Skip to main content

Back from the dead with a power supply repair: my BBC Micro Model B

A couple of weeks ago I plugged in my BBC Micro Model B and it went up in a plume of smoke.

Unfortunately, the 30 year old machine had undergone one of the most common failures. A capacitor that had been sitting there all these years failed, burst open, emitted smoke and leaked all over the circuit board.

Today, I repaired the damage. This blog post is for anyone who faces the same thing.

To fix the damage I replaced three capacitors that are likely to die over time (including the one that had burst) using a kit costing £2.70 from this site and a copy of the BBC Microcomputer Service Manual.

The kit has one electrolytic capacitor (which is polarized and has to be inserted the right way round) and two ordinary capacitors which can be inserted in any way. The three capacitors to be changed are marked C1, C2 and C9 on the circuit board. On my machine it was C2 that had failed.

The first step is to remove the power supply from its case. This is carefully described in the service manual (including two cable ties that need to be cut and replaced). Here's the supply without its case. I've marked the three capacitors to be replaced.

And here's the reverse of the circuit board (clearly hand soldered) and I've marked the six joints that have to be desoldered. I used my normal soldering iron and a solder sucker to remove all three capacitors.

And here's a shot of the three capacitors after they've been removed with a close up of the ruptured C2.

With the capacitors removed you can see the capacitor juice that had leaked onto the PCB. I cleaned it off with some alcohol and Q-tips. You can also see that C2 actually had four holes drilled for it. Two were used for the supplied capacitor and the other two are used in the repair as the new capacitor has leads that are slightly closer together (good of Acorn to plan 30 years ahead!).

Soldering the three in is not hard (make sure the electrolytic is the right way round). Here's my finished circuit board:

And the reverse side showing the three new capacitors in place:

And here are the two cable ties that need to be replaced. One holds the low voltage cables going to the main PCB in place (it's attached to the power supply casing), the other holds the mains cables in place (it's attached to the power switch).

And putting it all back inside the case... it works!

Comments

Glen Mackie said…
I've also just had this happen to me. But for some reason, the power switch seems to be reversed on my machine. i.e. 'On' turns the machine off and 'Off' turns it on. Do you have any idea why this might be?
Phil B said…
Hi Glen Mackie,
If the power is inverted it means the switch is in upside down (most likely replaced earlier in its life).
If the switch is a single pole then turn pull off the spade terminals and turn the switch upside down.
If its double pole then simply remove one of the AC leads and fit to its opposing end (if that makes sense!)

Best Regards

Phil Blythe (ex BBC Micro engineer from the eighties).
Unknown said…
It is quite difficult to fix broken power supply. My friend did it a few times but I failed. Have you ever tried to fix Dell PSU? Like this model for example http://hardware.nl/dell/2h35j.html? It would be interesting for me to know it.

Popular posts from this blog

Your last name contains invalid characters

My last name is "Graham-Cumming". But here's a typical form response when I enter it:


Does the web site have any idea how rude it is to claim that my last name contains invalid characters? Clearly not. What they actually meant is: our web site will not accept that hyphen in your last name. But do they say that? No, of course not. They decide to shove in my face the claim that there's something wrong with my name.

There's nothing wrong with my name, just as there's nothing wrong with someone whose first name is Jean-Marie, or someone whose last name is O'Reilly.

What is wrong is that way this is being handled. If the system can't cope with non-letters and spaces it needs to say that. How about the following error message:

Our system is unable to process last names that contain non-letters, please replace them with spaces.

Don't blame me for having a last name that your system doesn't like, whose fault is that? Saying "Your last name …

All the symmetrical watch faces (and code to generate them)

If you ever look at pictures of clocks and watches in advertising they are set to roughly 10:10 which is meant to be the most attractive (smiling!) position for the hands. They are actually set to 10:09.14 if the hands are truly symmetrical. CC BY 2.0image by Shinji
I wanted to know what all the possible symmetrical watch faces are and so I wrote some code using Processing. Here's the output (there's one watch face missing, 00:00 or 12:00, because it's very boring):



The key to writing this is to figure out the relationship between the hour and minute hands when the watch face is symmetrical. In an hour the minute hand moves through 360° and the hour hand moves through 30° (12 hours are shown on the watch face and 360/12 = 30).
The core loop inside the program is this:   for (int h = 0; h <= 12; h++) {
    float m = (360-30*float(h))*2/13;
    int s = round(60*(m-floor(m)));
    int col = h%6;
    int row = floor(h/6);
    draw_clock((r+f)*(2*col+1), (r+f)*(row*2+1), r, h, floor(m…

The Elevator Button Problem

User interface design is hard. It's hard because people perceive apparently simple things very differently. For example, take a look at this interface to an elevator:


From flickr

Now imagine the following situation. You are on the third floor of this building and you wish to go to the tenth. The elevator is on the fifth floor and there's an indicator that tells you where it is. Which button do you press?

Most people probably say: "press up" since they want to go up. Not long ago I watched someone do the opposite and questioned them about their behavior. They said: "well the elevator is on the fifth floor and I am on the third, so I want it to come down to me".

Much can be learnt about the design of user interfaces by considering this, apparently, simple interface. If you think about the elevator button problem you'll find that something so simple has hidden depths. How do people learn about elevator calling? What's the right amount of informati…