Category Archives: Uncategorized

Solidlights seal upgrade

It’s been nearly five years since I sold the last Solidlights product, but there’s still a steady stream of requests for spare parts, servicing and upgrades from loyal customers. Over the production run of Solidlights I made various improvements, especially to the water sealing. This turned out to be particularly important on the 1203D dynamo-powered light: who’d have guessed that commuters and tourers would give their lights a much tougher time than mountain bikers? Well, they did.

Nowadays I routinely fit the upgraded water seals to every light that comes in to the workshop. Because Solidlights customers include many enthusiastic DIYers, I thought it was about time I published details of how the upgrade is done, so here they are.

Parts for the upgrade will be available in the Solidlights on-line shop.

Seal Upgrade Instructions

These instructions describe how to upgrade the seals on a Solidlights 1203D, 1203DR or XB2 to the latest production specification, or replace damaged ones.


The following parts are required:

  • 1m silicone rubber sealing strip 0.5mm, part number 30092
  • 8 off O-ring 1.8” Viton, part number 30072
  • 8 off screw black stainless self-tapping #6, part number 30071

Tools Required

  • Cross-headed screwdriver (Posidriv No.2 or equivalent)
  • Marker pen
  • Fine rectangular engineer’s file
  • Vice
  • Jeweller’s screwdriver, tweezers or similar for scraping old seals away
  • Flush wire cutters or sharp craft knife


Remove both side panels from the light by unscrewing all eight of the screws.


Discard the old screws and o-rings, if fitted, but keep the two rectangular gaskets. One side panel will remain attached to the light by the wiring to the button. Take great care not to damage the wiring by pulling or bending it excessively.


Gently pull apart the front of the light by hand and slide out the front window.


Check what type of seals are fitted at the top and bottom of the front window.

If they are the pale yellow-coloured silicone rubber type, no modifications are necessary. Simply remove and discard the old seals ready to fit new ones later.


If the seals are the black neoprene foam type, they may have been crushed so much that they are almost invisible. The seals are self-adhesive and may be firmly stuck to the inside of the grooves in the light.


Carefully scrape away all traces of the old seals using a tiny screwdriver or tweezers. Gentle heat may help.


Modifying the front window

If the light had the black neoprene foam seals, it will be necessary to modify the front window. The new seals take up more room, so the window needs to be about 1mm smaller.


Use the marker pen to mark a line about 1mm thick along one long edge of the front window.


Clamp the window firmly in a vice, using some paper or similar to stop it getting scratched, and file away the marked area. Make sure the edge you leave is straight and smooth, because it has to form a good seal. Use fine emery paper or wet-and-dry paper if necessary.



Lay the rubber seal in to the bottom front window groove. Leave a few millimetres sticking out of one end, and cut off the other end also leaving a few millimetres sticking out.


Place the front window into the groove on top of the seal, making sure it’s lined up correctly with the sides of the case.


Now, holding the window in place with one hand, turn the whole case over and slide the other seal into the opposite groove.


Gently pull the casing apart to allow the window to get into the groove on top of the seal. The springiness of the case will hold everything in place.


Use sharp flush cutters or a craft knife to cut off the ends of the seals, leaving about half a millimetre sticking out.


Put one o-ring on to each screw.


Place the first side panel in place with its gasket correctly positioned.


Carefully insert each screw. You may have to squeeze the case together, either by hand or in a vice, to get the screw holes to line up with those in the side panel. As you turn the screws, make sure the gasket doesn’t get twisted out of place. Squeezing the side panel against the case will help to hold the gasket in position.


Do not overtighten the screws: tighten them only until the side panel is held snugly in position. Overtightening will push the o-rings out of place.


Transistors don’t do that

Last year, I spent some time restoring my Tektronix 549 storage oscilloscope to full working order. The other day, I wanted to use it, so I switched it on and let it warm up. As I tried to use it, it was misbehaving: the controls all did strange things, the spot wouldn’t focus properly, the vertical amplifiers were all out of balance, it wouldn’t trigger properly, and it was generally very sick. It’s been moved all across the continent since it was restored, so I suppose I shouldn’t be surprised that something had gone awry.

I took the side panel off to make a few checks, but I didn’t have to look far.  Down in the bottom of the machine I saw a valve with a white top. Here’s a picture: on the left, a normal, healthy valve. On the right, the one I saw.


The white top means one thing: the vacuum has escaped. There’s a patch of deposited metal (a barium compound, I think) on the inside of the valve whose job is to mop up any odd gas molecules by reacting with them. If serious amounts of air get in, it all turns to oxide and goes white.

I did the obvious thing and tried to wiggle the valve out of its socket.


Oops! How did that happen? The valve, a Mullard ECC83, had spontaneously broken. There’s no other physical damage to the scope, and it’s been moved about a bit but stored well. It’s a mystery. Still, at least it was easy to spot the fault.

The valve in question, V624, is the feedback amplifier for the -150V regulator in the scope’s power supply. The -150V rail is the reference for absolutely everything else in the scope, so if it’s acting up, there’ll be big trouble. And there was.

So, replace the valve. The ECC83, a double triode, was a common-or-garden cooking-variety valve in its day. Cheap and simple, useful in lots of places from radios to laboratory equipment. Trouble is, it was used in hi-fi equipment and guitar amplifiers which now have a cult following. Their fans will pay extraordinary money for valves with the right things written on them: which brand, which factory, what shape the glass is, are all highly prized. This little UK-made specimen from Mullard would probably be worth fifty quid or more if it was new in its box and not broken into two pieces. I’m not joking.

I didn’t want to pay the enormous price of a period replacement, so I was delighted to discover that RS Components sell new ECC83s for a very reasonable £4 or so. It’s amazing that they’re still made, but here’s one. A 12AX7 is the same thing as an ECC83 – American type number rather than European, but the same to all intents and purposes.



It’s Chinese of course. I think it’s made by Shuguang. Popped it in the scope, switched on, a slight tweak to the -150V adjustment and it’s back in action. Hooray. Here’s a gratuitous picture of some glowing valves.



Repairing a PC power supply

I seem to be doing a lot of repairs on things which ordinarily wouldn’t be worth repairing at the moment. This is another one.

In the workshop I keep an old PC. It’s mostly used for reading PDF files of circuit diagrams and data sheets, and a few sundry tasks like audio editing and EPROM programming. One of the things I use it for is scanning. The scanner I’ve got (a Canon LiDE 30) works well, but isn’t really supported on any operating system later than Windows XP. The workshop machine is still running XP, so it does the scanning. Simples.

This morning I urgently needed to scan a document, so I went to switch the PC on. Pressed the button and…nothing. Check the power cable, check the socket, all OK. PC is dead. Not even the fan starts. Now I have a choice: fix the PC, or try and get the scanner to work on another machine. Visions of out-of-date drivers and endless reboots swam before my eyes, and I opted to fix the PC.

The symptoms were of a dead power supply, so I pulled it out of the machine and gave it a hard stare.


An ‘Advance MXA-300PTF’. Well. I read the warning, “Hazardous voltages contained withln this power supply not user service. Able returnto service center for repair.”, but it didn’t make any sense. The German version is even funnier. So I removed the screws and looked inside.

The mains fuse was blown. Not a good sign. There were signs of a struggle, too. Here’s a picture taken after I’d removed a few parts.


There’s an ominous burnt patch on the right of the PCB, and a lightly toasted section below it on the other side of the transformer. These parts all turned out to be part of the standby 5V power supply. If it’s not working, nothing will work, which explains why the PC was dead. The scorched patch was caused by a diode, which fell in to two pieces when I prodded it. The lightly toasted patch was from the switching transistor, which was short-circuit between all pins. The smoothing capacitor on the output of the 5V supply was bulging ominously, though it still seemed to have some capacitance.

At this point I had a quick web search for likely circuit diagrams of PC power supplies. They’re mostly basically the same, especially at the (ahem) budget end of the market. Here’s the relevant fragment of the circuit diagram of a similar one (original source here from this very handy page of schematics). It’s remarkably similar to the DVD player power supply I wrote about a while ago:


Component designators I write here will refer to this diagram, not the original PCB. The burnt diode was D8, the short-circuit transistor was Q3, and the bulging capacitor was C12. I replaced all three, using a BU508A for Q3 – rather over-specified, but tough as old boots and I happened to have some. I fitted a new fuse, gingerly switched on and…nothing.

Since Q3 had clearly suffered badly, it would make sense if there was other damage on the primary side of the supply. Semiconductors are always suspect. Probing around, Q4 was dead, short-circuit all ways, and D7 was leaky in both directions. Replace them. I used a BC183 for Q4, with its legs crossed because it has a different pinout. Still nothing. Then I spotted that R12 (3.6 ohms in this power supply) was hanging by one lead because its other lead actually shared a PCB hole with the emitter of Q3! I hadn’t spotted it when replacing Q3. How cheapskate is that? It turned out to be faulty anyway, so I replaced it. By now the board looked a bit crazy with replacement components tacked on the back at all angles.


Switch on…and switch off again quickly! I had a meter connected to the +5V standby output. The meter was set to its 20V range and showed over-range. At least the thing was now working, but it clearly wasn’t regulating. I tried again, bringing it up slowly on the variac. With the mains voltage at about 30V, the output was already at about 6V, but at least it was stable and controllable.

I looked around at the feedback network. U2, a TL431 shunt regulator, was doing sensible things and seemed to be conducting when its input reached 2.5V, as it should. Using my bestest screwdriver to short the output (pins 3 and 4) of the optocoupler U1 should make the power supply think its output is too high and thus drastically reduce the output voltage, but it had hardly any effect. Odd. It turned out that R16 (220R in this unit) was almost open-circuit. Once it was replaced, my screwdriver test worked, but the output still didn’t regulate itself at 5V. This left only one suspect: the optocoupler U1. I scavenged a replacement from another scrap power supply (a cheapo USB phone charger) and fitted it. Success!


Now the output regulated at exactly 5V, according to the trusty AVO. I did a quick test of the rest of the power supply, pulling the PSON signal to ground. The fan started up and the output voltages were plausible. I decided that was good enough, and tidied up the repair.


Since the diode had clearly had a difficult time, I replaced it with a chunky 3A one (a 1N5420) on ceramic standoffs.

So what had gone wrong? Either the diode and output capacitor had died from a long-term overload (which reminds me, I must check how much current the PC draws from 5V standby) and the rest of the supply had burnt itself to a crisp trying to keep the output up, or the optocoupler had failed so the supply burnt itself and the output diode and capacitor to an even bigger crisp with a huge overvoltage. In the latter case, I’d be worried about the effect on the PC motherboard – the 5V supply could have been well over 5V, which wouldn’t do it any good.

With some trepidation, I reinstalled it in to the PC and switched on. Much to my amazement it worked, so the motherboard hadn’t been fried. Phew. All credit to Asus for an apparently indestructible board – an A7V266. Yes, it’s from 2002. That’s how old the PC is.

Just for fun, here’s a picture of the collateral damage.


Those nine components were all that was stopping me scanning my document.

Denon DVD-1720 DVD player power supply schematic and repair

A couple of weeks ago, my wife and I wanted to watch a movie. I went to put the DVD in the player, and was disappointed to find that the machine was dark and didn’t respond to any of its buttons. It was broken.


It’s hard to say that a DVD player is really worth fixing. This one, a Denon DVD-1720, is about 7 years old, and wasn’t expensive when it was new. However, I was curious to know why it had stopped working, and that alone was reason enough to delve inside.

Considering its low cost (I think it cost about £100 when new) it was very neatly constructed. Removing a few screws allows the top to come off (careful of sharp edges) and the plastic front panel just unclips with a bit of gentle persuasion, revealing the guts.


Before we go any further, I have to insert a health warning. Dealing with DVD player power supplies has health risks. There are dangerous voltages present inside. If you don’t have the right equipment or don’t know what you’re doing, you can get a nasty surprise, a terminally broken DVD player, a serious injury or be electrocuted. None of those are fun, especially the one which results in death.

Almost all the electronics are on one big motherboard, apart from a few buttons and lights at the front and the fiddly digital stuff to do with the actual DVD reading, which sits on the green board on pillars in the middle. Undoing the screws from the green board, a few on the motherboard, all the ones on the back of the machine, and unplugging three connectors, makes it possible to wiggle the board free. It’s a bit of a squeeze and the board is fragile, so don’t blame me if you break it.


The power supply is at the rear right corner. Denon have kindly marked in white the area with dangerous voltages in it.


There’s not much to it. It’s clearly a switch-mode flyback converter. The transformer is much too small to be an ordinary mains transformer, and there are no inductors on the secondary side. I did the obvious checks – the fuse wasn’t blown, and the three big pale blue safety resistors measured OK. All the diodes dioded too. Time to put my reverse engineering hat on and dig deeper.

The PCB is nicely marked with the component identifiers on both sides:


so it wasn’t too hard to draw out the circuit diagram.


It’s a pretty elegant design, using only three transistors. The circuit appears to be a blocking oscillator centred around Q1001 and the transformer. At startup, Q1001 is allowed to conduct, and current starts to flow in the transformer primary. At a certain current, the transformer’s core will saturate so the current can’t increase any more. The voltage induced by the changing core flux in the feedback winding at the bottom of the transformer will suddenly drop, creating a pulse which, fed via C1029 to the gate of Q1001, switches it off. At that point the energy stored in the transformer core has to go somewhere, and it finds its way out through the secondary rectifiers into the outputs. The basic scheme is very similar to the Joule Thief, a simple blocking oscillator for driving LEDs from batteries.

The two transistors Q1003 and Q1008 seem to be involved in regulating the supply. Q1003 can cut off the gate drive to Q1001 in response to three things: the current in R1001 getting too high, the optocoupler IC1001 conducting too much (which indicates that the output voltage is too high) or Q1008 stopping conducting. The latter seems to be a way of shutting down the supply if its output voltages get low, or it might be a cunning standby mechanism involving some more stuff on the secondary side which I haven’t investigated. C1032 seems to be there to make sure that the power supply starts up.

I powered the bare motherboard from an isolating transformer (don’t try this at home, folks, unless you know what one of those is and how to use it) and measured some things. There was precisely no activity going on at all, apart from 300V on the reservoir capacitor C1004 and Q1001’s drain, as expected. I first laid the blame on the little electrolytic C1032, since they’re the most unreliable components in any modern electronics, but tacking another 10uF across it didn’t help. Holding my breath and briefly shorting C1032 didn’t bring anything to life, either.

Then I measured some of the DC conditions. Everything around Q1008 was OK, with R1096 and R1034 merrily feeding electrons into Q1003’s base and keeping Q1001 switched off. High-value resistors are the next most suspicious components in a circuit like this, especially when they’re teeny-tiny ones. I checked around the 1.8 megohm R1005 and R1006 which pull up Q1001’s gate. Clearly it couldn’t work without them. Lo and behold, my 10 megohm input meter showed 250V at the junction of R1005 and R1006. That can’t be right – the voltage should be more like 150V, since the resistors are the same value and basically connected straight across the 300V supply. I pulled R1006 out of its hiding place and measured it – it was open-circuit. Gotcha!

Soldering in a replacement 1.8 megohm resistor for R1006 brought the whole thing back to life. The secondaries seemed to have sensible voltages on them (3.3V and 12V at a glance) so I reassembled it far enough to test. It lit up, accepted a disk and played it. Success!

It’s a credit to Denon’s neat design that there were no screws left over when I put it back together. The whole machine is now back in action for the sake of a 2p resistor.

Tapping a hole without a tap

No, this is nothing to do with plumbing. It’s much drier than that.

Today I wanted to screw some resistors to a heatsink. The resistors have 2.5mm mounting holes, so I looked in the M2.5 fasteners drawer and found plenty of screws but no nuts. That’s not going to work. And I haven’t got any self-tapping screws that small, and self-tapping screws are a bit horrid anyway. Unless…I could tap the holes in my heatsink so the screws would screw directly into them. That would be easy if I had an M2.5 tap, but I don’t. Only 6BA, M3, M4, M5, M6, M10x1.0 and 1/4″ UNF, since you ask.

What to do? Well, the heatsink is aluminium, so fairly soft. And the screws are steel, so harder. I decided to try making a tap out of a screw. I attacked one with a cutting disc on a Dremel, like this:

Img_6367If you try this at home, keep your fingers out of the way. Oh, and switch the Dremel on. It works better like that. The result was a screw with a couple of flutes in the end of the thread:


I took the photo after the screw had been used to tap half a dozen holes, so it’s a bit mangled, but you get the idea.

Just use the screw like a tap, turning it gently into the hole, reversing every now and again to break the swarf. It worked a treat, and all six of my holes take M2.5 screws very neatly. If only they lined up with the holes in the resistors. Sigh.

Action shot of tapping in progress. You can see the sharp edge of the flute which helps to cut the thread, and the authentically-mangled head of the screw.


NAT loopback and local DNS again

A while ago I had to do some fiddling to make my internet-accessible server also work from behind the Orange Livebox router at home. I described the problem and first fix in this post. Yesterday, it all stopped working, which was very annoying. It was my fault.

What was the problem? Well, the dynamic DNS entry was no longer updating, so when the IP address of the home router changed, as it does from time to time, the outside world could no longer find my server. It turns out that the little script which keeps the dynamic DNS updated relies on being able to find two addresses on the internet which are in the same domain as my dynamic address. For the sake of this example, we’re using ‘’ as my dynamic IP address. The dynamic DNS update process also needs to be able to find ‘’ (which finds out the IP address of my router) and ‘’ (which accepts the update itself). I’d got my local DNS set up to override the whole ‘’ domain so the update process simply wasn’t working – it could find neither ‘’ or ‘’.

The fix was delightfully simple. I just changed my DNS setup so it overrode only the specific address I was interested in. So now /etc/bind/named.conf.local has the following section in:

zone "" {
  type master;
  file "/etc/bind/";

and the zone file /etc/bind/ looks like:

$TTL 604800 ; 1 week IN SOA localhost. root.localhost. (
  2009060801 ; serial
  604800 ; refresh (1 week)
  86400 ; retry (1 day)
  2419200 ; expire (4 weeks)
  604800 ; minimum (1 week)
  NS sheevaplug

Problem solved – the dynamic DNS update works and my server is visible from the internet again. Now I can go to the office and use it.

Herrmans H-Track rear light teardown and schematic

I’m in the process of fitting dynamo lights to my wife’s bike and to mine. For our rear lights, I chose a light I hadn’t seen before: the Herrmans H-Track. It’s relatively cheap but seems to have good reviews. It meets all the relevant standards, if you care about that sort of thing. A quick test on the bench showed that it was nice and bright, and the illuminated ring round the outside is eye-catching.


The standlight works quite well but stays lit for a long time (around 15 minutes, I’d estimate), gradually dimming until it gives up. That’s not as nice as the B+M DTopLight I have on another bike, which stays lit for the requisite four minutes then switches off. I was curious to see if it was possible to improve it, so I took it apart.

The light is glued together but there are little gaps between the two halves at the bottom, which make it relatively easy to lever apart without damaging it. Here’s what’s inside.


A little PCB with not much visible – three LEDs and a capacitor. It’s nicely made, with a fibreglass PCB, but there’s no waterproofing or other sealing. The rest of the components are surface-mounted on the other side of the board. I noted in passing that neither of the wires is connected to the mounting screws, which is handy for lighting systems like the Solidlights 1203DR which need both wires to the rear light to be isolated from the frame. Here’s a closeup of the PCB.


Not a lot to it, really. You get what you pay for. For the curious, here’s the circuit diagram I traced out. It’s clear that the H-Track doesn’t have any protection against overvoltage, so you’re in trouble if your front light comes disconnected.


What about improving the standlight behaviour? Well, I’ve worked out a modification for that but haven’t tried it out in a real light yet. Watch this space.

SheevaPlug Power Supply – Sorted

From the department of over-engineering comes this: a SheevaPlug power supply to end all SheevaPlug power supplies. I’ve got fed up with power supplies dying. First the SheevaPlug’s infamous internal one only lasted a few months, then I think two wall-warts expired in turn. Most recent was the horrid little one at the bottom of the photo, which didn’t come back on after being switched off. It also generated so much radio interference that it actually stopped the touch pad on my laptop working. No joke.

The new one is at the top of the picture.


I made it from a power supply I scavenged from a scrap network switch or disc drive enclosure a few years ago, feeling sure it would Come In Handy. It seems to be well built, and has a bunch of features which are missing from most wall warts:

  • a fuse, so it shouldn’t go up in flames if something goes wrong
  • interference suppression, so I should still be able to listen to Radio 4
  • some attempt at protection against power transients
  • components made by manufacturers I’ve heard of
  • some space around the parts so they don’t cook themselves into oblivion
  • a circuit board made out of something more robust than compressed mushrooms

Its outputs are rated at 5V, 2.5A, and 12V 1A. The SheevaPlug draws about 0.8A from 5V at rest, so the new power supply should live a long, cool, reliable life. I hope. Also to help with cooling, I fitted it in a spacious metal enclosure bought from the convenient Warszawska Gielda Elektroniczna. Let’s see how long it lasts.

Overheated 300Tdi engine

This blog was originally supposed to be about what’s going on in my workshop. However, this post is about what’s going on in someone else’s workshop – the garage which is currently working on my Land Rover. I’m putting it here because it’s an interesting engineering lesson.

My Land Rover’s 300Tdi engine is very badly broken, and apparently it’s all because of the failure of the ‘P gasket’. This part, which costs £3.24 according to a web search I’ve just done, forms a seal between the engine block and the mounting bracket which holds the water pump, alternator and various other things. It’s supposed to play its part in keeping the coolant inside the engine where it belongs. Trouble is, sometimes it doesn’t. And when the cooling water escapes on the motorway, the engine overheats before you’ve even got time to look at the the temperature gauge. This is an object lesson in the damage an engine can do to itself when it overheats badly.

The first I knew about it was that the engine suddenly lost power and then died altogether. I was only able to drift to the hard shoulder in a cloud of steam and smoke. Once cool, the engine would restart but ran badly, making an odd tapping noise, and the Land Rover’s top speed was about 40mph. It’s no racing car, but it’s not supposed to be that slow.

Cutting a long story short, the head is now off the engine. Here are pictures of what’s inside.

Firstly, cylinder number 4. I think this is the reason that the engine stopped: the piston got so hot that it started seizing in the bore, scoring it badly. The score lines are more than just visible, they’re perceptible with a fingernail, too.

2013-12-05 10.39.12The other cylinders look OK, but one bad one is enough.

Sadly the head gasket has also failed, between cylinders 2 and 3:

2013-12-05 10.39.39That would explain the loss of power. Considering that only one out of the four cylinders was in anything like normal condition, it’s amazing that it still ran at all.

I’ve now spoken to several people who’ve said that if a 300Tdi engine has been overheated this badly, the head will be wrecked as well. The search for a replacement engine is now on.



The BBC Micro linear power supply

The BBC Micro is the machine that really got me into computing. It was designed by Acorn in a tearing hurry for the BBC Computer Literacy Project in 1981 – see the film Micro Men for a great dramatization of the story. Because it was done in such a rush, some things weren’t quite finished for the early machines. One of those things was the power supply.

Why is the power supply interesting? Well, later BBC Micros had a perfectly normal, reliable switch-mode power supply, not dissimilar to the one in a modern PC. But early ones had a linear power supply, which gained a fearsome reputation for overheating, exploding, and being incompatible with nearly everything. I recently dragged an early, Issue 2, BBC Micro out of the loft and realised it contained this elusive beast, the linear power supply. I thought I’d see what all the fuss was about.


First things first: does it work? Well, yes. Despite not having been switched on for probably 20 years, it powered up absolutely fine. Boop-beep. No smoke or flames. Good. Time to see what’s inside.


This particular machine started life as a model A, with just 16K of RAM and very little else. However, it got upgraded at some point into a model B, with the full complement of RAM, and has an unusual Opus double-density disc interface in it as well as a couple of extra ROMs including Wordwise, the word processor. That’s the sort of thing that’s supposed to be impossible with a linear power supply. You can see the black box on the left – later power supplies are a gold colour. Here are a couple of closeups of its labels:

DSCN8939  DSCN8931

I took it out and examined it. The first thing I noticed was that it doesn’t really fit very well: there are some odd washers sandwiched between the power supply and the case, and they’ve used nylon screws to fit it, for some reason.

DSCN8942 DSCN8943

You can probably see from the side view that it’s riveted together. The rivets were quickly dealt with, revealing the insides:


Pretty straightforward stuff: a toroidal transformer, rectifier and smoothing, and a row of regulators. Here are closeups of the circuit board and regulators. Note the little orange tantalum capacitors. I suspect that’s where the reputation for explosions has come from: when used like this, they do tend to fail short-circuit and go off like little fireworks from time to time. These ones, however, are rated at 35V, so with only 5V or 12V across them they should be fairly reliable.

DSCN8950 DSCN8949

It didn’t take long to trace out the circuit.


What is interesting is the way the 2.25A output at 5V is achieved. Rather than use one big regulator, they’ve chosen to use three 7805s, rated at 1A each. Three sets of wires leave the power supply and are delivered to three places on the Beeb’s motherboard. There is no connection between the three 5V rails on the motherboard according to my meter. They’re entirely independent. This is good, because it means that the three 7805s won’t end up fighting each other if their output voltages are slightly different.

I took a few measurements while the machine was running. The voltage on the 4700uF smoothing capacitors was 10.5V. The currents delivered by each output were:

VCC1: 0.49A
VCC2: 0.75A
VCC3: 0.84A
-5V: 15mA

I tried removing the disc interface to see what difference that made to the power consumption. It reduced VCC1 to 0.35A and had no other effect.

The total current flowing at 5V is just over 2A, which is sailing fairly close to the wind given that the power supply is only rated at 2.25A. However, nothing in the power supply is under great stress, and I see no reason why it should fail unexpectedly. Unlike a switch-mode supply, it would also be easy to repair. If it was going to be used for a long time, I’d be very tempted to replace the 1uF tantalum capacitors with modern ceramic or electrolytic ones, just because the tantalums do tend to commit suicide randomly with old age.

These machines are now becoming collectable, and early ones like this are worth preserving in their own right. If you’ve got an early Beeb, there’s no reason to fear the linear power supply and replace it. It’s part of the story, and it’s possible to keep it going more or less indefinitely.