Belated BBC B

Any 8bit repairs, images, cool things, upgrades etc.
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stephen_usher
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Re: Belated BBC B

Post by stephen_usher »

:dualthumbup:
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.
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rubber_jonnie
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Re: Belated BBC B

Post by rubber_jonnie »

Aaaand back in the saddle!!

IMG_20210710_141944.jpg
IMG_20210710_141944.jpg (253.61 KiB) Viewed 1992 times

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.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
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rubber_jonnie
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Re: Belated BBC B

Post by 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!!
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
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rubber_jonnie
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Re: Belated BBC B

Post by 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.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
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stephen_usher
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Joined: Mon Nov 13, 2017 7:19 pm
Location: Oxford, UK.
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Re: Belated BBC B

Post by stephen_usher »

And the continuity test show nothing wrong?
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.
User avatar
rubber_jonnie
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Posts: 10464
Joined: Thu Aug 17, 2017 7:40 pm
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Re: Belated BBC B

Post by rubber_jonnie »

stephen_usher wrote: Sun Jul 11, 2021 6:10 pm 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
BBC_No3_KBD_Bodge.jpeg (106.67 KiB) Viewed 1953 times

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.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
User avatar
stephen_usher
Posts: 5578
Joined: Mon Nov 13, 2017 7:19 pm
Location: Oxford, UK.
Contact:

Re: Belated BBC B

Post by stephen_usher »

:dualthumbup:
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.
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