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kiiwii

Macintosh IIsi Power Supply Diodes

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Hi all, been a while since I last posted here, about the same time I tried to use my IIsi(s)...

 

so dug them out but neither of them would power on.

 

I would get a brief click, fan and light then they would power off again.

 

Tried replacing the fuses, which might have done more harm than good since the guy sold me slow fuses by mistake which I stupidly tested anyway.

 

after getting the right fuses they blew straight away.

 

so did some continuity testing and found at least a couple of the zener diodes are allowing current in both directions, but there’s no way I know of to know their values.

 

Wondering if anyone would happen to know what diodes are used at D302 and D305? See attached photo.

 

Thanks in advance!

3D195671-CC05-4AEC-866F-070761B24553.jpeg

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  I'm pretty sure both of those are for standby supply to the computer that enables it to turn on with the keyboard switch.    In the several Mac IIsi power supplies that I have repaired, the main things that fail are the bunch of electrolytics at the output that filter 5 VDC .  Those are C 201 , C 211, & C212 (1200Uf/16 v ), AND THE 2200 / 5V. Check also the 220's at 16V.  Mine didn't blow fuses however.  Usually the 220's at 250 V are OK .  One time long ago i found that the STR 80145 had burned up,   I would also suspect the 220/ 250 V guys., in Your case. 
  Now returning to this post later ,  I tested those diodes in a power supply that was recently working.  They are both zeners, the bigger one testis  29.7 Volts (reverse zener voltage).. the 2nd smaller one tests at slightly under 17 Volts.  Depending on the test device You are using, they could measure , conducting in both directions as zeners are "connected backwards".  they are back biased to bring up the zener breakdown voltage.  The way to test non destructively is to supply them with DC voltage Plus side to B+  (higher than the expected zener voltage say 50 VDC), through something like a 470K resistor, and then measure the voltage across the diode only. Someone earlier on another thread mentioned that they had a schematic.  It would be great to see that !

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My IIsi PSU was blowing fuses after I recapped the daughter card, it turned out that the resistor at TH101 had blown!? Replacing that, and everything started to work as intended.

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Thanks guys for your input. Would love to get my hands on a copy of the PSU schematic! I read there was a guy on this forum that had them and was very handy at repairing them too! Will see if I can find his name.

 

i saw a broken IIsi listed for $300 locally but seems a bit high.

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Ok so the post I’m referring to is here:

 

The dude with the schematic is @techknight

 

And the repair guy is @hackdog

 

Would be great if either or both of them could chime in. I’ve got two dead IIsi’s and they’re one of my favourite 68k macs!

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The measurements of GeoffreyReed seem to be a good starting point. Isn't there anything written on the diodes themselves? If it immediately blows a fuse, I'd start checking if any electrolytics or semiconductors in the primary part of the circuit have blown / are shorted like the bridge rectifiers, the main switching transistor, the clever automatic bridge rectifier/voltage doubler changeover IC (STR 80145) etc.

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Hi hackdog, thanks for your fast reply!

 

to be honest, I’m not really sure what the problem is... one psu blows fuses straight away and another just clicks and shuts down.

 

i don’t suppose the schematic indicates the ratings of the parts?

 

the only things I tested were the diodes since I guessed that could be one reason the fuses blow so quickly.

 

everything looks pretty clean except I think I recall a bit of blackening around some parts of the circuit board.

 

if I upload some more photos could you take a look?

 

cheers and thanks again!

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Ok attempting to upload some images....

 

If anyone can spot anything or has any recommendations, I'd appreciate it. Have never repaired a PSU before. But it is discharged I believe... in that it hasn't been connected to power for at least a year.

 

@hackdog @techknight @GeoffreyReed

 

Thanks in advance!

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Fuse F101 appears to be missing, but there are end caps in the fuse holder?? Was there ever a capacitor installed at C116? It's not obvious from the picture if something was there previously. Maybe C302 is leaky?

Edited by cruff
Added question about caps.

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Yeah there was a fuse, when it blew that’s all that was left... not sure about C116, I can check my other PSU it’s the same model. I’ll check C302.

 

Thanks for your tips @cruff

 

Edit: just checked C116, nothing meant to be there. Checked both PSUs. Well spotted though!

 

Actually, I’m wondering if it’s the MB that’s causing the problem?

 

Is there any way to test the PSU outside the IIsi case?

 

My electronics knowledge doesn’t extend to PSUs...

Edited by kiiwii
More info

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You can test it. Some switching power supplies require a minimum load for their regulation to work correctly. I don't know anything specific about a IIsi power supply, it's been probably 25+ years since I had a IIsi in my collection. Often for small computer power supplies, the +5V line needs some fraction of the amperage rating drawn, and the other voltages will be regulated as a byproduct. You can attach a suitable resistor across the +5V and GND lines to achieve this. The resistors might be some combination of power resistors to draw an amp or two, or possibly a automobile lamp. Since the IIsi won't supply a lot of power, and you won't be doing this for too much time at a shot, you can probably get by with some ceramic power resistors and not needing to supply cooling for them.

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Ok thanks for the tips... I have some ceramic resistors, should I use the highest rated one?

 

I have a very basic multimeter but it should do the job.

 

Back when I opened my 68kmla account it was for a competition and I wanted to overclock the 68030 to 25MHz by changing the Quartz crystal but have had problems ever since (not the psu) despite putting the original crystal back in. Think one of the feet of the crystal is not attached properly and may have affected other components?

Edited by kiiwii
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Has nothing to do with what happened with the power supply. In this day and age, the power supply is a known weak point and needs completely cleaned and recapped. if a fault happens after a recapping, then its probably other semiconductors that has failed in some strange way.

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As to the resistors for a dummy load, you'll need to calculate the appropriate values and power ratings for the load you want to draw.

 

For example, if you want to draw 1 A continuous from the 5 V supply, you will need a 5 ohm resistance that can withstand 5 W. If this was a continuous service, then you'd likely spec a 10 W resistor or provide proper heat sinking/cooling.

 

For your test, where you won't be running it continuously, you could get by without cooling. You could also use 5 x 1 ohm resistors in series, and you could get by with 1 W resistors as each would have approximately 1 V across each one.

 

You might want to draw current from the other supply voltages also, to see if there are issues with regulation under load. In that case you'd add resistors between the other voltages and ground with the appropriate ratings. Are the voltages and currents marked on the supply somewhere?

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On 7/22/2020 at 12:56 AM, techknight said:

Has nothing to do with what happened with the power supply. In this day and age, the power supply is a known weak point and needs completely cleaned and recapped. if a fault happens after a recapping, then its probably other semiconductors that has failed in some strange way.

Hi @techknight

 

thanks for for your reply, didn’t notice it before... good to know it’s not the mobo, though ai noticed internal SCSI wouldn’t work after my mods only an external scsi Zip drive.

 

Ill try testing the caps before replacing them...

 

 

btw, do you still have access to the schematics for the iisi psu?

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Concerning the second PSU (the one just clicking and going off) replacing the caps is not an option, the majority are probably leaking by now.

 

The capacitors are Elna brand IIRC and bad leakers.

 

Another issue is the small daughter board where two smd caps are present, those should go as well, but there is also a main chip present in that small board... most of the time contaminated by cap goo.

 

IIRC this chip is responsible for kicking the PSU on and off (soft power)

 

The chip should be removed, pads cleaned underneath and chip soldered back.

 

The first PSU, hard to say, but probably an issue on the primary side if the fuse is blowing all the time.

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I can vouch for all the recommendations to recap the IIsi power supply. I just did one, and most all the caps in the secondary end had let go with goo. Also, the SMDs on the little circuit board that controls soft power had gone as well and spread goo around, and in my case the symptom was that the machine would power up when the cord was plugged in, and would not power down correctly- the cord had to be unplugged to shut it down.

 

After recap, everything worked as expected. The amount of leakage from the can caps was pretty impressive - and smelly! Glad I did the work along with recap of the logic board.

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I completely re-capped my first IIsi Sony power supply (it was a mess in there...) a while back with good results except for one odd issue. When first powered on and for ~3-5 minutes after (until everything warms up) if I issue a Shutdown command the IIsi does the shutdown, the power supply clicks but then immediately restarts. As I mentioned, after its been running a while start up and shutdowns work as expected.  

 

Does this behavior ring any bells for anyone?

Edited by Fizzbinn

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9 hours ago, Fizzbinn said:

I completely re-capped my first IIsi Sony power supply (it was a mess in there...) a while back with good results except for one odd issue. When first powered on and for ~3-5 minutes after (until everything warms up) if I issue a Shutdown command the IIsi does the shutdown, the power supply clicks but then immediately restarts. As I mentioned, after its been running a while start up and shutdowns work as expected.  

 

Does this behavior ring any bells for anyone?

Yes, it’s an issue with the main chip on the daughter board being contaminated. 

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8 hours ago, bibilit said:

Yes, it’s an issue with the main chip on the daughter board being contaminated. 

I have another one I'm getting ready to do and will take your advice on that one first:

 

12 hours ago, bibilit said:

The chip should be removed, pads cleaned underneath and chip soldered back.

If all goes well I'll go back and rework my first one.

 

Thanks!

 

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This is the chip I am talking about. 

 

This board was heavily coated with cap goo, making the reflow process hard to manage. 

 

 

84FAF0DA-43A6-453D-B110-B6AB81C12059.jpeg

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@bibilit Finished recapping my second IIsi power supply, this one wasn’t as bad cap leakage wise as the first but I did take your advice on the chip on the daughter card. 
 

This picture is after an initial IPA Q-tip cleaning:

87FEF868-1C85-4A6F-80AD-1096E3342CCF.jpeg.9b98cc4dd7fc94287d76d344e57ac2e2.jpeg
 

This is after removing the old caps (which had clearly leaked), cleaning their pads, removing the IC (I used chip quik), and putting some UV mask near the lower cap pads where the copper was exposed:

86D7CBEE-0F87-4C09-9D33-85ED9E3CA10E.jpeg.3902f37e1f1722638a5b0460db6a1928.jpeg

 

And the final results after further IPA cleaning, soldering the IC back on, and new tantalum capacitors:

059E4625-A75A-44C9-92E1-3419F62A6A84.jpeg.f657bc2544118836bf6c6800be631db2.jpeg
 

Initial testing after finishing the rest of the recap of the PS looks good, no weird issues with shutdowns!

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