I bought a Kapok 6400 the other day thinking it would be an easy fix, I have the PSU for it ready so it should be easy to get it going right?
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It's a super cool (though rather noisy) pentium era laptop with a game port and built-in wavetable chip.
When I got it though, it didn't do much, my bench PSU adapter wouldn't hook up so I had to hook it directly to the original PSU. Turned it on and it doesn't do anything...
Took the lid off and uh this zener diode on this top PCB is really hot?
Oh wait that's the DC-DC power supply board and that's the 3.3v rail... I have a Kapok 6200AT (later design, supports MMX CPU voltages properly) and can check the voltages and zener diode on that. It took me a while to learn how to check the voltage limit of a zener diode, seems it was limited to 4.3v by that zener diode that burned so... it was more than 4.3v going through the CPU and anything else on the 3.3v rail...
I happened to have a spare kapok 6200/6400 motherboard without the VRM board and plugged the VRM from the burned board into the spare board. Ah, that burned the zener too... ok let's take a look. Found the power input, ground and output of the removable DC-DC VRM module. Probing around the LTC1435 switching regulator and the SI4410 n-channel mosfets, those are all working. The frequency reading on my multimeter is reading 16khz+ on the gate of both mosfets and the DC-DC board is running cool so no failed parts that are obvious.
It had a foam pad over some of the board though and checking deeper the foam pad was covering two resistors - those resistors just so happen to be the voltage divider resistors that the VRM controller chip uses to determine the output voltage and this module should be outputting 3.3v.
Checking it though, I put in 5v and it outputs 5v - so has the regulator failed??? Well, then I used the soldering iron to move those voltage divider resistors off from the circuit so I could measure them. One of them reads at the value the datasheet specifies, but the other reads infinite. That's BAD. I think the foam pad adhesive may have corroded the resistor perhaps, or it happened before this laptop was put away in storage...
If the voltage divider resistor that hooks to VCC is measuring as infinite, that means the regulation voltage that the VRM is aiming for is not right. So when it should be giving 3.3v, it's instead gonna give as much as the input voltage, which is 18 volts!!
See the discolouration on the pins around that upside-down pentium CPU? That's the original CPU which has VCC and GND shorted so it won't do anything good now.
That does explain why on the thermal camera, one of the cache chips was getting up to 170C. And one of the nearby chips which I think is the keyboard / embedded controller, has a hole in the middle.
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Fixing that broken voltage divider resistor puts this melted board to much more normal temperatures though I haven't replaced the zener diode of the DC/DC board yet. I really really hope it didn't nuke the chips on the spare board but who knows...
The wavetable chips were safe so at least I can do something cool with the broken mainboards.