Help with (hopefully not completely) fried Apple IIc

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Help with (hopefully not completely) fried Apple IIc
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Hi Folks,

I recently plugged in my old IIc for my son to play with. He was surprisingly captivated - until it just quit: the screen went black, and the whole thing was totally unresponsive.

I'm not much of an electronics expert, and certainly not an expert in the much about the IIc. I do have the IIc Technical Reference Manual, some basic tools, and enough knowledge to get me in trouble.

Here's what I know:

  • The first thing I investigated was the external power supply. It's rated for 15 volts, but is putting out just shy of 20. Maybe this is the root of my problems.
  • The fuse F1 on the motherboard appears to be blown. I get no continuity across it. Power supplied to the input plug from a bench supply seems to get nowhere.
  • The internal power supply appears okay. If I supply it with 15 volts, I get just shy of +/- 12 volts and a nice 5 volts on all the appropriate rails.
  • If I bypass the fuse (did I mention getting in trouble?) and supply 15 volts, I do get a reaction. Specifically, the green power light comes on, but there is no beep and the disk doesn't spin up. I do get something on the screen, but it's garbage (see the attached picture). The computer consistently draws ~.65 amps.
  • I have pulled, blown out, and reseated all of the socketed ICs and ribbon cables I can find.

So, any suggestions on how to fix this or next steps in debugging? Or should I be looking for a replacement on everyone's "favorite" auction site?

Many thanks!

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Re: Help with (hopefully not completely) fried Apple IIc

Hey Dr Zook,

I'm sure someone here has better answers than I have. But first thing I would do is replace that fuse - if the //c isn't dead yet, it might be soon without one!

15 volts @ .65 amps is 9.75 watts, I think that draw is a touch lean? Is that off the original PSU when bypassing the fuse, or off the bench power supply? The original //c brick PSU puts out 18 watts (15 volts x 1.2 amps), maybe a little less due to heat loss.

Measuring the rails when not under load won't tell you the whole story, you should try measuring them while the //c is under load and see if any are undervolt. If so, that could explain the garbage on screen. You could then slowly increase the voltage on the bench power supply to see if it made any difference (after replacing the fuse!). Maybe a few extra watts is all it needs. The internal switching converter should operate anywhere between 9 volts and 20 volts, with less amps needed at the higher voltages.

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Re: Help with (hopefully not completely) fried Apple IIc

Hello,

I think you're right on next steps. I'll Mouser me up a 10A (!) fuse and swap it out. A good point on the PSU measurements, too -- if the fuse doesn't fix things, I'll measure the voltage in situ.

The .65A was the draw off of the bench supply while bypassing the fuse. I'm limiting it to 2A, but it's not drawing that much. BTW, I did notice what you suggested: if I varied the voltage up a bit, the amp draw was less.

Many thanks -- give me a few days to get the parts and I'll report back.

-DZ

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Re: Help with (hopefully not completely) fried Apple IIc

Okay, progress to report!

I replaced the fuse F1. The switch SW1 was also jammed in the on position - it is now unjammed.

Unfortunately, nothing much else seems to have changed. I get (roughly) the same screen that I posted earlier when running either off the external brick or my bench supply. Still no beep or drive activity. The //c also still pulls about .65A at 15V, with amps dropping a bit as I increase the voltage up to 19V.

I did test the voltages on the internal PSU while wired in, and they seem about right: I get 5V on the 5V rail, -11.5V on the -12V rails and +12.25 on the +12V rails. These vary very little as I vary the input voltage.

Thank you for the help so far - any other ideas would be very much appreciated.

-DZ

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Re: Help with (hopefully not completely) fried Apple IIc

Not an expert - but I think -11.5 volts is just a little too far outside tolerance, definitely could be a place to check.

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