VOGONS

Compaq Portable III - Power Supply Fried!

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First post, by Grizzly Adam

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Good morning all,

I have finally begun working on my Compaq Portable III several years after it first went down.

What I found: The capacitor right by F2, I think it was C5, was visibly toasted. The IC next to it (SN7406N) also shows signs of overheating, but I have not confirmed the damage. The chip seems to be a collection of NOT gates. The power supply was completely toasted with one of the upright boards physically blowing surface mount components off of the board. The -12v and +12v pins on the power supply connector are both grounded out.

Currently: I need to replace the capacitor again ( I did that once before I realize the PS was bad). The capacitor is shorted, so I am willing to bet that this is one of the issues. Last night I hacked up a atx power supply and connected it to the motherboard. Without the +12v and -12v connected in (due to the short) I have power to the board and the LED illuminates. I have nothing connected except the main board. Update: This cap was good, caps 109 & 110 were causing it to appear shorted when in circuit

Questions:
1. Pin 1 on the power supply is sense, I assume 5v sense. Is this pin looking for 5v or was it just feedback to the old power supply and should be left disconnected?

2. Does anyone know what all the +12v and -12v are used for? I am working under the assumption that +12v is used only for drives, but am unsure. -12v I have no idea. Solved, Caps 109 & 110 caused the short

3. The plasma display- I am looking at ordering a 12v to 200vdc board to power the display. I am told that the molex connector has TWO different 200vdc lines with resistance between them. Does anyone know what the deal is there, why there are two, do I need to have two separate circuits?

Thanks for any help,
Adam

Last edited by Grizzly Adam on 2023-08-29, 23:04. Edited 2 times in total.

Reply 1 of 4, by weedeewee

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2. most likely those voltages were used for the serial ports. and likely also for the expansion unit ISA slots.

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Reply 2 of 4, by Grizzly Adam

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weedeewee wrote on 2023-08-29, 16:47:

2. most likely those voltages were used for the serial ports. and likely also for the expansion unit ISA slots.

Thanks. I will target those areas tonight after I change out the cap and further examine that area.

Reply 3 of 4, by Grizzly Adam

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I did a little more poking around and caps 109 and 110 were the ones causing the issue. I would like to put back in close to what I took out, can anyone make a recommendation? All I have are ceramic and electrolytic.

Reply 4 of 4, by Grizzly Adam

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I think I am to the point of trying to replace the power supply when I get back home. I am using a ALLIED SL-275TFX, a very small ATX power supply that matches the wattage requirements of the system. I have already rigged up on ATX and saw the red led come come, so this is promising. I am still shooting in the dark a little regarding the following two items:

1) The yellow/sense wire-- what voltage is supposed to be here and is it originating from the power supply or flowing through the board and returning as a check value? I assume what we want to use 12v+, perhaps the Power Good atx value. Does anyone have any idea?

2) The red molex connector that powers the plasma screen. Different people are reporting different voltages on this. The markings on the board show the following:
(from top to bottom)
Yellow wire: DPY GROUND
Black wire: 200t
Red wire: 5t
Green: 200

Someone has suggested that the T may indicate negative. Can anyone check on their system and report back? Everything I have seen conflicts and I don't want to fry my screen.

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