IIsi power supply: triumph and tragedy

[hackdog]

Hi there, it's my first post!

Some time ago, the PSU in my IIsi suddenly turned itself off while running. Recently I opened it up to try and fix it and found 4 leaked caps on the secondary stage. The electrolyte had corroded through the legs and affected the solder mask and some pins on the bottom side. No unrepairable damage though. Cleaned the mess off, resoldered the oxidized wires and replaced the bad caps.

Then tested the bare PCB on my desk with a fog light as load on the +5. It worked absolutely fine again: briefly touching /PFW to PSTRICL turned it on, touching /PFW to GND turned it off. After ten minutes it still worked fine, so overjoyed I put the thing back in its enclosure. Then tested again: /PFW to PSTRICL still turned it on, but as soon as I let the connection go, it turned off.

Puzzled by this sudden change in behavior, I touched /PFW to GND to see if that would reset something. Or so I thought, because it wasn't GND -- I took the wrong hole in the connector and instead touched /PFW to -12... No smoke but then it didn't start at all anymore. My heart sank when I realized my mistake -- all my meticulous work, ruined by a single lapse of attention! About a minute later the main fuse F101 suddenly blew, completing the misery. I was quite upset by my stupid mistake and I'm still mad at myself right now...

I later reverse engineered most of the main PCB and the daughterboard PCB. All parts on the main PCB seem to test OK, so I'm not sure why the fuse blew. On the daughterboard (https://cdn.pbrd.co/images/5dxpafFDF.jpg) I found three SMD parts with one open PN junction each. Alas Q1 and especially Q2 have such unclear markings that I can't guess what they are, thus halting my resuscitation endeavour.

Hence my questions:

• Can someone identify Q1 and Q2? Their markings may be clearer on your own unit than on mine. Q1 may be "18" or "183". Q2 is very unclear but might be "13" or "132". Q3 is definitely "14" (etched) although it's not visible in this pic.
• Does anyone have a spare PSU for sale, or the daughterboard from a broken one?
• Converting a regular new ATX power supply to IIsi may just involve some rewiring and a logic inverter between /PFW and PWR_ON. Indeed I found this post: https://68kmla.org/forums/index.php?/topic/28901-microquadra-630/page-3 Is this approach known to work for a IIsi?

 

[techknight]

Well the 2 caps on the daughterboard are known leakers and show the same thing in that photo. Tons of corrosion caused by that, and it needs a serious acetone bath. That may bring her back around again. If it pops the fuse again well.... 

Anyways, they have to be changed. 

Fuse will pop if the main switching FET or NPN depending on design shorts or gets held on. 

the enable signal is probably opto-isolated so look on the secondary side of the optoisolators for damage. 

If not, then the main PWM IC is probably dead which could hold the switching fet full-on and pop the line fuse. It usually damages the main FET too so check that. Many variables here. 

I also have the schematics for that power supply kicking around somewhere. I rebuilt mine, and another one. Easy as pie. 

 

[techknight]

Looking at the schematic, the main switching FET is transformer-coupled so thats not blowing the fuse. That also rules out the PWM. 

But, there is an STR IC that generates your standby voltage that can short out and blow your fuse. 

Check that circuit. Q301, Measure shorts on D301, etc... 

 

[techknight]

What you label Q1 isnt a transistor, its an 18V Zener diode, SMD code 183. 

Q2 is SMD code 14, PNP. 

Q3 also isnt a transistor, its a 13V Zener SMD code 133. 

Transistor with code 45 is your switch-on transistor. Also, transistor code BR which is coupled to a 4.7K resistor and the 100uf 6.3V leaky capacitor forms a feedback network for voltage regulation. Which also is the same line as the enable signal is sent on. without that +5V the SMPS wont run, when its present, NPN 45 turns on which starts the supply but the two BR transistors keep it regulated by using the +5V as a voltage for reference comparison. 

 

[james_w]

hey Hackdog, I have a non-working IIsi PSU plus most of the replacement caps that would be needed to fix it. Happy to ship it to you for a small amount to cover the cost of the caps and shipping - PM if you want to talk further

 

[hackdog]

Thank you!

I've tested Q101, Q301 and D101 out of circuit with a component tester; they're OK. IC101 has no short between T1 (pin 2) and T2 (pin 3). D301, all diodes and resistors measure OK in circuit. None of the caps or transformers have shorted. I'll try and desolder the two daughterboard caps tomorrow.

Regarding Q3, yours has SMD code 133? Because mine's etched 14: https://cdn.pbrd.co/images/QzeJlakf2.jpg I'm surprised that Q1 and Q3 are diodes, as their three leads follow different tracks. The A6 diodes have pins 1 and 2 connected for example.

 

[techknight]

Not looking at a power supply, I am looking at a schematic. 

Yours could be a slightly different revision. 

 

[MrGasS27]

If you want to temporarily use your IIsi without original PSU you can attach an ATX PSU to it.

I'm linking here a video that I filmed some months ago

https://youtu.be/mL1V_604thY

 

[hackdog]

techknight, you had hit the nail on the head concerning Q1, Q2, Q3 and the SMD electrolytics!

Q1 is indeed a Zener diode "183", but the board designers have routed a track through its unconnected pin 2. So from the layout it appeared that all three pins were in use, while in reality only pins 1 and 3 are used for the diode and pin 2 is soldered to an unrelated track. Same story for Q2: it's a zener diode "133" but unconnected pin 2 is connects to an unrelated track. Q3 is a digital PNP transistor "14".

I've replaced the two SMD electrolytics on the daughterboard and they were indeed completely shot; both measured about 200 pF. Yes, picofarad!  After triple checking everything I plugged it in. Nothing dramatic happened, so I touched /PFW to PSTRICL and lo and behold: it turns on and stays on! Moreover, all output voltages were correct and practically identical to what I measured before my mistake. So the chip and most of the other bits have survived.

However, there's one problem: once on, it doesn't turn off anymore when touching /PFW to GND; I have to pull the plug. More investigation needed, I suspect a BR or the 44 may have been fried by the accidental -12V connection as they are connected to the /PFW line. The diode in front of the 45 will have protected it and the turn-on still works. Not sure how that subcircuit with the BRs and the 44 works though.

 

[techknight]

Might need to make sure the board is squeaky clean and rid of all cap goo, because it can cause parasitic resistances which will lead to an outcome you dont want. 

AT least it works, I figured thats what it was. 

To be honest, I dont think -12 was even hot when in standby so you might be ok. 

 

[hackdog]

The PSU lives again and so does the Mac!

I've thoroughly cleaned both PCBs with IPA and removed most of the dusty flux residue. Then tested it on the bench; now it reliably turned on and off. But strangely, when put back in its housing, it did its old trick again, turning off immediately when /PFW is removed from PSTRICL. So the housing definitely affected the behavior of the circuit. At first I suspected a short circuit but eventually I discovered that the effect was caused by the earth connection. The PCB rests on three big tabs inside the housing. The one near the output wires connects the circuit ground to mains earth via the underside of the PCB. When circuit ground is not connected to mains earth, the PSU remains on after /PFW is disconnected. When circuit ground is connected to mains earth, the PSU immediately shuts off when /PFW is disconnected; I guess this is the intended behavior.

I put it back together and installed it in the IIsi. The Mac starts again from the keyboard, runs perfectly and shuts down from the Special menu. A happy end after all! Many thanks to all involved for your support!
 

 

[kiiwii]

Hi, sorry to resurrect an old post but need some help with my IIsi power supply...

see here: 







thanks!

 

[FrancescoDS]

If you want to temporarily use your IIsi without original PSU you can attach an ATX PSU to it.

That could be very helpful for diagnosing an old Mac, can you provide the pin-out you used?

 

[MrGasS27]

That could be very helpful for diagnosing an old Mac, can you provide the pin-out you used?

 

[FrancescoDS]

Very helpful, I'll give it a try tonight, thanks!

 

[GeoffreyReed]

Hack Dog, Thanks so much for the post on  repairing, Mac IIsi PSU problems.  I am also new to this forum, but 2 years later.   I have been puzzling about exactly the same problem You had  for a week.  I had repaired 4 Mac IIsi PSU's in the last several months  by replacing the 1200 /uF's & 2200 uF's on the output and then also several of the 220's at 16V (for 12 V/, and restored them to use for a few weeks or a month, only to have them fail again (but by simply failing to turn on.).  It seems the daughter board electrolytics have likely failed as well (33 uF / 25V and the 100 uF / 6V).  Thanks again.  Those daughter boards were corroded, but I had only cleaned them, and not recapped.  Really hoping this puts things right !  If anyone can post a schematic of the PSU it would be greatly appreciated.  I have accumulated a surplus of parts for recapping Mac IIsi PSU's , if anyone needs help finding parts message me.

 

[kiiwii]

So I tried to link to my post back in 2018, still haven’t gotten anywhere...





@techknight and @hackdog if you have any suggestions would be most appreciated!