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.

35 thoughts on “Re-Capping An Ancient Apple PSU

  1. Can’t second the part about those huge hot side capacitors. *Do* discharge them right after disassembly, preferably with a resistor, of course. Some PSUs have bleeder resistors – but not even most of them do, especially the cheapest ones. Also, don’t assume the heatsinks are grounded – they may be not, they may be even connected to V+ – some PSUs are weird, yeah. I do the whole “initial disassembly” in gloves made for working on mains.

    1. I agree with you,CRImier-definitely respect the capacitors and work with caution. Like you say even if the circuit has some sort of grounding failsafe, THAT may be the part that is fried so things can act very differently or plane delamination which is catastrophic in PSU.
      I definitely prefer to remain in the low volt and amp world and respect the fact that I will never be faster than electricity lol.

    1. Yep, caps have a weird logic-defying ability, where when you discharge them, after a while a charge can return. No idea how, but it does. Prob best to short them every time, just before you touch them, just in case.

      1. The charge isn’t removed uniformly out of the plates due to the resistance of the foil, and the ions in the electrolyte don’t move instantly, so there’s a surface charge that gets depleted and replenished by re-distribution of the remaining charge carriers.

  2. Tipping and ripping loose the trace of the other terminal, Most surface mounted caps need TWO irons to lift pads both at once.
    I had a repair, two caps the size of the end of a pencil eraser leaked enough crap to ruin a square inch of both sides of a board. They were bypass caps on the op-amp area of a digital keyboard. Decon then rebuild traces and vias.

  3. I would have been inclined to replace the 3 or 4 (or so) secondary filter caps as well. Sure the ones replaced present a “go / no go” situation but faulty filter caps can cause damage to the logic board.

    Also I bought one of those desoldering tools from China. It came with a plug adapter and was rated at 110 Volts. We have 240 Volts here. I turned it on expecting it to get very hot. If I had of captured the smoke that came out I could have fixed many many transistors. It’s the Volts squared relationship. Twice the Volts id four times the power.

    1. Yes, the capacitor filtering the AC line just has to deal with 60Hz. In the old days, their decay would hum in the output of a radio, annoying but not fatal.

      The DC side filter capacitor have to filter the much higher frequency which the switching supply is operating at. That’s harder, and is the reason so much focus on the replacement of electrolytic capacitors in recent times, the capacitors are working harder at the higher frequencies. Their decay may present less than optimal voltage to the outside world, but since there is feedback, their failure may cause other problems, not initially apparent.

      The high voltage filter capacitor could go bad, but it is the secondary ones that will be a problem first.


    1. I just burn the old smt off with the iron. Not gonna need it anyway.
      We get it-you stress the pads anytime you add or remove a component. Folks here are trying to offer up cost-effective ideas that have worked for them in the past. Something this old would give you far more tolerance than say a Galaxy 5 in terms of plane heat dissipation and size of component. If you start with the negative side you are going to have that juicy ground to dump heat to and it makes getting to the positive side easier. With PSUs you will have larger caps and the modern ones have the plastic stabilizer at the base that melts right off to make removal easier and with little worry of pad separation or loss. I use silver solder when I work with em to keep the heat lower and get in and out a little faster. Not had any problems so far (knock on wood). Hope this helps someone with removal :)

    1. I call BS! Capacitors store DC and that has little effect on the heart unless there is dire per-existing condition in which case he probably wouldn’t have been able to take the PC apart in the first place.

      1. DC can clearly cause death in healthy individuals. It all depends on when the heart receive that energy. Theres a small time window in the cardiac cycle in which a DC shock can cause ventricular fibrillation that will cause death unless someone can initiate CPR and has access to a defibrillator (which, ironically, is basically a capacitor that is discharged thru the heart).

      2. My experience getting more than my fair share of the volts left in capacitors tells me that I wouldn’t want it myself. Maybe you differ, but I’ll leave that to you.

      3. Feel free to probe around and get back to us. Like the other guy said, defibrillators and so forth…
        Also, I would like to add that different folks have different tolerances to it and galvanic skin response probably plays a decent role in it. Same thing as tasers killing folks, bro. It is really all about amperage anyway. You have to outdo grounding and body capacitance and overcome the neural current in its micro-cycle (the things that machines read in the hospital to give us vitals info).
        After my last serious electrical accident due to an inattentive coworker I know my days of sloughing off crazy extra voltage are over if I want to make it without heart problems. Anything that leaves you shaking for three days, outside of childbirth, probably isn’t good for you.
        Let’s all have fun and be safe out there :) this world may need us some day pretty soon :(

      4. Universal PSUs can have 300 VDC in the capacitors, even in the 110 Volt models because they use a voltage doubling circuit. The energy stored is between 10-50 Joules, which is enough to directly kill a small animal, and certainly can cause fibrillation or a heart attack when you discharge it through yourself.

      5. Well as a reply to [All]

        I started electronics as a hobby and had lots of DC zaps in the valve era. Even those big high voltage valves that have the anode on top – well especially those.

        Many years later I had a domestic electronics repair center so I have had my share of DC zaps. TV anode voltages over 20,000 Volts.

        On one occasion a a fault appeared when I had my hands in the back of a set. It was described by the customer as “TV is flashing”. I expected problems with the color drive signals or tube voltages.

        Well it flashed all right. The SMPSU went maximum output with no regulations. The crowbar circuit was dead (I later discovered) and the whole box lit up with plasma. The normal anode for that set was about 27kV so it must have gone over 50kV. I wouldn’t be surprised if it was much higher. I was launched backwards and landed face down in the fetal position 3 meters away from the set.

        My xray is probably permanently fixed into a wall there lol.

        Anyway, that was the worst DC shock I have ever had. And the only one of any magnitude and I was fine. It was also the last DC shock and it was 20 years ago. I have been at electronics first as a hobby then a career for 45 years and in that time there certainly is the opportunity for something to go wrong.

        Sure, not trying it again just to make a point lol.

        1. @RÖB
          Haha, Jesus. It’s all laughs decades down the line but I bet at the time it gave everyone a right scare.

          Sidenote: if I’m gonna die in an accident, I’d prefer it be *spectacular.* Visible skeletons and carbon shadows certainly qualify!

        1. EXACTLY why smps works in the first place !

          transformers do NOT work on DC, thats why we “chop” it into pseudo-AC,
          which is what you get when you touch DC and your hands fly off the terminals: ON then OFF, with a slope that determine harmonics… basic radio theory :) or in this case, shock theory?

          1. PS: electrocution (death DURING electric schock), is NOT the usual way of dying from electric schock!
            aside from breathing or heartattack, it can cause many other internal injuries.

            DC can cause you to hold on in a simillar (NOT “same”) way as AC, if you hold on, you GET COOKED, boiled blood does not do it’s job, which is fatal

  4. Aluminium (or aluminum) SMD caps can easily be taken off by just screwing them off. Push, don’t pull and turn. after a half turn, they just fall of. You can clean the pads with your regular solder iron.

  5. Agreed, desoldering tweezers are fine tools, I’d use ’em myself. But your average Joe does not possess a set, the best he has is usually a cheap non-temp-controlled iron.

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