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Acorn BBC Micro B & B+ PSU Repair

Whilst powering up a newly aquired BBC Micro model B+, I experienced what many collectors of retro computers have already experienced: The machine suddenly emitted misty smoke and the unmistakable smell of a ruptured capacitor (pungent burned sweetish electrical smell).
The machine continued to operate despite this, so I could only assume it was one of the small number of filt capacitors which had gone. Usually it's C1, C2 or C9 (or a combination of these).

So! It was time to open her up and take a look. I've documented the experience as a guide so that if you need to try this yourself then hopefully my experience will be of some help (click images to enlarge). If you do try this then I highly recommend that you photograph each stage prior to unplugging or remoing a component using a digial camera so that you can easily refer back to how things were if you get confused during reassembly. Please note that I took some of the photos below on both my computer table (black) and my mobile computer desk (wood).

The following will work for both a model B and B+, but there will be differences for the BBC Master & Master Compact.


Although the repair doesn't involve working on live mains circuits, it does deal with capacitors which have the capability
to store mains voltage and which, if you're unlucky and incautious, may cause you serious harm or even death.
I cannot be held responsible if this happens. I also cannot be held responsible if you botch the repair and permantently damage
your PSU or anything else.
The bottom line: You do the following at your own risk.

A little bit of safety [IMPORTANT!]

The following is a short list of safety precautions. It's well worth reading as it might save your life.

! Don't work on live mains. Unless you are an experienced and qualified electrician please do not plug in the power cable whilst working on the PSU. See the above warning regarding serious harm or death. 'nuff said.
! Using a multimeter. If you decide to test the PSU using a multimeter please be very sure to use a high quality mains-certified meter. The small/cheap meters (generally costing under £100) will not be properly constructed for this and could be permantely damaged or even explode (causing you harm) if exposed to mains voltage. This EEV Blog video covers multimeters and safety (as well as being a good buying guide). You can use the cheaper/non-mains multimeters on the +5V and -5V cables going from the PSU to the mainboard though.
! Don't solder live circuits. This seems a bit obvious, but please don't ever solder live circuits for obvious reasons.


You'll need:
! a philips screw driver
! a half decent soldering iron using a flat screwdriver type tip (don't use a pointed tip)
! some solder (60/40, leaded or leadless, 0.5mm diameter or smaller if possible)
! solderbraid/wick and if you can manage it a solder sucker (not essential)
! wire cutters
! a clear work bench/table
! felt tip pen or pencil

! Replacement capacitors:
C1 (0.1uF (100nF) X2 class capacitor) @ 250VAC (or higher)
C2 (0.01uF (10nF) X2 class capacitor) @ 250VAC (or higher)
C9 (220uF electrolitic capacitor) @ 10V
I tend to pick mine up at Farnell or Mouser. I don't recommend eBay for anything involving mains electronics (components or tools) unles you happen to know that the eBay shop you're using is very good and reliable: You have no idea generally speaking where the components/tools have come from and if they're safe to use or not.


Now that's out of the way, unplug the mains power cable from the wall socket and then switch the power switch to the on position on the computer for 5 seconds. Repeat that a few times. This will discharge any power which may still be lurking in the capacitors. If you want to be ultra safe then leave it like off and unplugged for 30-60 mins.

Before proceeding, make a space to one side to put any spare parts. I cannot tell you then number of times that I've taken something apart and then wondered where the bl**dy screws had gone, or module/component X, etc.. It also stops you kneeling, sitting or standing on them which saves you a bit of pain and potentially saves damaging things.

Open the computer & remove the keyboard

Remove the screws from the underside and rear of the chassis. Note that one set of screws anchor the keyboard and that on most chassis' these should unscrew fine:

RemoveScrews RemoveScrews2

However, on some chassis' you'll have remove the two larger screws, remove the lid, then put an adjustable spanner or pliers on the nuts that the small screws go into to hold them in place while you unscrew the screws underneath::
keyboardscrews keyboardscrews2

Note: Removing the keyboard isn't absolutely necessary, but can make things a bit easier when removing the PSU.
Remove the lid (which should come away easily) to reveal the mainboard. The PSU can be located to the left.
Ease the keyboard connector off (don't yank it!) and the two-wire speaker connection from the mainboard. Please note that on the BBC model B the speaker connection is along the 'bottom edge' of the PCB underneath the keyboard. On the BBC model B+ it's between the PSU and the keyboard connector.

speaker_unpluggedBplus speaker_unpluggedB

Remove the three sets of power connections which go from the PSU to the mainboard. I highly recommend taking a photo of exactly how these are connected before removal. It's essential that they go back on correctly later.


Turn the computer the right side up and put the keyboard off to one side, turn the computer upside down and remove the three screws holding the PSU to the chassis.
Remove the PSU from the computer chassis. You'll need to feed the power cable through the power switch hole and negotiate the plug through as well. It does come out, but is just a little awkward.

undersidescrews PSU_removed

Remove the three screws anchoring the PCB to the PSU chassis and also unscrew and detach both earth wires. Each earth wire is at the opposite end of the PSU from the other, the cables being green and yellow (be carefull to keep hold of the washers).

PSU_topside_chassisscrews earthconnection1 earthconnection2

Remove the power switch. To do this you'll need to cut any cable ties which are anchored to the switch (don't cut any wires!) and the remove the brown and the blue power connections. I highly recommend taking a photo of exactly how these are connected before removal. It's essential that they go back on correctly later.
Push the power switch through the chassis and out of the 'back'. You'll first need to force the retaining plastic clips which are above and below the switch against the body of the switch. I found a screw driver helped a lot, but it's still quite fiddly.

powerswitchremoval powerswitchremoved

Now for another fiddly bit: Gently ease the 6 pin auxillary power port (used mainly by floppy drives) into the the PSU, through the chassis. You'll need to hold the side clips on the port in and be careful of the electrolitic capacitor behind it. Once the port is free, carefully lift the PSU out of its metal chassis.

auxpowerport PSUoutsidechassis

Now the fun bit. We need to determine which capacitor has gone south (died). In the vast amount of cases it's C1, C2 or C9. At this stage you might just want to replace all three considring that if one has gone, then the rest might possibly go soonish (they are now roughly 30 years old!). Here are the specs on each:

Component ID Value Notes
C1 0.1uF (100nF) @ 250VAC (or higher) Must be an X2 class EMI suppression capacitor for mains voltage. Not polarised.
C2 0.01uF (10nF) @ 250VAC (or higher) Must be an X2 class EMI suppression capacitor for mains voltage. Not polarised.
C9 220uF @ 10V Electrolitic. Polarised.

Locating these is easy as each is marked next to it on the top side of the PCB. If you see one with brownish gunk around it you've just located the one which went pop !! If you do spot it use a cotton bud to mop up as much of the gunk as possible once the capacitor has been removed. If you leave it there then it'll just keep smelling your room out when in use. Now, the best way to proceed is to locate each of the above capacitors then mark the legs on the underside (solder side) of the PCB so that you don't accidentally desolder the wrong component(s).

Note that C1 and C2 are not polarised, which means that you can fit them any way around that you wish.
C9, however, is polarised and you MUST get this the right way around. If you don't it will probably violently explode when the power is turned on. The top side of the PCB normally has a plus ('+') sign marked next to the positive leg hole and the capacitor itself normally has a minus ('-') sign next to the negative leg (so don't put the negative leg in the positive hole):

capacitor_negative_leg C9_position C9_fitted

Position the PSU upside down so that you can see the solder side and make sure it's stable (doesn't slide about). Heat up your soldering iron and use it and the solder sucker & solderbraid to remove the solder around each leg. When done, removed the capacitor, fit the new one and solder each leg.
Clip the legs of the capacitor so that they are as short as the other components on the PCB using your wire cutters
Repeat this for each capacitor. Here's a picture of the underside of my PSU PCB after I had finished (and before I cleaned the residue away - so it looks a little messy!):


The next stage is to double check your work. Look very carefully at each leg of each capacitor and ensure that the solder joint is a good one. Also, look out for any solder bridges that you may have introduced. If you find a poor solder joint then either reflow the joint and add a little bit of solder or simply remove the sodler and redo that joint. If you discover any solder bridges that you've accidentally created use your solder braid to remove them.

Fit the PSU back together, reversing the above instructions. Pay attention to this as reconnecting things such as the mains live and neutral connections the wrong way around to the on/off switch can be dangerous and damage or destroy your PSU. Also, be very sure to reconnect the yellow & green earthing cables.

One test that you could try (if you have a multimeter) is to check the three +5V (red & black cabling) and the -5V (purple cabling) connections before connecting them to the mainboard to ensure that you get the right voltage.

When all is re-assembled, plug in your BBC and turn it on. Hopefully you'll get that great 'Burp-beep' and a display.

Here's the results of my repair:

As you can see, C2 was the culprit


Well, that's it. I hope that if you follow this guide to repair your own that it goes well.
All the best !

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