Apple IIgs power supply on the fritz MWP-303

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Joined: Sep 21 2014
Posts: 74

Ok, people , looks like ive got a power supply issue on the 2gs

I noticed it was having difficulty turning on. You can hear tht fast audible chirping from the power supply. Thinking it was the PS, I pulled the memory card.

With the memory card pulled the computer will turn on. However it ticks several times as the voltage ramps up then it turns on.

I can get it to turn on if I remove most everything from the unit

If it hasnt been running for a few hours it will come on with the 1 mb ram card. If you shut it after a few minutes and try to restart it, it just does the chirping noise

The measured voltages without the memory card are

+5
+10 should be +12
-12
-5

By recommendation Ive replaced the 4 small capacitors C6 C7 and C8(3 near the transformer) as well as C12

any suggestions? Any testpoints I can check to determine component failure?

Jordan

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Offline
Joined: May 27 2013
Posts: 792
Re: Apple IIgs power supply on the fritz MWP-303

Any power supply that begins with model number "MWP" is not an Apple power supply, it is an aftermarket power supply. These MWP power supplies have more amps than the regular Apple power supplies.

You would need to approach the repair process like you would do to any other power supply.

If you replace capacitors near the transformer that would probably not correct a voltage problem. If the power supply has a bridge rectifier circuit then the capacitors are used to clean up the signal and would not have anything to do with the power output.

When approaching the repair process on a power supply just as in any other type of electronics repair, test the semiconductors first. Diodes, transistors, IC's etc.,

If there are voltage regulators then test those. Verify the input voltage vs. the output voltage.

That's the best advice I can give you. This is general advice that I heard when trying to diagnose electronic circuits. I don't know what level of technician you are so I have no idea if you have heard this stuff or not. I confess that I have not ever been successful in repairing a power supply except the one inside the Philips DVP-642 DVD Player. (red flashing/blinking standby/power light - C316 bulging capacitor problem)

So, I admit that I am not very good at this stuff although I would love to learn. I'm a lifetime student, I want to know everything.

Offline
Joined: Sep 21 2014
Posts: 74
Re: Apple IIgs power supply on the fritz MWP-303

I found the problem with this particular power supply is 76 reads and one reply. Same thing on the Apple II facebook group.

Not a discredit to anyone , mind you. But apparently this item is one not popularly understood in the apple ii circles like other equipment.

I gutted the power supply and replaced it with an ATX board at twice the wattage rating.

I consider this problem now solved

thx

Jordan

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Joined: Oct 2 2014
Posts: 9
Re: Apple IIgs power supply on the fritz MWP-303

I don't know how to fix it. However, if you want to replace the internals I would go with either:

http://www.onlinecomponents.com/eos-power-lfmvlt804000.html?p=43406272

or

http://www.onlinecomponents.com/eos-power-lfmvlt1104300.html?p=43406252&ref=OctopartFeed

They fit inside the existing IIgs case so from the outside it will look the same, but you will get either an 80 ro 110w 85% efficient power supply.

Thanks,

__________________

Apple IIgs ROM3; TransWarp GS, GG-LABS 4MB RAM; CFFA3000 v1.0 rev C; Apple II High-Speed SCSI, System Saver GS; Two 3.5; Two 5.25

Wanted - SuperDrive controller

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Joined: Sep 21 2014
Posts: 74
Re: Apple IIgs power supply on the fritz MWP-303

Ive since replaced it with an ATX power supply