Metal Cased BBC B Micro

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1024MAK
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Metal Cased BBC B Micro

Post by 1024MAK » Sun Jan 29, 2017 3:03 pm

Metal Cased BBC B Micro

I received this Beeb from a kind forum member. Details of where it came from are in this thread.

I was busy with work and real life in the run up to Christmas and over the holiday period, so have only recently had a chance to look at it.

Due to the number of wires in it, the first thing was to document it by taking lots of 'photos just in case a wire freed itself!

All the 'photos (uncompressed versions) are here.

After taking the 'photos, I carefully noted what was connected where and made notes.

Then I disconnected the PSU. After the normal disassembly, it was possible to see that the 100nF X2 capacitor had started blowing apart. So I desoldered it. Thankfully it had not leaked too much of its innards over the PCB and very little over the nearby heatsink. After a clean up of the area, I soldered in a brand new 100nF X2 capacitor. I also renewed the 10nF X2 capacitor and also C9 (220uF).

I then reassembled the PSU and refitted it to the Beeb.

Now I turned my attention to the home made ROM RAM board and the associated wiring and connections. I traced out what was connected to what. Then I studied the PCB. I have reverse engineered it, so now know how it should work.

It takes the outputs from the sideways ROM selection latch chip on the Beeb main board, then decodes these four digital signal lines to allow the eight ROM/RAM sockets to be mapped to the top eight ROM slots.

In addition, it picks up four outputs from the VIA chip used for the user port, outputs PB0, PB1, PB2 and PB3. The normal configuration of the board is for the number on these lines to be used to select the RAM chip when the CPU writes to the sideways area. Once I have drawn the schematic in a schematic capture program, I'll post it in this thread.

After finishing the reinstallation and connection, it was time to power up!

I got the burr-beep :D
But, the picture on the TV used as a monitor (using a RGB to SCART lead) was breaking up, it looked like a problem with either the video processor or the sync circuitry. So I checked the sync polarity link/jumper S31. It was set East instead of West. After sorting that, I had a good picture :D

Next, I put everything back in the case. Yep, still works :-)

More soon...

Mark
For a "Complete BBC Games Archive" visit www.bbcmicro.co.uk NOW!
BeebWiki‬ - for answers to many questions...

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trixster
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Re: Metal Cased BBC B Micro

Post by trixster » Sun Jan 29, 2017 3:12 pm

Tremendous! Well done Mark, I knew you'd get her working again!
A3020 | A3000 | A420/1 | BBC B + 128K RAM/ROM + 20K Shadow + Pi0 + VideoNuLA
Master Turbo + DC + BeebSID | Atom | A4000 060 | A3000 060 | A1200 060 | A500
Atari Falcon 060 | Saturn | PS1 | SNES | CPC6128 | C64 | 3DO | MD | Jaguar

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1024MAK
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Re: Metal Cased BBC B Micro

Post by 1024MAK » Sun Jan 29, 2017 3:15 pm

See also this event thread :wink:

Mark
For a "Complete BBC Games Archive" visit www.bbcmicro.co.uk NOW!
BeebWiki‬ - for answers to many questions...

DigitalDunc
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Re: Metal Cased BBC B Micro

Post by DigitalDunc » Sun Jan 29, 2017 3:19 pm

Those pictures are rather interesting! What extra tricks does this beeb do, since there's so much tinker under the hood?

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sweh
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Re: Metal Cased BBC B Micro

Post by sweh » Tue Jan 31, 2017 12:58 am

Using the user port to select the RAM bank is the old trick used by the original Solidisk SWR boards and allowed easy loading of ROMS

Code: Select all

   
?&FE62=15
?&FE60=bank
*LOAD ROMIMG 8000
Rgds
Stephen

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