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Old 18th Sep 2020, 8:46 am   #1
cathoderay57
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Default Acbel API1PC36 from an old Mac

I was asked by a friend to attempt a repair on an Acbel power supply from an old Apple Mac. He had highlighted some useful tips for repair from the Web that I have included here. It is a complex (to me, anyway) switch-mode power supply. I could not find a circuit. The received wisdom, mainly from the "badcaps" website, is that these power supplies are robust and usually repairable by replacing electrolytic caps, but which one? https://www.badcaps.net/forum/showthread.php?t=25253 . I decided to extract the list of most likely suspects and test them one by one using my analogue Avo. The first two checked out OK - slow charge to a resistance exceeding 1M and retention of charge for a long time, depending on capacitance. The board is packed with components and heat is evidently a problem as witnessed by the use of 2 cooling fans. Eventually I picked on C60, a 1000uF 10v 105C cap made by Teapo. The PDF attachment shows its location in a group of 4 electrolytics glued together with potting compound above the upper large aluminium heatsink. Close inspection showed the top of the can was very slightly bulged. It had lost most of its capacitance since it charged almost instantaneously and discharged within 2 seconds. I looked for a replacement with greater that 105C spec but since the original was merely 8mm diameter I could not find a higher spec component that was less than 10mm. Even a 10mm item wouldn't easily fit in the space available. I could maybe have shoe-horned one it but it would have been touching 2 adjacent high wattage resistors that were no doubt producing heat and therefore would have defeated the object. I rejected using extender leads owing to the risk of introducing instability. Having replaced the cap with a Panasonic component I needed to understand how to power up the PSU on the bench, since I didn't have the Apple Mac to go with it. This link was incredibly helpful because it shows all the pin-outs and the fact that 2 sets of pins needed bridging, one with a 10-30 Ohm resistor to defeat the 3.3v sense: http://haertle.ch/tinker/ps/ Thankfully, all went well, the PSU powered up, and I got all of the expected outputs. The PSU hasn't been refitted to the computer yet, and so I am tempting fate by posting a success story but, hopefully, installed performance will be OK. Cheers, Jerry
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Old 18th Sep 2020, 9:52 am   #2
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Default Re: Acbel API1PC36 from an old Mac

Look also for a high value resistor from the rectified mains like. The resistor charges a start up capacitor which gets the whole thing going. These go too high and the supply then doesn't start. These used to fail before the capacitors got old enough to become problems.

David
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Old 18th Sep 2020, 1:29 pm   #3
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Default Re: Acbel API1PC36 from an old Mac

Looks like a PSU from a PowerMac G4. I think they had a bad reputation for leaking capacitors. Maybe they ran too hot with those 2 40mm fans?
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Old 18th Sep 2020, 1:30 pm   #4
cathoderay57
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Default Re: Acbel API1PC36 from an old Mac

Hi David, thanks for the additional tip. I will warn the recipient in case the problem recurs. The original fault was that on power-on the Apple on-light illuminated briefly, then extinguished. Hopefully it was just the dodgy cap that I have replaced but, if not, then at least I now have another option to explore. For anyone attempting a similar repair, it's maybe worthwhile if I add a few tips on dismantling. The cover is removed by first extracting 4 cross-headed screws. The short edge of the cover has locking tabs that slide into slots so the cover has to be slid sideways to release the tabs. Inside, the PCB is secured to the chassis by another 4 cross-headed screws. Three are simple to unscrew, but the fourth is partly concealed by a large rectangular filter capacitor soldered to the mains IEC connector terminals, and access to the nearby screw is made more difficult by a liberal blob of potting compound used to stick the capacitor to the PCB. Once the 4 screws are out, the filter capacitor and the blue and brown mains input leads need to be de-soldered from the IEC connector, and the earth lead unscrewed from the chassis. The locking clips on the IEC connector can then be pushed in temporarily with a screwdriver sufficiently to slide the connector out of the chassis hole. Then the 2 pairs of red and black leads from the fans need to be released from the coiled components on the PCB to which they are stuck with more potting compound. Once the wires are released the locking tabs on the fan plugs at the remote ends of the wires can be held open and the 2-pin plugs released. The cable tie around the fan wires and another tie restraining the large multi-coloured wiring harness to the chassis hole need to be snipped off. Then the PCB can be lifted clear of the chassis. One other thing to note - after removing power, the 3 x 100uF 450v electrolytics remain charged up to in excess of 150v for a long time and that's enough to give a painful shock - so worth checking and discharging them before handling the PCB. Some of the electrolytics are glued to the PCB with yet more potting compound but on mine it was still fairly pliable and easy to cut free with a sharp penknife. Cheers, Jerry
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Old 18th Sep 2020, 5:07 pm   #5
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Default Re: Acbel API1PC36 from an old Mac

A friend had a sideline in looking after a number of Macs, he was from the IT/software side, and I had a number of power supplies passed my way as fairly young failures. So I got used to them. 90% were simply startup resistors gone high.

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Old 20th Sep 2020, 10:02 am   #6
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Default Re: Acbel API1PC36 from an old Mac

Quote:
Originally Posted by cathoderay57 View Post
I am tempting fate by posting a success story but, hopefully, installed performance will be OK.
I was indeed tempting fate. While on the bench, the PSU fires up reliably with a shorting link to emulate the power switch and a 22R to defeat the 3.3v sense; however, when it was re-fitted into the Mac, it refused to run. When I get it back I will have a look for the high value resistors that David referred to, downstream of the mains diode bridge and, if necessary, go for a shotgun approach of replacing the electrolytics that I can get at since they are relatively cheap. I only tested 3 on the previous attempt, one of which was definitely duff. Cheers, Jerry
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Old 26th Sep 2020, 3:47 pm   #7
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Default Re: Acbel API1PC36 from an old Mac

Since I am now seeking help here, maybe the Mods want to move the thread to Vintage Computers? I'll press on here for now. I have now got the parent computer that the Acbel API1PC36 comes from. It is a Power Mac G4. When connected to the motherboard, pressing the power switch just gives a twitch on the main fan then the thing stops. However, when run up on the bench with a 22R dummy load on the multi-plug relevant pins to defeat the 3.3v sense, and a jumper across another pair of pins, the PSU starts OK and keeps running, delivering the specified voltages. The "Badcaps" Forum suggests block replacement of electrolytic capacitors, which I am beginning to do. However, in Post #2 David mentioned a high value resistor connected to the rectified mains line sometimes goes high. I have struggled to find it. I cannot locate an exact circuit for this model, but the mains HT end and many other parts appear to be very similar to the one I have uploaded. R4 is only 0.04R and is OK. R3 is 10k but measures 4.9k in circuit. R67 is also 10k and similarly measures 4.9k in circuit. These are probably low because of other paths or devices in parallel - I didn't bother to des-solder them and re-test. The links from B+ go all over the place. It's a big ask, but please could David give me a clue as to which resistors I should be looking at? The only high-ish value I could find is R12 which checks out OK at 180k. Cheers, Jerry
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Old 26th Sep 2020, 4:53 pm   #8
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Default Re: Acbel API1PC36 from an old Mac

Not something I work with but, have you checked the computer side of the power rails for any dead shorts just in case it is bringing down the PSU?
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Old 26th Sep 2020, 7:01 pm   #9
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Default Re: Acbel API1PC36 from an old Mac

The startup resistors on the mains side of SMPSUs have to drop a lot of voltage so it is common, even normal, for the 'startup resistor' to consist of a series chain of highish value resistors all with the same value so that each resistor in the chain shoulders an equal fraction of the total voltage drop.

There can sometimes just be a single startup resistor, but if you can spot two or three equal-value resistors in series they are definitely worth homing in on.
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Old 26th Sep 2020, 10:35 pm   #10
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Default Re: Acbel API1PC36 from an old Mac

Thanks SiriusHardware. I am very comfortable with thermionic valve technology, but freely admit that I am stumbling in the dark with solid state SMPSU issues. Please can you indicate by resistor numbers from the attachment in Post 7 which ones are start up resistors? Also I'd have thought that if there were a problem with startup resistors, the PSU would also fail to start on the bench; since it does in fact start reliably on the bench, maybe there is a problem that only occurs when it's on load. Thanks Timbucus, another useful tip. Cheers, Jerry

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Old 26th Sep 2020, 11:29 pm   #11
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Default Re: Acbel API1PC36 from an old Mac

Yes, sorry, I was being generic about SMPSU faults rather than specific about yours. If it starts at all, it is probably not a startup chain problem - if it was it would be unlikely to start up under any circumstances.

With that out of the way, since it appears to run OK with a dummy load you may, as has been suggested, have a problem on the computer which is causing the PSU to shut down - excessively low resistance on one of the rails for example.
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Old 27th Sep 2020, 3:45 am   #12
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Default Re: Acbel API1PC36 from an old Mac

OK, thats definitely a later supply.

There are three switchers and the main controller chip gets rectified mains directly. No R/C startup circuit,

Q5 is a little ancillary switcher running off rectified mains (labelled B+ to keep the Americans comfortable) It's a flyback converter. Notice that there is a thyristor crowbar on its output.

The rectified mains does NOT feed a reservoir, the voltage is allowed to drop between half cycles. If there had been a reservoir, it would have held the voltage, the rectifiers would drop out over a lot of the half cycle and the mains current would drop to zero.

Q3 and Q10 with an inductor and a diode make a boost converter and this drives the real reservoir. While the mains voltage is low the boost converter boosts it to take some power. The controller chip looks at the mains voltage and massages the current the booster takes to sinusoidally follow the sinusoidal mains... so the power supply pretends to take current just like a resistive load. This is called power factor correction. It's now required on many higher power things. You'll find it in some Linn hifi amps, my TIG welder in the garage has it. It gives the alternators wherever the mains comes from a less damaging experience.

Q1 Q25 T1 are the main power converter, turning the PFC boost voltage into all the outputs. T1 is where mains isolation happens.

A protection chip monitors the main outputs and if something is wrong shuts down the main control chip via an opto isolator.


Overall, this supply is a bit more comprehensive and a bit better protected than most computer supplies.

David
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Old 27th Sep 2020, 9:51 am   #13
cathoderay57
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Default Re: Acbel API1PC36 from an old Mac

Thanks to David for the explanation of how the thing works and to SiriusHardware for the update. Since the thing is so complex, doing any diagnostics with it powered up is frankly beyond me and my test equipment. Therefore, my plan is to replace virtually all of the electrolytics (except the 3 large 100uF 450v main smoothers for now) and see if it will then work on load. If it still fails to run on load then it would appear either there is a fault on the motherboard that is taking down the supply or something in the protection chip circuit has failed causing it to wrongly detect a fault condition when there is none. Cheers, Jerry
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Old 27th Sep 2020, 2:37 pm   #14
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Default Re: Acbel API1PC36 from an old Mac

I checked all of the voltage rails at the motherboard and no shorts. And then, well, it's a weird one. I changed 12 of the electrolytics before lunch, and did a quick check run. The symptoms were exactly the same - fan twitch, a red LED on the motherboard (D518) flashed once and nothing else happened. So I had lunch and before changing any more components I did another check run. The darn thing started and ran for 5 seconds. Two more restarts and it kept running. Why that should be the case I have no idea. There is a backup battery on the motherboard that currently tests as 3.65v, so I guess that can't have been a problem.
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Old Yesterday, 4:50 pm   #15
cathoderay57
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Default Re: Acbel API1PC36 from an old Mac

The Power Mac G4 is still starting reliably. I have elected to simply replace the small electrolytics (19 in all) except for those on the fan controller daughterboard. I dislike using the shotgun approach to component replacement but, in this case, I had little choice since, as I said in Post #13, a diagnostic approach, without service data for this thing, is beyond me. Cheers, Jerry
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