The English Patient - A Beeb of My Own - BBC Model B Worklog

Discussion in 'Retro & Arcade Worklogs' started by aXLe, May 23, 2013.

  1. aXLe

    aXLe Member

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    The BBC Microcomputer was a computer designed and built for the BBC by Acorn in the very early eighties. The BBC required a computer that they could use in a new computer TV show that they were planning, and they approached a few vendors. Acorn threw the BBC design together in a week, basing it off another machine that they had been working on (the Proton). They demo'ed it to the BBC, and the rest is history.

    They sold around 1.5 million units from 1981 through till 1994.

    The model B runs a 2MHz MOS 6502 cpu, with 32KB of RAM. It was capable of 640 x 256 res graphics with 8 colours and 4 channel audio.

    My earliest memory of the BBC Micro is from the very early 80's when staying at a friends holiday shack. I remember my friend and I heading out in the morning for a days fishing whilst his cousin stayed behind, beavering away at the keyboard of a BBC Micro. He was still working on it in the evening when we returned. It's the row of red function keys that really stands out in my memory of that machine. A few years later, when I started using the schools Apple ]['s and purchased my own Apple //e, the BBC was more or less forgotten.

    My interest in the machine was rekindled recently after refurbishing the power supply in Xanex's BBC Micro (here), and thanks to forum member rade, I have managed to aquire a BBC of my own.


    The English patient :


    [​IMG]


    Rade received this as part of a large haul of retro computers, mostly Commodores. It is a little grubby, but absolutely perfect for a worklog - the before and after photos should show a marked improvement.

    My planned schedule of works for this BBC Model B Micro :
    • Refurbish the PSU, replacing all AC and electrolytic capacitors
    • Give the whole unit a good clean
    • Retrobrite the case to bring it back to the original colour (or close to)
    • Swap out some of the ROM's - reprogramming them as required
    • Install an MMC based disk solution so as floppies are not required
    • Enjoy some retro computing

    Should be fun!
     
    Last edited: Oct 21, 2019
  2. callan

    callan Member

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    I bought a chunk of the commodore stuff from rade, too.
    Worked on the 1571 disk drive today, and after a complete PSU rebuild (it was STUFFED!) and cleanout of the rest of the drive, it's working perfectly!!.

    The BEEB - the ultimate "Elite" game machine!

    Callan
     
  3. OP
    OP
    aXLe

    aXLe Member

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    I started work on the Beeb - PSU capacitor replacement first up.

    Disassembly is pretty easy - just 4 screws (2 in back, 2 in bottom) to remove the top of the case, plus another 3 in the bottom for the PSU :


    [​IMG]


    Then the top just lifts off :


    [​IMG]


    This was a pretty interesting step as it revealed that the motherboard was a US 223,000 Issue 1 board - quite a different layout to the UK ones. Also the ground wires on the PSU are large braided cables that connect back to tabs on the PSU housing. Again quite different to the UK setup where the ground wires connect back to the PCB. It turns out that this machine was assembled in Hong Kong along with around 49,999 others that were destined for the US market. Apple and others had that market pretty well wrapped up though, so the Beeb wasnt a great seller there, and many of the units were returned to UK and sold there and in other countries.

    2 bolts secure the keyboard :


    [​IMG]


    Unplug the speaker and the ribbon cable for the keyboard and it lifts away :


    [​IMG]

    [​IMG]


    The PSU provides only a +5V rail to the motherboard. There is an auxilary connector that actually comes out on the underside of the Beeb and that has +12V, +5V and -5V as well, but the motherboard just has the +5V requirement. As the motherboard draws a few amps, Acorn used multiple connection points on the board so to reduce the required track size on the PCB - makes for a kind of messy set up with the wires going to multiple points, but does the trick!

    With the PSU unscrewed (when removing the case screws earlier), and with the motherboard power connectors removed, it just lifts out :


    [​IMG]

    [​IMG]


    A nice view of the motherboard :


    [​IMG]


    Note the 5 ROM sockets. The Beeb is pretty interesting in that of the 64KB address space of the MOS 6502 CPU, 32KB is mapped to RAM, and 32KB to ROM. A 16KB OS ROM would be installed in the leftmost socket, leaving 4 sockets for user ROM's. Through the OS you would select which of the user ROM's (known as Sideways ROM's) was active (the OS would enable the chip select on one of the 4 Sideways ROM sockets, and that ROM would therefore be mapped into the 16KB address space reserved for it). This means that you could purchase a word processor app for example on a ROM, saving you from loading it from disk, and also from taking up valuable RAM. A popular add-on was a Sideways ROM expansion board that plugged into one of the 4 Sideways ROM socket and provided the chip selects and sockets for up to another 14 ROM's! Of course only one of the Sideways ROM's would be active at any one time (in addition to the OS ROM).

    [​IMG]


    After a quick visual of the PSU I could see that the X2 caps were still in once piece, so I took the risk of testing the PSU before removing and replacing the caps :


    [​IMG]


    Pretty damn close to 5V there!

    The PCB was then removed from the housing and I also labelled the tops of the caps so I could identify their original positions when testing them later. I then started to remove all electrolytic and X2 caps :


    [​IMG]


    The PSU manufacturer bent the leads over on the back of the pcb and allowed a large solder pad for a better mechanical connection :


    [​IMG]


    This makes it a bit more painful to desolder! I used a combination of applying heat while carefully prising the lead straight, or using solder wick, then straightening the lead so I could get my desoldering tool onto it :


    [​IMG]


    [​IMG]


    All done! The naked PCB :


    [​IMG]


    The crazing in the plastic of the X2 caps is seen here - 30 years old :


    [​IMG]


    This PSU used a different manufacturer for the 250V 100uF caps to the last one I did (for Xanex) - Marcon. A couple of the caps also had heatshrink over them :

    [​IMG]


    Next up was measuring all the various caps for capacitance and ESR to see how well they've stood up to the passage of time.

    One of the 25V 100uF caps had fairly high ESR, and I'd call this one bad :

    [​IMG]

    The same cap was down a little on capacitance too :


    [​IMG]

    In the end most were still pretty good - impressive for a psu that is 30 years old.


    [​IMG]


    [​IMG]

    I sourced all the replacement capacitors from Panasonic.

    The X2 caps are Panasonic ECQUL series. For the 250V filter caps on the primary side I ordered Panasonic EE series, and all the rest are low-ESR Panasonic FR series.

    For all of the FR series caps I have gone way up in the voltage specs in order to get a large diameter body and suitable lead spacing for the PCB. In general, the higher voltage large diameter caps have higher operating hour ratings, which is a plus, but If I were to stick with the same voltage rating as the originals then many of these modern caps would have only a 3.5mm lead spacing (8mm diameter body). Out of interest, I measured the capacitance and ESR of the new caps too :

    [​IMG]


    Replacements fitted :


    [​IMG]

    [​IMG]


    Note the link on the PSU board to select 115V or 230V - all components are identical - just solder a jumper on the back for 115V supply.



    [​IMG]


    Now to put it back together and test!
     
    Last edited: Oct 21, 2019
  4. aloysius

    aloysius Member

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    The different power supply setup and location of the ROMs to mine is interesting. I think that makes yours the model produced for the US market? Don't know the machine well enough to say for sure.
     
  5. OP
    OP
    aXLe

    aXLe Member

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    Yep - I'm still editing the post, but this one that I have is assembled in Hong Kong, and was indeed originally designated for the US market :)

    The German one was the same too.

    Apparently they never really took off over there (no doubt thanks to Steve Jobs' marketing skills with Apple!), and many were brought back to the UK and sold there and to other countries.
     
    Last edited: May 28, 2013
  6. aloysius

    aloysius Member

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    Interestingly though it has no Econet...My understanding was that all the US specced models had that.
     
  7. OP
    OP
    aXLe

    aXLe Member

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    Im guessing that some of the HK assembled units were built to standard spec for local market and possibly NZ and Australia too, but just a guess.

    Hard to find info on the HK assembled ones!
     
  8. badmofo

    badmofo Member

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    Mate for some reason I am fascinated by your PSU refurbs! Thanks for all the great pics, I'm looking forward to seeing this thing cleaned up and running. I'm particularly interested in how the retrobrighting goes, I've never had much luck with that.
     
  9. OP
    OP
    aXLe

    aXLe Member

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    I reassembled the PSU last night - looks good as new :)


    [​IMG]


    Tested it again, loading it with a 2R2 10W resistor - happy with that :


    [​IMG]


    I then dropped it into the BBC for a proper test. Now the BBC uses a BNC connector for video plus an RCA style connector for UHF. Not having used one of these before, and not having a suitable BNC connector to RCA that I could use to plug it into my TV, I rigged up a very temporary video connection and turned it on.

    Booooop Beep!

    Sounded good, but what the?!


    [​IMG]


    Quite apart from the shocking video quality (which I will look into later!), it was readily apparent that :

    1. This computer belonged to the Victorian Library at some point (most likely the original owners)
    2. Custom ROM's had been installed

    This is what I was expecting :


    [​IMG]


    Hmmmm. The machine has 4 ROM's installed, and none are very well labelled. I tried pulling out the last 2 ROM's :


    [​IMG]


    And after re-testing, there was no change. I knew that the 2 ROM's left in the machine would have to be the OS and Basic ROM's, so it seems that the OS ROM is a custom job.

    Nothing for it put to pull them all out, erase them and reprogram them :D


    [​IMG]


    Peeling off the labels reveals the windows in the EPROM's :


    [​IMG]


    Two different types of EPROM have been used : NEC 128kbit (16KB) and Hitachi 256kbit (32KB). I'm pretty sure the BBC will only address 16KB of the ROM so I'm guessing that half of the 256kbit ROM is blank.

    Nice close up of the window through which you can see the bonding wires to the silicon die :


    [​IMG]


    I dragged out my rather ancient (but trusty) EPROM eraser, set it for 30 minutes erase time, and dropped the chips in the drawer. This thing could do with a clean and retrobrite too!


    [​IMG]


    EPROM's like these are erased with UV light - the eraser basically has 3 small UV tubes in there and a timer. Here is a sneak peak inside with the drawer open a little :


    [​IMG]


    Once the 30 minute erasure cycle had been completed, I dragged out my equally ancient EPROM programmer. This thing has DOS based software (I must get a new one!!), so I have an old Dell laptop that I use specifically for this purpose.

    Old School :


    [​IMG]


    [​IMG]


    Through the software I selected the manufacturer and model of EPROM, and did a blank test to confirm that they had been properly erased - firstly each of the 3 NEC EPROM's :


    [​IMG]


    And then the Hitachi one :


    [​IMG]


    All ROM's are now confirmed blank and awaiting the next step - reprogramming. I've downloaded the OS 1.2 ROM and the Basic II ROM from here, so I'll burn these 2 ROMS in my next update, and re-test!
     
    Last edited: Oct 21, 2019
  10. aloysius

    aloysius Member

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    Did you have a good play with the library rom? Was the wordprocessor and spreadsheet just View attached to the menu system?
     
  11. OP
    OP
    aXLe

    aXLe Member

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    Actually no I didn't, and I'm kicking myself for not backing it up before erasing it, but ultimately its not what I want in my Beeb so it had to go :D

    It could explain the size of the ROM though - 32KB rather than the usual 16KB - maybe all those apps were shoehorned into the one ROM. Hmmmm. I'll have to do some reading now to see what size ROM the BBC can address.
     
  12. aloysius

    aloysius Member

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    I was going to say did you back it up for nostalgia reasons but after reading your post I decided to not ask in case you didn't and regretted it :p Seems my assumption was right :p
     
  13. rade

    rade Member

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    Good to see the progress so far! Didn't realise it was so straightforward to flash/reprogram the roms like that
     
  14. OP
    OP
    aXLe

    aXLe Member

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    Tonight I programmed 2 of the 16K EPROM's, one with OS 1.20, and the other with BASIC 2 using the ROM images I downloaded earlier.


    [​IMG]


    [​IMG]


    After labelling them so as not to mix them up...


    [​IMG]


    ... I popped them back into the motherboard :


    [​IMG]


    With fingers crossed, I plugged it in and fired it up.

    Boooop Beep!

    Success!


    [​IMG]


    [​IMG]


    So now I have a Beeb with stock ROM's and 3 empty sockets for others. Next up will be a full stripdown of the case (remove the motherboard) and give it a good scrub while I work out where I can get the ingredients I need to mix up some Retrobrite :D I'll strip and clean the keyboard down too - hopefully get the cleaning done over the weekend and the Retrobrite some time later.

    Then I'll need a solid state storage system so I can load up some software and use it!
     
    Last edited: Oct 21, 2019
  15. OP
    OP
    aXLe

    aXLe Member

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    I spent a little bit of time over the weekend cleaning the keyboard. It was very dusty when I received it :


    [​IMG]

    [​IMG]


    Nice to see that the keys are double shot injection moulded - not seen so much these days due to the expense, but was very common during late 70's and eighties.


    [​IMG]


    Every key requires a separate mould, as the white lettering is injected firstly using one mould, then the reset of the key injection moulded around it using another. The great thing is that there is no chance of wearing the text off when cleaning the keys.


    I removed all the keycaps, exposing the key switches that are clipped into a painted steel deck:


    [​IMG]


    Then into a warm soapy bath for a bit of a light scrub:


    [​IMG]


    While they were drying off, the deck was dusted down:


    [​IMG]


    And the keycaps put back on, squeaky clean:


    [​IMG]


    Quite a contrast:


    [​IMG]


    I gave the top cover a quick clean too - getting there!


    [​IMG]
     
    Last edited: Oct 21, 2019
  16. andyroo54

    andyroo54 Member

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    Wow this is awesome!

    My late uncle actually wrote a few programs for the BBC Micro in the eighties!

    http://trove.nla.gov.au/work/19056610?versionId=22378380

    I only found out recently, I've been using it on a BBC micro emulator. It's been uploaded to a ton of emulator sites all over the net, he actually thought it was lost years ago, so he was really happy when I showed him it working.

    You can download it from tons of sites, but here is one below.

    http://nitroroms.com/show/file-info/L19535/Acorn_BBC_Micro/Silicon Voyager, The (1984)(John Kingston)[bootfile].ssd.html
     
  17. Ohmigosh

    Ohmigosh Member

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    Awesome stuff mate. I owned one of these back in the early 80's. My first compy. Bought it with savings from my first job. Got an amber screen and a disk drive with it. Ahhh the days of typing in assembler from magazine listings! Interestingly enough on my machine there was some interference to the on board speaker that allowed you to debug code by sound. When you had an infinite loop you hear the pulsing through the speaker. Good days. Loved playing Elite and Aviator. :thumbup:
     
  18. OP
    OP
    aXLe

    aXLe Member

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    I've just ordered the MMC disk drive solution (which uses an SD card for storage - 128MB SD card stores the equivalent of more than 500 floppies!) so I'll post an update when that arrives :)

    I'm still trying to track down some ingredients to retrobrite the case, which is the main task to complete to bring it back to looking new again.
     
  19. flu!d

    flu!d KDE Neon 5.16

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    They were a good PC in the day, they had a far better OS than the Commodore's of the time, they even had their own serial networking system using 5 pin DIN style connectors and a central server - I remember working out exploits to hack into various accounts at school.

    * I am [username] [password].
    * dir$

    Ha! Some things you just don't forget.

    Actually wouldn't mind a BBC as well as the Econet server to do up and have a bit of a play.
     
  20. Flamin Joe

    Flamin Joe Member

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    Is that the HxC Floppy Emulator? I've been looking into one of those myself for an Amiga 500 restoration project I plan on starting up soon. Looks great but it's pricey!

    Interested to hear how you go as I'm planning to do the same.
     

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