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IIsi Fuse blows in recapped power supply?

timoog

Member
Recapped the power supply and main board a couple of years ago and all working well, not used for 6months or so went to boot up last week and there was a pop from the psu, I’ve taken it apart and had a clean round no obvious faults, replaced fuse and pop again, any ideas or advice on where to even start diagnosing what I presume is a short somewhere?
 

joshc

Well-known member
I assume your PSU is the Sony variant? (There was another make of PSU in the IIsi but most are Sony).

There's a short/dead component somewhere.

If you don't have a component tester, I'd highly recommend one - I use a TC1 component tester, they are cheap. It will identify a component and tell you what it can about it, it's how I found some dead diodes on my IIsi PSU.

See here:


Other places to look, other than D105 and D106 is checking D301, TH101 and TH301. I would also check the resistors on the high voltage side.

You may also find the schematics useful. The IIsi PSU schematics are included in this download: https://archive.org/details/Macintosh68kSchematics

Hope this helps
 

timoog

Member
TH101 was blown apart! have replaced, diodes read ok and resistors check out - and still blowing fuse, some corrosion on the board from around D301 C302 see picture, maybe from leaky cap previously could this cause short? E8DBBAD9-D95A-4E90-A1F2-AA514DB90090.jpeg
 

joshc

Well-known member
Some of those joints don't look too good, probably an idea to reflow them with fresh solder. Did you get any further with it?
 

timoog

Member
Hi Joshc, on pause for a few days, figured out how to test continuity on those traces, all seems good. Ordered some diodes and a couple of thermistors will re-flow those joints at the same time and see. Watching videos on repairing switching PSUs!
 

timoog

Member
Replaced the diodes at 105 & 106 same issue, just starting to replace components now! Following this thread https://68kmla.org/bb/index.php?threads/macintosh-iisi-power-supply-diodes.32114/ I’ve ordered a STR80145, not sure how to test these so ordered replacement, probably D101 too. - any advice on testing these multi pin diodes With the TC1? Getting no or broken component on most pins and diode on 1+5 for the 80145. Perhaps D302 & 305 next looks like 30v & 33v Zener diodes from the schematic and post info, anyone know how important is the Max power dissipation? RS have these with 500mW dissipation. thanks !
 

timoog

Member
So following this guide on YouTube looks like the bridge rectifier D101is shorting - I understand from this that both middle ac pins shouldn’t give a reading but I’m getting .6 from pin 3(- lead) & 2(+ lead), can anyone confirm?
 

techknight

Well-known member
short means .000 or close to it. That seems normal to me.

You should start checking all components instead of throwing parts at it.

Such as, check the main regulator for shorts. check the switching transistors for shorts, etc... Sounds like the power supply grenaded itself and thats that.
 

timoog

Member
Yeah appreciate that @techknight, not skilled at testing some of these more complicated components - hence the video, trying new parts is a way forward. Even with the schematic I'm not sure which components you're referring to, can you give me a reference at all? Hope it's not grenaded itself, my fist mac, had it since '93 & quite attached!
 

techknight

Well-known member
Yeah appreciate that @techknight, not skilled at testing some of these more complicated components - hence the video, trying new parts is a way forward. Even with the schematic I'm not sure which components you're referring to, can you give me a reference at all? Hope it's not grenaded itself, my fist mac, had it since '93 & quite attached!

You do you, so if you wanna just start throwing parts at it, be my guest... but, what you demonstrate in the video is normal.
 

timoog

Member
Thanks I will, you too. So yeah the video is a tutorial on testing bridge rectifiers, my question was is it normal to get a reading on the two ac pins? I was getting .600ish in one configuration, see first two pics below.

So as it happens the new rectifier turned up today and no, it’s not good to get a reading. Good news is the psu is fixed and the iisi lives to bong another day!
 

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joshc

Well-known member
@timoog congrats on fixing it. Seems there's a few common components that go in these, so I'm glad you documented this.
 
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