Zenith Supersport 386SX Power Supply Repair Question

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Zenith Supersport 386SX Power Supply Repair Question

Postby T-Squared on Wed Dec 19, 2012 5:08 am

I recently acquired a Zenith Supersport 386SX laptop as a gift to myself for completing another semester of college.

However, one half-expected problem popped up: The hard drive had failed.

But the problems didn't stop there:
  • When I turned on the computer to test the leads for the hard drive power cable (to modify and replace the hard drive), the computer stopped working, the keyboard lock (Caps, Num, Scroll) lights started blinking, the PC Speaker started clicking, and the fuse blew on the power supply.
  • A few of the capacitors apparently blew, because I found some weird green liquid on one of the capacitors, and on the connector that joins a switch and plug inlet board to the power supply.

I went to Radio Shack to pick up a few capacitors and a new fuse, but it doesn't seem to be working. (I've been using capacitors that were of higher voltage, but of same capacitance.) Now the computer won't even turn turn on.

Even more, two of the capacitors had more of the green liquid directly under their rubber stoppers when I replaced them, and another one of them seems to have more resistance than usual. (About 300 Ohms, rather than the 5 Ohms they usually display on the multimeter.)

I used a 12V 1.3A power supply to test if the computer would even work, but now I've been using a power supply rated at 19v, 4.74A, and the computer can accept at least 16.5v 2A.

I was carrying the power supply along with me as my dad and I went out running errands yesterday, and I got a few static shocks from when I got out of the car.

Now, the question is, can a computer's power supply be fried by ESD?
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Re: Zenith Supersport 386SX Power Supply Repair Question

Postby Gibsaw on Wed Dec 19, 2012 8:52 am

I would be very surprised. You possibly still have repairs to do in the PSU, but what I think you'll find is that when the power supply has died, it has put high voltages through to the computer and done some damage.

Have you cleaned the main board of the PSU thoroughly? That electrolyte solution is conductive.
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Re: Zenith Supersport 386SX Power Supply Repair Question

Postby T-Squared on Wed Dec 19, 2012 9:10 am

Gibsaw wrote:I would be very surprised. You possibly still have repairs to do in the PSU, but what I think you'll find is that when the power supply has died, it has put high voltages through to the computer and done some damage.

Have you cleaned the main board of the PSU thoroughly? That electrolyte solution is conductive.


Yes, I've read Mr. Stewart's blogs about how he cleaned up some leaky capacitor residue on one of his Mac Classic II's. I did the same thing as soon as I lifted off the capacitors I wanted to replace, cleaning the residue up with some isopropyl alcohol and Q-tips. (Also, I thought these old 386 computers were about as robust as computers that used a 6502 CPU or any large IC chips, like Sorcerer, Apple II, Dick Smith, etc.)

Also, I think I may have just stressed the board a bit. If I'm repairing a computer, and the system does not immediately turn on, I immediately turn it off to prevent any sort of damage as best as I can.

By the way, I also forgot to mention that the 3.5" floppy drive also made a weird screeching noise for a second the first two times I switched the computer on after removing the hard drive, like the read-write-head motor drivers were receiving temporarily-bad data.
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Re: Zenith Supersport 386SX Power Supply Repair Question

Postby SpidersWeb on Wed Dec 19, 2012 9:40 am

386 desktops are strong, but the laptops generally not so much. After working on my old Toshiba 386 laptops, I cross my fingers when I apply power again. It's not ESD I've had issues with but caps wear out, boards get flexed, solder joins come apart (especially if SMD based which is likely) but first thing is to make sure the right voltages are getting to the board.

Capacitors should charge and then stop conducting - so you might get a burst of low resistance but then it should go up very high, if you have capacitors that are showing 5 ohms continuous then either they or something in parallel is causing a strong short. (It's usually the caps, but you never know)
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Re: Zenith Supersport 386SX Power Supply Repair Question

Postby T-Squared on Thu Dec 20, 2012 5:36 pm

Well, the first time around, I replaced two suspect capacitors on the board in the hope that it would fix the problem, but to no avail, and the system won't even respond with lights or clicks. (This is why I asked if it was possible to damage any computer's power supply with ESD.)

Today, I've replaced all but 4 capacitors on the power supply (technically two, actually, but the other two I replaced are non-polar and might not work correctly), but still nothing. I did actually have a few that were leaking electrolyte (not enough to spill out onto the board, though. I bought some caps and they should get here tomorrow. The good thing about the computer is there is no obvious damage to the motherboard. No magic smoke, nothing.

I've actually seen symptoms like this before with the system not starting up after capacitors have failed. My Sega Game Gear developed it back in 1998, when the capacitor chemical mixtures inside spoiled and the system would turn off every so often, eventually never turning on at all, that is, until 13 years later in 2011 when I replaced all the caps. It's now in perfectly-playable condition. I'm wondering if replacing all the capacitors on the power supply board will fix the problem and restore the power. (It's about 8"x1.5"x.5", so it's small.)
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Re: Zenith Supersport 386SX Power Supply Repair Question

Postby T-Squared on Fri Dec 21, 2012 4:33 pm

The computer's power supply has had all of its capacitors replaced, and I got something interesting I don't remember getting before: The light on the power supply brick that I'm using is blinking, as if it's constantly resetting. I don't smell any smoke or hear any hissing from the computer board itself.

EDIT: Scratch that! I wanted to test the power supply with just the screen attached (without the board itself being attached) to see if the screen or the backlight would work, even if no data was being sent to the screen, because you should NEVER use a power supply without a load, and I'm getting the exact same symptom! The light on the adapter is blinking, and no backlight on the computer! (This is leading me to believe something is wrong with the power supply board itself!)
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