Return to Grannies Garden, Restoring a BBC Micro

Discussion in 'Retro & Arcade' started by aloysius, Apr 28, 2013.

  1. aloysius

    aloysius Member

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    I was given a BBC Micro Model B about 3 years ago which upon testing made a nasty crackling noise and released the magic smoke. All the while it was still displaying a prompt on the screen so I ripped the power as quickly as possible and shelved it to investigate later. Now is later :D

    After talking to aXLe on here he has kindly offered to give the power supply a once over for me replacing any caps that are over the hill. I have photographed the dismantling and will document the process of getting this beautiful old machine running again.

    Apologies for camera phone pics

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    Awesome how software was loaded via ROM chips. What you can see here for those that aren't familiar with the system is the equivalent of Microsoft Office and a couple of other programs. Very cool that the daughter board they are mounted on was manufactured in Adelaide :D


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    The poor 33 year old power supply unit.
     
  2. OP
    OP
    aloysius

    aloysius Member

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    Click to view full size!

    While I am at it here's a pic of another system I was given at the same time as the Beeb. Acorn 3000. This one has video issues, and I may get around to fixing it up in the future, but the beeb has far fonder memories for me from childhood so it has priority.
     
  3. BeanerSA

    BeanerSA Member

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    My high school was part of pilot program back in the 80's. We had a room with 10 BBC Micros in it, with a server against the back wall.

    It was my first experience with personal computing on a larger scale. Grannies Garden was a must at lunchtime.
     
  4. aXLe

    aXLe Member

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    good stuff - I'm looking forward to pics of it running again once we've sorted the PSU out :)
     
  5. kbekus

    kbekus Member

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    awesome, then you'll be able to play the original and best version of Elite!
     
  6. OP
    OP
    aloysius

    aloysius Member

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    You read my mind :D
     
  7. aXLe

    aXLe Member

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    I received the PSU from Xanex last week. I thought the Rice Bubbles box, specifically the "Snap", "Crackle" and "Pop!" was a perfect packaging choice for this old PSU :D


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    The patient - 33 years old - complains of smoke leaking out the back side :


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    A quick visual inspection confirmed that one of the X2 AC mains suppression capacitors had failed, which is very very common in these old supplies. In this case it was the 0.1uF X2 :


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    After removing the PCB from the casing I noted that a couple of the electrolytics show signs of internal pressure - none had actually vented from the top, but a couple had slight bulges on the tops that could be felt :


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    The plan was to replace all the electrolytics along with both X2 capacitors. I was curious though as to how well they all had aged ie. what their current capacitance is, and also their ESR (Equivalent Series Resistance) as most of the secondary side caps are Low ESR rated.

    This was of particular interest to me as I've seen a number of BBC repair kits on the internet that sell you just the X2 caps and the startup electrolytic cap (C9) - basically just the 3 caps that are the main cause of failure in these PSU's.

    However, it has has always been my opinion that ALL the electrolytics should be replaced as they generally have a limited lifetime anyways (in the order of 1000 to 5000 hours), plus they don't always age gracefully. This means measuring each one and recording the values. So first up, all X2 and electrolytic capacitors were removed from the board :


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    The failed X2 cap - the plastic cases get very brittle with age, but these things have a very hard life anyway - living their life at 240V across the AC line. This one had blown out the bottom of the case. By regulation, they are built to be self healing to a degree, and if they do fail then they are designed not to burn :


    [​IMG]


    Next, all the electrolytics were measured for ESR and Capacitance. This was interesting as the startup cap did show higher ESR that it should have (around 0.4ohms), though its capacitance was in spec, however one of the 1000uF caps was shown to be very high ESR (0.641ohms) :


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    This same 1000uF cap was down to a capacitance of 675uF which is well outside the tolerance :


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    Thus, even though it was found that most of the caps were still in spec (see table below), which is a real credit to the OEM (Nippon Chemi-con), I'm sticking with my opinion that if one is going to go to the effort, then all electrolytics should be replaced.

    Here is the board layout, and my table of capacitors to be replaced, including measured capacitance and ESR :


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    Next comes the sourcing of the replacements. For the C2 X2 cap, 2 lead spacings are available (15mm and 20mm) which is handy - the rest just provide a single lead spacing so I have to match the new components up as best as I can.


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    Also of note is that during original manufacture, each solder joint is inspected and QC'd with a red marker


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    For reference, PSU schematic is shown below :


    [​IMG]
     
    Last edited: May 7, 2013
  8. aXLe

    aXLe Member

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    The following replacement components have been ordered - all Panasonics :


    [​IMG]


    The X2 caps are Panasonic ECQUL series. For the 250V filter caps on the primary side I have 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).

    When they arrive, I'll update this post with the installation pics :D
     
    Last edited: May 7, 2013
  9. OP
    OP
    aloysius

    aloysius Member

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    Awesome work, I can almost hear the Booop Beep already :D
     
  10. rainwulf

    rainwulf Member

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    Whats the feedback element? On most modern SMPS they either use an optocouple or a signal transformer.
     
  11. aXLe

    aXLe Member

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    Yep - in that schematic you can see transformer T3 with a dotted line between primary and secondary sides
     
  12. Master PooBaa

    Master PooBaa Member

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    OMG Boop Beep!

    I grew up with a BBC Micro Model B. Ahh the memories.
    Rev's, Speech, Elite, REPTON!

    Scuse me all, ill just be off on a stroll down memory lane.
    Back later :)
     
  13. elvis

    elvis Old school old fool

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    Nice work fellas. :thumbup:
     
  14. kbekus

    kbekus Member

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    Yah for Ocean and Acornsoft!
     
  15. aXLe

    aXLe Member

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    I received some of the caps today, but the rest are coming from another warehouse and may not be here till this time next week.

    Once I have all the caps I'll post some comparison shots with new for old before I fit them :)
     
  16. aXLe

    aXLe Member

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    Finally today I received the last of the caps :)

    Here is a photo of old cap (left) versus new cap (right). The size is generally pretty close as I specced much higher voltage replacements for most parts so as to maintain the lead spacing.


    [​IMG]


    The progress in technology is most noticeable with the 250V 100uF caps (like for like replacement), and the 2200uF cap of which the 35V replacement part is half the size of the original 16V part. Also the X2 replacements are much smaller than the originals.


    [​IMG]


    Next up will be fitting and initial testing before shipping the supply back to Xanex :D
     
    Last edited: May 17, 2013
  17. OP
    OP
    aloysius

    aloysius Member

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    Woo! Anticipation is killing me.
     
  18. aXLe

    aXLe Member

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    All done :D

    Capacitors fitted :


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    Reassembled :


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    And Tested OK :


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    For testing, I used a 2R2 10W resisitor to load it up. I tested all the various output voltages on the aux power connector and all good. I'll be sending it Express to Xanex on Monday so hopefully he will receive it Tuesday!
     
  19. COMman

    COMman Member

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    wow, cannot wait to see that up and running :) nice work..
     
  20. OP
    OP
    aloysius

    aloysius Member

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    Thank you so much for your amazing work aXLe. So excited to get this back. Will post pics and possibly even a vid to share the beautiful boop beep!
     

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