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Mac Plus keeps blowing fuse


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Hi all,

 

I have a MacPlus 1MB UK model that would not turn on. I had a quick look inside and noticed that the internal fuse had blown so I replaced it, just hoping it was just a simple fix.

Anywaym, this immediately blew and I have searched through the Mac repair gudes and could not find anything that could lead me to a solution.

 

Any ideas ?

 

StephenM

 

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Check whether the diodes in the bridge rectifier are still diodes and haven't decided to become shorts.  You might also want to check that the main switching transistor is still a transistor—on mine, it was fine and two of the diodes had gone, but if the diodes go, then it is likely to take out the transistor too.  I can't remember the component designations, I'm afraid (and my boards aren't entirely accessible at this precise moment).

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9 hours ago, cheesestraws said:

Check whether the diodes in the bridge rectifier are still diodes and haven't decided to become shorts.  You might also want to check that the main switching transistor is still a transistor—on mine, it was fine and two of the diodes had gone, but if the diodes go, then it is likely to take out the transistor too.  I can't remember the component designations, I'm afraid (and my boards aren't entirely accessible at this precise moment).

Cheers,

I pulled the diodes in the rectifier and they are ok. I  checked the thyristor which may have shorted by that seems good as well. I was reading late last night and it could be some of the other diodes that may have shorted which could benefit from beefing up.

 

 

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1 hour ago, StephenM said:

I  checked the thyristor which may have shorted by that seems good as well

 

That's very good news; that thyristor is extremely difficult to find a modern replacement for.  All the other diodes and transistors are reasonably common, as far as I know.

 

1 hour ago, StephenM said:

it could be some of the other diodes that may have shorted which could benefit from beefing up.

 

That whole power supply suffers from being rather underspecced, nearly all the components really should have been the next size up.  I am told that this was reasonably early days for this kind of PSU, and the designers underestimated the stress that would be put on the components.

 

You have probably seen it, if you have been reading up, but in case you haven't, this PDF contains a schematic of the US PSU.  The International one is not identical, but I found these schematics useful for finding my way around the analogue board regardless, even though the component designations were often different:

 

https://vintageapple.org/gamba2/images/plus_analog.PDF

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12 minutes ago, LaPorta said:

I am interested to know: what is a thyristor?

 

It's a kind of semiconductor device related to the transistor; you might also hear it referred to as a "silicon controlled rectifier", which I think is the more common name on that side of the pond.

 

A transistor is an electrically controlled switch, essentially.  It is a device with three wires, and applying electricity to one terminal of the device controls whether electricity can flow between the other two terminals (pedant's note: the precise definition of "applying electricity" varies with the type of transistor, but for the purposes of this discussion, we can gloss over that).  When you apply electricity to the control terminal ("base" or "gate"), current can flow through the other two terminals.  When you stop applying electricity, it cannot.

 

The really useful thing about this is that it allows a comparatively small amount of electricity to control the flow of a rather larger amount of electricity.  So, they find common homes in things like amplifiers, as well as anywhere you might want a fast remote-controlled switch.

 

A thyristor is like a transistor, but it has a kind of "memory" to it.  When you apply electricity to the control terminal (the "gate"), it allows electricity to pass between the other two terminals, like a transistor.  But, unlike a transistor, when you stop applying current to the control terminal, it keeps allowing the current to flow between the other two until that current stops of its own accord.  Then it resets and prevents current flowing again.  It's a bit like a locked door that needs to be opened from the other side, but once it's been opened, each electron passing through it can hold the door open for the electron behind it, and once no more electrons are coming through it will slam shut again.  So it kind of has a latching action; while electricity is flowing, once it has been turned on, it holds itself on until the current stops flowing,

 

This makes thyristors really useful in AC applications (which is, I assume, why there's one in the power supply here), and in fact they are the component behind most light dimmers.  If you think about a sine wave (which is roughly what AC is), it hits 0 twice per cycle:

 

 

1881216905_ScreenShot2021-05-06at11_04_16.png.237bc6ffa6fc305c547b89aa3c9c27ae.png

 

If you trigger the thyristor on once per cycle with a small poke, it will stay on for the rest of that hump of the AC waveform and then turn itself off at the zero point.  If, for example, you set it to turn on at the very peak of the wave, only the second half of each half-cycle will be passed between the other two terminals:

 

913228861_ScreenShot2021-05-06at11_13_29.png.d70d1bf928f690b998694a6956db816f.png

 

(pedant's note: this assumes the thyristor works with current flowing in both directions.  Most don't; some do.)

 

This means that whatever is connected in series with the thyristor's two current terminals will only see about half the power that it would do if it were getting all the electricity, and if it's a simple thing, like an incandescent lightbulb, that means it will shine noticeably less brightly.  (This, incidentally, is why a lot of dimmers don't work very well with LED bulbs, because they're more complicated and don't entirely like having their power supply mucked about with like this).  By using a thristor like this, the knob and the other components in the dimmer can be a lot smaller and will get less hot, because they only need to pass enough electricity to turn on the thyristor, not the whole current that will be used to power the bulb.

 

Does that help, or have I just muddied the waters still further? :D

 

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Well I think I can safely say I've buggered this one. It turns out it's a 120v Mac not a 240v, so no wonder the fuse blows. Is there any way to turn the analogue board into a 240 or am I on the hunt for a new board?

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Posted (edited)

Oh dear.

 

International 120V boards can be turned into 240V boards.  US ones cannot.  (There are two different 120V ABs, because why make it simple?)

 

However, I'm not sure what the survival rate is for these when overvolted.  If you do end up on the hunt for a new AB, though, don't be disheartened—they're not particularly rare, I picked up a spare one this year for not too much.  And hard-to-find components like the flyback (and perhaps the thyristor, if it tested good) are likely to be salvageable from that board.

Edited by cheesestraws
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