Belated BBC B

[stephen_usher]

:dualthumbup:

[rubber_jonnie]

Aaaand back in the saddle!!

IMG_20210710_141944.jpg

Just a temporary setup, but at least I can do some stuff, as in a RIFA smoke bomb replacement on the Belated BBC B :)

Had a quick look at the keyboard and the switches are actually fine, so will have to trace the tracks, but I have at least fixed the LEDs, which as I suspected had been pushed down on breaking the tracks, but a quick solder bridging exercise has them back as normal.

[rubber_jonnie]

Ok, so the BBC tests fine with another keyboard, so for now I'm buzzing the PCB out to see what's what. So far, all the dead keys actually have working switches, and I have replaced all the ICs with good ones, but no change.

Since there is evidence of rework, and it's not the best soldering I've ever seen, I'm going to have a clean up and reflow, and since one of the sockets is 2 pins too big, I'll pull it and fit a properly sized socket.

It's nice to have my decent tools available again!!

[rubber_jonnie]

Looking at the schematic, the KBD matrix is fairly simple.

It seems that all of the failing keys are connected to the row looked after by D5, pin 14 of the 74LS251 Multiplexer.

Since the other rows and columns are clearly working, I am surmising that it's that row that has the problem, otherwise the rest of the keys would have a problem. All the diodes check out too, though they are connected to the columns anyway. I need to check out the row pull up resistors though.

Since I have replaced all the ICs with new, I don't believe the 74LS251 is a fault (or any of the others either) so time to check thePCB again from those switches back to pin 14. Bit of a head scratcher.

[stephen_usher]

And the continuity test show nothing wrong?

[rubber_jonnie]

And the continuity test show nothing wrong?

Actually, they did, and found a problem between the row trace shared by Return and :/*. This seemed to be giving an intermittent connection between the two, and it meant the row wasn't being properly pulled up.

Twisting the keyboard made it kinda sorta work.

The rest in the row were fine back to the 74 LS251 so I put a bodge in for the trace between the switches as follows:

BBC_No3_KBD_Bodge.jpeg

And all is now well.

Once I've found my microscope I might have a close look at the traces, but to the naked eye and minor magnification, they look normal. The KBD PCBs seem pretty weak and easily damaged, but for now this'll do.

I also removed the switched ROM because the soldering and general installation was pretty poor. I have lots of photos and will revisit it at some point in the future.

Otherwise it boots up fine, has Acorn DFS and a Wordwise ROM that works just fine. I seem to be getting the hang of the Beebs.

[stephen_usher]

:dualthumbup:

[rubber_jonnie]

OK, so it turns out that going through the box of stuff that came with the my number one BBC, there was a DFS ROM in the bottom of the box!!! It's quite an old one, however the number 3 BBC, the hero of this thread also has a DFS ROM. Both are quite old versions.

It also oddly has another ROM installed, however the *ROMS command doesn't work, it just hangs when I hit enter. The other ROM is Wordwise plus something I can't read written on the ROM.

Also, the number 3 machine is the one with a 8271 installed.

I've tried hooking the FDD up, but nothing, so we'll see what happens when I get the 1770 up in the number 2 BBC.

[PhilC]

@rubber_jonnie it's possibly worth checking the logic chips under the 8271 and the floppy socket for bent or corroded pins. I think it's 74F38s for the logic chips IIRC

[rubber_jonnie]

@rubber_jonnie it's possibly worth checking the logic chips under the 8271 and the floppy socket for bent or corroded pins. I think it's 74F38s for the logic chips IIRC

Thanks, I'll take a look.