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View Full Version : Compaq Portable II, Shorted Rails



Conmega
December 7th, 2013, 03:24 PM
Well I got a Compaq Portable II off ebay and upon trying to power on it just cycles its power on and off, after looking into the problem I found a few things to have shorted power rails to ground... The main one as of now is the motherboard...
It first had its 5v, 12v, and -12v rail shorted to ground, the first thing I tried was pulling one of the ram chips and the 5v rail was now magically not shorted anymore, even after putting the chip back in it was fine.
So now the 12v rail and the -12v rail is left shorted...
I went around and found most if not all of the tantalum capacitors which where on the 12v rail and I tried to narrow it down to one but ended up just snipping one of the leads of all of them and there is still a short!
This is the first machine that I've had to do this much repair on so I'm running into dead ends here and am not quite sure where else to look or what else to try.
I'd love to get this machine running as its my first true 'luggable'.
Any and all help is appreciated!
Thanks!

(This is an earlier version of the machine with the larger motherboard if that helps at all, also the power-supply seems to work fine as it will power the fan in the case when not hooked to the board.)

Stone
December 7th, 2013, 03:45 PM
Did you pull all the cards and turn it on?

When the fan is on see if the 12v is there. If you don't have a meter you can see if it will power a hard drive when the board is not attached.

Conmega
December 7th, 2013, 06:06 PM
I checked the rails while the motherboard was disconnected from everything, cards out, power going to the HD/FD disconnected, power to the CRT driver board disconnected...
I just checked the supply with a meter, seems there is 12v out where the fan connects and then on the edge connector to the motherboard there are 5v 7v 10v 17v from a quick reading, there where some that where negative and such, I have no idea of the pinout but 4 pins in a row where connected together to a thick trace so I assumed it to be GND. I think the power supply is ok... From places I've read there is an PSU OK line on the supply and i saw two 5v outs I think so it should mean the supply is ok, just the board is shorted somewhere...
Also a few people seem to think the HD and FD get connected at the supply for power but the only outputs from the supply is a connector for the fan and a card-edge style female adapter that slides onto the side of the motherboard and the motherboard redirects power to everything else in the machine.

Oh I also remembered I took a photo of the motherboard the other day if it helps at all.
http://goput.it/tmh.jpg
The quality isn't the best, if you need a closeup of some area let me know and I'll be sure to try and get it asap.

iff69
December 8th, 2013, 12:41 AM
Do You have a service manual?
http://www.minuszerodegrees.net/manuals/Compaq%20Portable%20II%20-%20Maintenance%20and%20Service%20Guide.pdf

Conmega
December 8th, 2013, 03:13 PM
Yes I did download this, it helps with knowing what connectors are what but I do not think it had anything past simple troubleshooting.

shaker68
August 31st, 2016, 05:24 PM
Well I got a Compaq Portable II off ebay and upon trying to power on it just cycles its power on and off, after looking into the problem I found a few things to have shorted power rails to ground... The main one as of now is the motherboard...
It first had its 5v, 12v, and -12v rail shorted to ground, the first thing I tried was pulling one of the ram chips and the 5v rail was now magically not shorted anymore, even after putting the chip back in it was fine.
So now the 12v rail and the -12v rail is left shorted...
I went around and found most if not all of the tantalum capacitors which where on the 12v rail and I tried to narrow it down to one but ended up just snipping one of the leads of all of them and there is still a short!
This is the first machine that I've had to do this much repair on so I'm running into dead ends here and am not quite sure where else to look or what else to try.
I'd love to get this machine running as its my first true 'luggable'.
Any and all help is appreciated!
Thanks!

(This is an earlier version of the machine with the larger motherboard if that helps at all, also the power-supply seems to work fine as it will power the fan in the case when not hooked to the board.)


Just picked up a couple of these and one has the 12v rail shorted somewhere. the other works great. :D
Poking around to see if I can find anything obvious. Took the board out of the chassis to make sure nothing is touching
Did you ever get yours working? I realize this is an old post

shaker68
September 15th, 2016, 02:05 PM
Just picked up a couple of these and one has the 12v rail shorted somewhere. the other works great. :D
Poking around to see if I can find anything obvious. Took the board out of the chassis to make sure nothing is touching
Did you ever get yours working? I realize this is an old post

Just in case anyone is following. I found the short.
Since I knew the 12v rail was shorted I hooked up a 12v UPS battery to the floppy drive power connector.
Sure enough one of the orange capacitors popped
I replaced it, put it back together and it works fine