VOGONS


First post, by Kahenraz

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I bought this board because it's a Socket 7 ATX, has three ISA slots, uses SDRAM and has an onboard Yamaha sound chip. A very nice set of features.

Unfortunately, it does not work. Powering it on provides no video. Removing the video board, CPU, or memory does not provide any error beeps which suggests a deeper problem. There is some minor corrosion on some unpopulated traces but the board is overall in excellent condition with no missing components that I could find or damage to any traces.

One thing of note is that the power MOSFET adjacent to the ATX power connector gets extremely hot. The processor also heats up despite nothing happening at power on. The only sign of life is that the power supply fan will spin and the board is responding to the power-on pins.

Is anyone aware of what might be a common point of failure? Maybe the MOSFET?

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Reply 2 of 39, by Kahenraz

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I may have found the problem. It looks like this capacitor by the ATX power connector has leaked. The others now all equally suspect as well. And they are surface mount which is going to make repair more difficult.

I will get a POST card as well. Thanks for the suggestion. Does it matter if it's ISA or PCI?

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Reply 3 of 39, by BitWrangler

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Has it? Looks more like flux residue to me.

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Reply 5 of 39, by mr.cat

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LewisRaz wrote on 2021-08-11, 20:55:

I think getting a post card would be a good first bet!

It's not a bad idea, but since there are no beeps that could mean the BIOS can't run, so no POST codes either. Doesn't hurt to try though!
Caps are usually the top suspect in the boards of this vintage.

Reply 7 of 39, by BitWrangler

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No CMOS battery in pic, did you put one in it? Really freaking annoying that some boards act dead without one, but it happens.

Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.

Reply 8 of 39, by Nexxen

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Could you get a reading on that mosfet?

left centre right legs

What is the cpu in the socket? Vcore can be determined and maybe the mosfet is just busted.
I have a board with mosfet+heatsink at it's unbearable hot but in specs.

Edit: give it a good clean. All sorts of filth can prevent the board from booting.

Last edited by Nexxen on 2021-08-11, 22:59. Edited 1 time in total.

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Reply 9 of 39, by Kahenraz

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BitWrangler wrote on 2021-08-11, 21:54:

No CMOS battery in pic, did you put one in it? Really freaking annoying that some boards act dead without one, but it happens.

The CMOS battery that came with it was very dead. I did try replacing it with no improvement.

Nexxen wrote on 2021-08-11, 22:14:

What is the cpu in the socket? Vcore can be determined and maybe the mosfet is just busted.
I have a board with mosfet+heatsink at it's unbearable hot but in specs.

The board came with an AMD-K6-233ANR.

Nexxen wrote on 2021-08-11, 22:14:

Could you get a reading on that mosfet?

How do I read a MOSFET? I have a multimeter and a capacitance meter. Can I get a reliable reading without removing it from the board?

I also tried replacing two capacitors, one by the CPU socket and the other by the ATX connector. The removed capacitors read correctly at 22uF as labeledand there was no change after replacement. There are two more capacitors between the PCI and ISA slots which I did not replace as they are in a precarious location and I don't think are related to this issue.

Predator99 wrote on 2021-08-11, 21:50:

Does it power on at all? There seem to be extra pins right to the ATX connector? Look here
A Permanent Solution to the Dell 'Fake ATX' Power Supply Problem?

It appears to be the standard ATX pinout. The power connector is not shifted as indicated by the photos.

Last edited by Kahenraz on 2021-08-11, 22:48. Edited 1 time in total.

Reply 10 of 39, by Nexxen

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Yes, it's easy.
Black probe on ground and red probe on DC voltmeter.
Touch the three legs one after the other and report the reading.

K6-233 ANR Vcore 3.2 volts and I/o 3.3, readings, except middle leg, should be around this.
Unless I'm totally wrong and will be chastised.

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Reply 11 of 39, by kaputnik

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Had a somewhat older socket 7 board acting exactly like you describe due to a busted keyboard controller. It initializes very early in the boot sequence, before video etc, and the BIOS wouldn't get past it.

AT/PS2 don't support hot plugging, wasn't all that uncommon that ppl killed their mobos by pulling/connecting the keyboard with the computer running back in the day.

Would also suggest getting a POST card if you can't find anything obvious.

Also agree that the residue close to the cap looks like flux rather than electrolyte.

Reply 12 of 39, by Kahenraz

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That would be unfortunate. The keyboard controller should be pretty well integrated in a large chip somewhere.

Nexxen wrote on 2021-08-11, 23:02:
Yes, it's easy. Black probe on ground and red probe on DC voltmeter. Touch the three legs one after the other and report the rea […]
Show full quote

Yes, it's easy.
Black probe on ground and red probe on DC voltmeter.
Touch the three legs one after the other and report the reading.

K6-233 ANR Vcore 3.2 volts and I/o 3.3, readings, except middle leg, should be around this.
Unless I'm totally wrong and will be chastised.

Facing the MOSFET as in my photo I read the following voltages from the pins left-to-right:

1.6v, 2.85v, 4.95v.

I don't know how tight the tolerances need to be but this seems pretty close to 3v. I don't think the transistor is bad.

https://josvandijken.nl/downloads/pdf/datasheet_lt1584ct.pdf

Reply 13 of 39, by zapbuzz

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the age of this motherboard makes me think deeply. It may benefit from new caps but I would check the chips with a oscilloscope. especially the quartz crystal. Given the heatup commented I would automatically replace caps and mosfet/s. Schematic diagrams are hard to get for these but thats essential because some backyard hobbyists subtitute components often in disreguard to ethical substitution. Substitution of mosfets can be done with technicians substitution guides many hooyist component supplies outlets keep them handy and can help people get them.

Reply 14 of 39, by Kahenraz

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The board only has four capacitors and I replaced the two by the PSU and ATX power connector. The other two are positioned between the PCI and ISA slots and are unlikely related to the problem.

The transistor is getting 4.95 input voltage, 1.6v on the modifier, but only 2.85 on the output when it should be closer to 3.3v.

Maybe 2.85 is too low? I'm testing it in-circuit and under load and I don't know how this may affect the reading.

I also found the super I/o chip. It's huge. I could replace it but sourcing a chip would be another $20 into this hole and I'm not even certain that it would fix it.

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Reply 15 of 39, by Nexxen

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Can you measure your PSU voltage rails?
5( red), 12 (yellow), 3.3V (orange)

Maybe the voltage isn't correct, are there pins for voltage selection?

Maybe the PSU has low voltages... (guess)

4.95 if directly from the psu looks a bit low to me.
I'm no pro at repairs 😀

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Reply 17 of 39, by Tiido

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The heatsinked part is a voltage regulator not a MOSFET. Since it gets super hot it means there's probably a short on its output, and most probable culprits are one or more of the tantalums nearby, if the heatsink still gets really hot with no CPU installed. There's a chance the regulator itself is dead but that is unlikely, they have overcurrent and other protections that prevent destruction even with dead shorts on their output.

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Reply 18 of 39, by Kahenraz

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I checked all of the voltages on the 20-pin connector and they all looked good. The 12v rail on pin 10 was a little low at 11.66v. All of the other ones were only slightly higher than their expected voltage.

I checked all of the tantalum caps and none were shorted.

Reply 19 of 39, by snufkin

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Does that regulator get hot even with a bare board (no cpu+ram)? If yes, then do any other components on the board get hot? The regulator will be producing about 2.5W/Amp, so to get a heat sink that large extremely hot there must be a few amps flowing somewhere. With the board off, what's the resistance between the output pin 2 and ground (mounting posts or connector shields are convenient places to use for a ground)?