BBC B Issue 7 factory B/32K with disc interface.
Problems: disc interface was removed at some point in the past and the piece of paper inside the machine suggested that the disc interface was faulty. I didn't need disc, so I installed Econet - the machine worked perfectly well before adding Econet.
After adding Econet the machine would not boot, a single beep from the speaker and the cursor (in MODE7) would sit in the top left hand corner flashing away, no keys on the keyboard had any effect.
Starting by pulling the DNFS ROM, the machine immediately booted up. This suggested to me that the NMI was not being cleared. Scoping the 6502 I could see a lot of activity around the NMI. I traced it back and found that the CTS and DCD on IC89 (68B54) were filled with high frequency waveforms and this was causing it to not respond properly. Following back from these pins, it became immediately apparent that IC87 (74LS123) was producing rubbish.
A quick look at it and the ah-ha moment, it had become corroded, a white and green powder was all over the legs of the IC and the socket. I pulled the IC and the contacts in the socket turned to powder. I removed the socket and replaced it, however, several of the tracks were corroded and had to be rewired, one of these went to the disc interface and that explains the original fault. Testing the 74LS123, it was just plain dead, I replaced that and now the machine boots with DNFS and can see the network, yay!
Problems: disc interface was removed at some point in the past and the piece of paper inside the machine suggested that the disc interface was faulty. I didn't need disc, so I installed Econet - the machine worked perfectly well before adding Econet.
After adding Econet the machine would not boot, a single beep from the speaker and the cursor (in MODE7) would sit in the top left hand corner flashing away, no keys on the keyboard had any effect.
Starting by pulling the DNFS ROM, the machine immediately booted up. This suggested to me that the NMI was not being cleared. Scoping the 6502 I could see a lot of activity around the NMI. I traced it back and found that the CTS and DCD on IC89 (68B54) were filled with high frequency waveforms and this was causing it to not respond properly. Following back from these pins, it became immediately apparent that IC87 (74LS123) was producing rubbish.
A quick look at it and the ah-ha moment, it had become corroded, a white and green powder was all over the legs of the IC and the socket. I pulled the IC and the contacts in the socket turned to powder. I removed the socket and replaced it, however, several of the tracks were corroded and had to be rewired, one of these went to the disc interface and that explains the original fault. Testing the 74LS123, it was just plain dead, I replaced that and now the machine boots with DNFS and can see the network, yay!
Statistics: Posted by maniacminer — Tue Apr 16, 2024 10:47 pm