VOGONS


First post, by maestro

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My 486 isn't powering up and I think it's the AT power supply.

When I switch the power on, the CPU fan spins about 1/20 of a rotation and I see the power LED on my CF2IDE adapter briefly flash on and off. When I switch the power off, the same things happens.

- This configuration was booting the last time I had tried it.
- I've removed all of the components except the CPU and I get the same result.
- I've tried reseating the socketed Dallas, which is dead and which I had removed/inspected between last boot and this boot.

No change in behaviour is observed during my troubleshooting and unfortunately I don't have another CPU or PSU to test against. I'm still hoping it's a simple fix.

Does this sound familiar to anyone?

Reply 2 of 22, by maestro

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You're right, or at least hot on the trail. I disconnected P8 & P9, left the CPU fan plugged in, powered up and both the CPU and PSU fans roared to life. It's a great lead. I'll see if I can figure out what's being shorted. Thanks for your help!

Reply 6 of 22, by chrismeyer6

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On older psu's without short circuit protection you usually get a nice fireworks display as the shorted tantalum cap blows up. But usually after that the system works again.

Reply 7 of 22, by creepingnet

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You might have a blown Tantulum capacitor (hehehe, TANTRUM capacitor) on the board that produced a short. This exact thing happened with my 286 about 10 years ago. All I had to do was remove the offending shorted capacitor and it has been powering on to this day trouble-free.

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Reply 8 of 22, by maestro

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Here's the pics, I had to retake them which delayed their posting. I couldn't see anything obvious but mine isn't a trained eye, just let me know if there's any area you want to see close up.

8bitbubsy says I could try 'measure the resistance between 12v and ground, and 5v and ground, on the PSU connectors (going to the mobo).' Is that a test for a short in the PSU?
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Reply 9 of 22, by digistorm

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If you plug your PSU into the motherboard, you can generally measure on one of the 4 pin molex connectors the resistance between black and red, and black and yellow. You could also lookup the pinout of the AT header and measure on those header pins. You should never have a resistance that is close to zero (few ohms) unless the header pins carry the same voltage (for example ground; therefore you need to consult the pinout of the AT connector).

Reply 10 of 22, by wiretap

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I'd check for bent pins on the QFP's to see if any are touching, check all tantalums for shorts, and test the voltage regulators.

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Reply 11 of 22, by Deunan

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maestro wrote on 2021-05-12, 20:23:

Ah nuts. This board's covered in tantalums.

Usually it's just the +12V line that shorts out, sometimes also the -12V one. You'd have to be unlucky to get any other shorts, but it's not impossible.
Get an ohm meter and test all the power inputs to the mobo vs ground, anything below 1 ohm is very suspect. If you do find a short, it's most likely the caps right next to the power connector (in fact these are usually the only ones for 12V rails). You can just remove those for testing, the mobo should boot without them, but obviously long term stability is not guaranteed (that being said even Pentium mobos don't use these voltages except maybe serial ports, these only go to the ISA slots).

Reply 12 of 22, by weedeewee

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http://www.win3x.org/uh19/motherboard/show/815

Another option... when you found the voltage line that has the short... either +12v, +5v, -5v or -12v, or more than one.
Is to take a power supply that is current limited and insert voltage on the line.
So let's say the 12v line has the short, you set your power supply to deliver a max of 1A at 1V and deliver that to the 12V line, now the shorted component will be dissipating at most 1W and will get hot enough to feel it getting hot and identify the shorted component.

Reply 14 of 22, by maestro

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I'll try to make time this upcoming long (holiday) weekend to diagnose those tantrum caps, this is an excellent guide you've all provided, I want to put it use and reward your efforts with a continuation of the story. Thanks everyone for your contributions.

One thing though, on this page: http://www.win3x.org/uh19/motherboard/show/815

The official bus speeds are 25 MHz and 33 MHz. There are unpopulated jumper locations for undocumented speeds but as I recall this is only on the 1.6 version. I have a thread bookmarked where somebody unlocked a 40 MHz bus option through experimentation although the author was unsuccessful at finding 50 MHz, and any speed above 50 is not supported.

Additionally, I've successfully used a single 32 MB EDO SIMM but when I tried two EDOs it wouldn't boot. EDO isn't officially supported and possibly only unofficially supported for the 1.6 version.

Reply 15 of 22, by evasive

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Fixed the speeds (these were CPU speeds still, not FSB speeds, we switched halfway through so a lot of boards need fixing now).
Added your comments.
Thank you for the info 😀

Reply 16 of 22, by maestro

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Hey guys,

Well, party weekend is over. Anyone want to help me diagnose this motherboard?

Foremost, I have looked for bent pins that could be touching and haven't seen any, I will keep checking though.

I tried powering it up again tonight with different RAM this time to the exact same result: The fan spins about one rotation upon power-on, then nothing, and then about one rotation upon power-off. I did notice a quiet electrical noise rising in frequency just before the fan spun, a build up of juice? None of the major components get warm when the power-on state is applied for some time. I guess consistency in this case is not the best result but not the worst either?

Using Skyscraper's guidance in another post, I've measured the resistance of the fuse, the tantalums and the AT pins (P8 & P9) shown in my attachment. Values are in Ohms with a 'k' or 'm' to represent kilo or mega respectively.

* varies: Means a stable reading wasn't achieved. When I watched one, the values appeared to go down and then up, starting around 200 m, then decreasing to about 3.1 m and then increasing from there.

Does this tell us anything? Am I doing it right? 😜

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Fuse
1.18

Tantalums (left to right)
0.3831 k
~65 m varies*
~3 m varies*
open circuit
0.3830 k

AT P8 (left)
varies*
0.3828 k
varies*
varies*
0.23 gnd-gnd
0.30 gnd-gnd

AT P9 (right)
0.14 gnd-gnd
0.15 gnd-gnd
open circuit
0.3827 k
0.3827 k
0.3827 k

Reply 17 of 22, by maestro

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Deunan wrote on 2021-05-14, 14:32:

Usually it's just the +12V line that shorts out, sometimes also the -12V one. You'd have to be unlucky to get any other shorts, but it's not impossible.
Get an ohm meter and test all the power inputs to the mobo vs ground, anything below 1 ohm is very suspect. If you do find a short, it's most likely the caps right next to the power connector (in fact these are usually the only ones for 12V rails). You can just remove those for testing, the mobo should boot without them, but obviously long term stability is not guaranteed (that being said even Pentium mobos don't use these voltages except maybe serial ports, these only go to the ISA slots).

Ever see the movie Sunshine? Look at my values ... What do you see? *Epic music commences*

weedeewee wrote on 2021-05-14, 15:14:

Another option... when you found the voltage line that has the short... either +12v, +5v, -5v or -12v, or more than one.
Is to take a power supply that is current limited and insert voltage on the line.
So let's say the 12v line has the short, you set your power supply to deliver a max of 1A at 1V and deliver that to the 12V line, now the shorted component will be dissipating at most 1W and will get hot enough to feel it getting hot and identify the shorted component.

I'd like to try this. I bought a mediocre variable power supply from 'the big jungle' marketplace for testing electronics, laptops, and other stuff that I pull out of thrift shops. The ability to apply controlled current for the purpose of fault detection by waste heat is another interesting use. Did you see the movie Sunshine?

Reply 18 of 22, by weedeewee

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maestro wrote on 2021-07-10, 03:45:

Ever see the movie Sunshine? Look at my values ... What do you see? *Epic music commences*
...
Did you see the movie Sunshine?

yes. Can't say I liked it. Left me with a lot of... meh.

Reply 19 of 22, by jakethompson1

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maestro wrote on 2021-07-10, 02:41:

I tried powering it up again tonight with different RAM this time to the exact same result: The fan spins about one rotation upon power-on, then nothing, and then about one rotation upon power-off. I did notice a quiet electrical noise rising in frequency just before the fan spun, a build up of juice? None of the major components get warm when the power-on state is applied for some time. I guess consistency in this case is not the best result but not the worst either?

Be careful and don't stand over the board if you continue doing this. If it is a tantalum capacitor and it finally explodes it could get in your eyes.
I assume the short is still there with no CPU in? Just to eliminate something easy if unlikely.