Feb 23, 2025 12:32 am