HELP: 120V Macintosh Plus analog board blowing F1 fuse

[Will Cheyney]

I'm trying to rescue a US 120V Macintosh Plus that was gifted to me. The previous owner accidentally connected it to a 240V UK mains power supply without a step-down transformer. Whoops!

The original 2.5A 250V fuse had clearly blown so replaced it at the same time as doing a complete recap.

When I did the "smoke test" just now (and this time I made sure to the use the correct step-down voltage converter), the fuse blew again so something somewhere isn't right. Would anybody be able to help me diagnose what the issue could be? Just as a side note, the logic board works fine in another Plus so luckily that wasn't fried by the original mishap.

Thanks for any help in advance

Will

 

[jjclay]

Hi Will. Just taken delivery of a Mac Plus 120v here in the UK, and plan to convert it to 240v (the “International” ABs we’re designed to support both 120v and 240v, by simply changing some wire links and a few other components). Can you tell us which specific model of AB you have? There were at least 4 designs.

With regards your fuse blowing, I’d suggest you start by checking the “safety capacitors”. These are designed to blow open, but the Y caps connect line and neutral to earth, and if these are bad it can trip the RCD (earth leakage device connected to your mains fuse board). Depending on your AB model, you may find them at C33 and C36 (square RIFA Y capacitors), and C37 (large RIFA X2 capacitor which if the original can produce “magic smoke”  ).

Hope that helps...

 

[Will Cheyney]

Hi Will. Just taken delivery of a Mac Plus 120v here in the UK, and plan to convert it to 240v (the “International” ABs we’re designed to support both 120v and 240v, by simply changing some wire links and a few other components). Can you tell us which specific model of AB you have? There were at least 4 designs.

With regards your fuse blowing, I’d suggest you start by checking the “safety capacitors”. These are designed to blow open, but the Y caps connect line and neutral to earth, and if these are bad it can trip the RCD (earth leakage device connected to your mains fuse board). Depending on your AB model, you may find them at C33 and C36 (square RIFA Y capacitors), and C37 (large RIFA X2 capacitor which if the original can produce “magic smoke”  ).

Hope that helps...

Thank you! @jjclay

The analog board I have is the 630-0102- J dated 1986.

I replaced the RIFA paper caps at C33 and C37 but the larger cap at C38 was already a red poly cap so decided to leave it as was.

I didn’t get any magic smoke just a flash of the fuse blowing.

I understood that these board can’t be converted to 240V? 

 

[bibilit]

No, the 120 Volt cannot be converted to the 220/240 variant, only the AB with the label "International" are able to do so.

The 120 v is a single voltage only.

 

[jjclay]

Hi Will

Bilbit is spot on - regardless of AB variant designation, your Mac Plus AB needs to say "International" for it to be able to be converted between 120v and 240v (see first pic below).

My 120v Mac Plus (which was made in California!) does say International on the AB, and FWIW the AB model number is also 630-0102 (with no letter designation after it - there were many) and alternate part number on other side of AB is 820-0107-D (yours probably says 820-0082-E). Mine was made by Capetronic (Taiwan) during 40th week of 1988, a couple of years after your 1986 board. I've attached a few pics below for interest.

There was an earlier thread from 2007 which compared a bunch of Mac Plus (and Mac 512k etc.) AB variants, which I can't find a link to but it was titled "Mac 128k ~ Plus Analog Board Analysis" (neither Google or 68kmla search can find it, but I kept a copy in Evernote, phew!).

In terms of diagnosis, I'd first start by making sure your transformer is setup right. Check its settings and use a multimeter to double check what mains voltage its feeding the Mac Plus. Maybe its not 120v after all?

Then make sure you have the correct fuse replacement. Even though you've a 120v machine, you need a 250v 2.5A fuse (specifically, according to my AB a normal blow UL fuse for 120v - I can see some of this marked in your AB pic). By comparison my International AB operating on 240v needs a 250v 1.6A Type T fuse, and jumper W12 removed. See pic below for this guidance from my Mac.

Assuming you have a multimeter, once the input voltage and fuse check out I'd then check diodes & look for one that may have failed as a short in the rectifier area CR23,24,25,26, then check the other diodes. R55 can also be an issue - but normally this fails open and just results in a blank screen, not a blown fuse. Also check for other shorts (e.g. due to sloppy re-soldering following a recap). Then I'd focus on other components in the mains voltage area.
 

That's all the ideas I have for now, but I'm sure others who've been at this more than me can offer more advice.

JD 

 

[Will Cheyney]

@jjclay Being based in the UK I’m used to the international boards - this is the first time I’ve come across a US 120V board. They’re are a number of differences in the layout and components.

I replaced the fuse with an exact match to the original (and what is stated on the board). 2.5A 250V.

The voltage converter I’m using is brand new and the 120V output checked out just now.

All the resistors you mentioned I check all gave the same reading on the multimeter so I assume we can rule those out.

I did a good job with the soldering and reflowed the usual places that can suffer from dry/cracked joints.

This is all helpful though - I’m learning!

Pictures of the front and back of the board attached.

 

[LaPorta]

In the midst of all this...why don’t you guys just get a converter and plug the board into it?

 

[Will Cheyney]

In the midst of all this...why don’t you guys just get a converter and plug the board into it?

@LaPorta that’s what I’m doing and it’s causing  the fuse to blow every time. I’m using a 240V to 120V step down voltage converter (with an adequate wattage rating).

 

[LaPorta]

It’s possible there is something else wrong with the board other than that, for sure.

 

[Will Cheyney]

I’m hoping someone with past experience in a similar issue may be able to make some suggestions

 

[jjclay]

Hi Will. You mentioned the previous owner had connected it to 240v instead of 120v, and that had blown the fuse. I think it likely that some other component has been fried before the fuse went. I checked the failure mode of X2 and my money is now on that:

When a Class-X capacitor, also referred to as an "across the line capacitor"—the capacitor placed between line and neutral—fails because of an overvoltage event, it is likely to fail short. This failure, in turn, would cause an overcurrent protective device, like a fuse or circuit breaker, to open.

Ref: https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/

I’d test for a short across X2’s pins next...it might be new looking but you didn’t replace it, right?

JD

 

[Joe]

The photo seems to show a black mark on top of LF1, if that is a burn mark then maybe LF1 has a short. Also, is the hightlighted resistor burnt or just black in colour?



If all good then my next step would be to desolder the switching transistor (and remove it from the heat sink), then switch on the power and see if the fuse pops or not. If it does, then it is likely you have a fault somewhere from the AC input to the 170V rail.



If not, then most likely the switching transistor (yellow circled in photo above) and/or associated circuitry is shorted out.

 

[JDW]

Will, do you have a multimeter to test the voltage coming out of your step-down converter?  And is the frequency of 60Hz correct too?  Even if the voltage and frequency are correct, what happens when you connect another 120VAC device (other than your Mac) into that converter?  I am trying to make sure you are getting good power going into your board.  If you are, then of course diagnosing your analog board will make sense.  And the things others have suggested are quite good.  You basically have to troubleshoot the board on a component level to find which component is causing a short, since that would be the only thing on the board itself that would repeatedly blow the fuse.

 

[Will Cheyney]

@Joe This is a HUGE help. Thank you.

The black mark on the top of LF1 isn't related to it being burnt. It's some kind of marking that appears on this component.

The highlighted resistor is just black in colour. In fact, no component is burnt/scorched. No funny smells either.

I'll give the rest of your trouble shooting a go and report back.

@JDW Thank you for your reply too.

I've checked the voltage output of the converter and it's right on the money. It's also at 60Hz and has a rating of 100w. No no issue there.

 

[Paolo1960]

Hi, this my first post here.
I resume this old thread because I have same problem faced by Will. A macintosh 512k 120v Connected to a 240v outlet.
I replaced the fuse and the following caps: C33, C37 and C38 plus CR26. But when I turn the circuit on R44, R45 and R48 went on Fire. Now my idea is to replace the transistors Q11 (vbc and vbe is equal to 0.18V) and Q9 ( here all the three coltages are the same more or less equal to 500mv) and CR29. What do you think? Is there any chance to have it alive or not?
thanks

 

[lfletche]

I'm also facing a similar issue with a 110V Plus AB that was plugged into a 240V supply - about to start diagnosing the components. @Will-cheyney did you end up having any luck solving your problem here?