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Zenith Supersport 386SX Power Supply Repair Question

T-Squared

T-Squared

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Joined
May 29, 2011
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657
Location
San Antonio, TX
I recently acquired a Zenith Supersport 386SX laptop as a gift to myself for completing another semester of college.

However, a few half-expected problems popped up.

  • The hard drive had failed.
  • 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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Is the 300 ohms between the capacitor's lead and the PCB trace, or is it between the two leads of the capacitor? If the resistance is between the lead and the trace I would recomend de-soldering the capacitor, clean the solder pad with alcohol, and re-connect the capacitor. If the reading is between the leads of the capacitor the capacitor could be bad. Although if this is an in-circuit test there may be other paths that connect the two leads, giving the resistance reading. Be sure when measuring electrolytic capacitors to put the black probe on the negative connection on the capacitor. The resistance reading on the capacitor should be infinite.
 
After deciding that the power supply might be the problem, I replaced all of the capacitors in it, 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. Even more, later 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!)
 
The flashing light on the PSU, based on what I know, does not mean that it is bad. I have a power brick that does a similar thing. The blinking stops once I turn on the laptop. You might want to check its output voltage to make sure that the PSU is the problem.
 
Old Computers said:
The flashing light on the PSU, based on what I know, does not mean that it is bad. I have a power brick that does a similar thing. The blinking stops once I turn on the laptop. You might want to check its output voltage to make sure that the PSU is the problem.
Click to expand...

No, no, the laptop itself doesn't have a power board with a blinking light. What I'm saying is that the power board (the one that powers the screen and the motherboard all at once) may be faulty. The power supply brick-on-a-leash (which DOES have a light) that connects to the board through the back works just fine.

Oh, and I almost forgot, I did try to insert a Conner hard drive that was originally too big for the computer. I took off the chassis of the hard drive (not the casing, just the special chassis separating the board from the drive housing.) and got barely enough room for when I would have put the cover back on. I used some longer screws to secure it, and I would have left it alone, except the screws could screw down further than I had anticipated, so I kept going. I'm hoping that the hard drive didn't press down on the motherboard and cause this failure. :( If I can get this to work, I'm going back to using a regular desktop hard drive with a modified power cable.)

I don't think it did, though. I seem to remember that it elicited the same symptoms (clicking speaker, flashing lights, and the first two times, the backlight clicking on for a second, then turning off.) when I removed the drive. Now with the replacement capacitors (higher voltage, same capacitance), it won't even turn on. The fact that even the screen or its backlight won't work when they're the only ones connected to the board leads me to believe there's something wrong with just the power board and nothing else. (Again, which is why I asked if a power board can be damaged by ESD. It has only one transistor and a bunch of power-management-only components like resistors, transformers, capacitors, and potentiometers.)

I'm just now testing this with the same working 3v-12v/USB variable power plug I used to see if it worked when I first got it (it would strain with the backlight when it would first start up, but it WAS working, even at such a low voltage!), and it's giving me the same problems the 19v power brick did, except the light totally goes out with this one. In fact, the light goes out when NOTHING is connected to the power board.
 
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I just thought of something last night, based on what someone told me when the computer first failed.

Can a computer system not work if it's not grounded?

I had a similar symptom when building a cable for transferring disk images to C64 disks, and a few pins on the parallel connector had to be grounded. I tried at first to run it without the pins grounded (only one was) to save time, but it caused non-destructive trouble for the disk drive and it wouldn't transfer anything. The drive worked fine after I went back and soldered those pins together.
 
I have had old laptops that appeared to have power supply problems. When I removed the battery, the unit powered up fine. It turned out that the bad battery was loading the power supply down.
 
PROGRESS! I GOT THE BLINKING LIGHTS AGAIN! :D (On the COMPUTER'S power light, AND on the keyboard!)

I added more peripherals to the system (the screen and the floppy drive), and the lights on the keyboard blinked EVEN BRIGHTER!!

2 more questions:
Is it entirely possible that the system was not starting because there was TOO MUCH power on the internal supply (thus no power good signal), and it was trying to protect itself from an overload?
Could the peripherals create a draw of power balanced enough that the power would be stable enough to start the system?
 
Ok, not so much progress. I tried turning it on with the rest of the peripherals connected (IDE and power also), but it won't start (still blinking lights). However, when I tried the 12v power plug, the lights became steady on.
 
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MORE PROGRESS!!!

In a desperate move, I poured isopropyl alcohol on the power supply (then dried it off with a hair dryer) to see if the problem was caused by a short similar to this one here: http://www.classic-computers.org.nz/blog/2009-04-13-mac-classic-2-dishwasher-fix.htm

When I plugged it in, the computer's power light started to blink again, but the BACKLIGHT STARTED FLASHING in time to the clicks! I'm getting closer!
 
Never mind, I'm giving up. The power supply may be too far gone. I tried. I think ZDSParts may have another power supply, but I can't be sure.
 
UPDATE! I bought a clunker Heathkit Supersport SX to see if the power supply was the problem. Turns out it is! The Zenith's power supply displayed the exact same symptoms on the Heathkit, but without the speaker clicking. The power kept resetting, the keyboard lights blinked, and I heard a very VERY high squealing chirp (like the kind that comes out of your TV without any background noise; a lot of people can't hear it because it's so high-frequency.) each time the lights blinked. HOWEVER, I put the original power supply back in, and it worked just fine!

I don't think the motherboard on the Zenith has actually been damaged. What I think is still happening is that darn fuse on the power supply. I had problems while trying to solder it in the first time, and I think the soldering itself caused a disconnect in the fuse (The inside of the fuse becomes pressurized from the heat, and causes the metal endcaps to lift off from that pressure, possibly disconnecting the endcaps from the fuse wire inside.) I'm going to go to Radio Shack tomorrow to to get a new batch of fuses and a screw-in fuse holder, just to keep from soldering in a new fuse directly. I hope that'll fix my problems.
 
Old Computers said:
Congratulations on figuring out the problem. Hopefully the fuse will work for you.
Click to expand...

Yeah, the Heathkit clunker cost me nearly $50. The weird thing is, the power supply is actually worse on the Heathkit. I smell a really foul odor when I turn it on, which I already know is the leaked capacitor electrolyte heating up, because I smelled the same odor while desoldering the legs of the original factory-applied capacitors I ripped off of the Zenith PSU board.

But I digress; the Heathkit is actually worse because there's something wrong with the internal video circuit. (The display will not darken and show characters. I already tried it with the known-working Zenith display) However, all is not lost: I got 3 bonuses with the clunker: a proper brick-on-a-leash power adapter. an internal modem, and a 387 MATH COPROCESSOR! (The coprocessor was mentioned in the sale, but I assumed that the seller was talking about the regular 386 processor and had made a typo. My big surprise was that the internal modem wasn't mentioned!)

The clunker itself still works, though, despite the flawed and aged PSU.
 
I haven't a clue what could be wrong, but there might be some schematics floating out in the internet somewhere. That is pretty awesome that you got the extras.

What are the voltages coming out of the power brick, and what is the polarity of the plug? I have a Zenith laptop, but no PSU, and I cannot find the info anywhere. Thanks.
 
Old Computers said:
I haven't a clue what could be wrong, but there might be some schematics floating out in the internet somewhere. That is pretty awesome that you got the extras.

What are the voltages coming out of the power brick, and what is the polarity of the plug? I have a Zenith laptop, but no PSU, and I cannot find the info anywhere. Thanks.
Click to expand...

The power requirements for the computer are 16.5 v, 2A. The polarity seems to be center pin negative. BTW, is this computer that you own a Supersport? (If so, you can probably get the power supply from the trashed one that's selling on eBay right now.)
 
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