Re-Capping An Ancient Apple PSU

It sometimes comes as a shock when you look at a piece of hardware that you maybe bought new and still consider to be rather high-tech, and realise that it was made before someone in their mid-twenties was born. It’s the moment from that Waylon Jennings lyric, about looking in the mirror in total surprise, hair on your shoulders and age in your eyes. Yes, those people in their mid-twenties have never even heard of Waylon Jennings.

[Steve] at Big Mess o’Wires has a Mac IIsi from the early 1990s that wouldn’t power up. He’d already had the life-expired electrolytic capacitors replaced on the mainboard, so the chief suspect was the power supply. That miracle of technology was now pushing past a quarter century, and showing its age. In case anyone is tempted to say they don’t make ’em like they used to, [Steve]’s PSU should dispel the myth.

It’s easy as an electronic engineer writing this piece to think: So? Just open the lid, pop out the old ones and drop in the new, job done! But it’s also easy to forget that not everyone has the same experiences and opening up a mains PSU is something to approach with some trepidation if you’re not used to working with line power. [Steve] was new to mains PSUs and considered sending it to someone else, but decided he *should* be able to do it so set to work.

The Apple PSU is a switch-mode design. Ubiquitous today but still a higher-cost item in those days as you’ll know if you owned an earlier Commodore Amiga whose great big PSU box looked the same as but weighed ten times as much as its later siblings. In simple terms, the mains voltage is rectified to a high-voltage DC, chopped at a high frequency and sent through a small and lightweight ferrite-cored transformer to create the lower voltages. This means it has quite a few electrolytic capacitors, and some of them are significantly stressed with heat and voltage.

Forum posts on the same PSU identified three candidates for replacement – the high voltage smoothing capacitor and a couple of SMD capacitors on the PWM control board. We’d be tempted to say replace the lot while you have it open, but [Steve] set to work on these three. The smoothing cap was taken out with a vacuum desoldering gun, but he had some problems with the SMD caps. Using a hot air gun to remove them he managed to dislodge some of the other SMD components, resulting in the need for a significant cleanup and rework. We’d suggest next time forgoing the air gun and using a fine tip iron to melt each terminal in turn, the cap only has two and should be capable of being tipped up with a pair of pliers to separate each one.

So at the end of it all, he had a working Mac with a PSU that should be good for another twenty years. And he gained the confidence to recap mains power supplies.

If you are tempted to look inside a mains power supply you should not necessarily be put off by the fact it handles mains voltage as long as you treat it with respect. Don’t power it up while you have it open unless it is through an isolation transformer, and remember at all times that it can generate lethal voltages so be very careful and don’t touch it in any way while it is powered up. If in doubt, just don’t power it up at all while open. If you are concerned about high voltages remaining in capacitors when it is turned off, simply measure those voltages with your multimeter. If any remain, discharge them through a suitable resistor until you can no longer measure them. There is a lot for the curious hacker to learn within a switch mode PSU, why should the electronic engineers have all the fun!

This isn’t the first recapping story we’ve covered, and it will no doubt not be the last. Browse our recapping tag for more.

Vintage AM radio restoration


Instructables user [knife141] enjoys restoring vintage electronics in his spare time, especially old radios. AM radios tend to pique his curiosity the most, and in this tutorial, he discusses the restoration of an old radio from the early 1940s.

While people would likely assume that the vacuum tubes in a radio this old are the source of poor performance, he has found that most units he repairs suffer from bad capacitors. He says that the old electrolytic, paper, and wax caps used in these radios were never meant to last more than a few decades, let alone 70 years.

He always starts the process off by discharging the caps and replacing the power cord, both as a safety measure. He was pretty sure the capacitors were bad in this radio, so he swapped all of them out, regardless of condition. All of the internal wiring was then checked over, and any damaged cables were replaced or covered with heat shrink tubing.

With that done, he powered on the radio and was happy to find that the distortion he previously experienced was completely eliminated. With the electronics taken care of, he tackled the radio’s asbestos insulation by encapsulating it with varnish. Attention was then turned to the exterior, where he cleaned and buffed the leather, refinished the face plate, and polished the dial’s cloudy glass.

While it’s not exactly a hack, we always like seeing vintage electronics given new life, and we’re always cool with saving these sorts of things from rotting in a landfill.

Hackaday Links: April 13, 2011

Oven parts scrounging


In response to last week’s post about parts scrounging with a heat gun, Hackaday forum member [BiOzZ] decided to try doing the same thing in his oven. It seems to work quite well, but we’re wondering if there should be any concerns over the lead content of the solder. Anyone care to chime in?

Spill-proof parts holder


Have you ever been in the midst of disassembling something and knocked over your container full of screws onto the floor? [Infrared] has a simple solution to the problem which also happens to keep a couple of plastic bottles out of the landfill.

Easy button stops abuse of the word awesome


Do you often repeat a word ad nauseam? Make author Matt Richardson does, and he hacked a Staples “Easy” button to help him break his addiction to the word “Awesome”.

Cheap Remote-controlled baseboard lighting


[Sean] scored a pair of LED deck lighting kits for a steal and decided to install them into his newly renovated kitchen. They are currently remote operated, but he plans on adding an X10 interface as well as PIR sensors for automatic triggering in the near future.

Yet another LCD recapping guide


It starts with a finicky backlight, or perhaps a high-pitched whine from the back of your display – by now, we’re sure that everyone knows the symptoms of an LCD panel that’s just about to die. [Eric’s] Syncmaster recently quit on him, so he pried it open and got busy recapping. It’s running again, and he wanted to share his repair process in case others out there own the same display.

Switched mode power supply repair guide


[Erich] spotted a broken DVD recorder at a local amateur radio meeting and decided to see if he could restore it to working order. While he was fortunate enough that someone labeled it as having a bad power supply, things aren’t always that easy. He gives a broad explanation as to how switched mode power supplies work as well as discusses some of the reasons these devices tend to fail. He identifies a few common components and areas that one should check while diagnosing a non-functioning power supply. While obvious bulging capacitors are easily identifiable, he discusses the need for an ESR meter and uses a kit-built model to test capacitors that do not have any visual signs of damage. While some of his walkthrough might be basic knowledge for readers who have experience in recapping circuit boards, it serves as a nice guide for those who are new to the world of electronics troubleshooting and repair.