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Compact Portable dead power supply debug and fix

Cmolson

Member
Joined
Apr 5, 2019
Messages
20
Location
Ottawa, Canada
Hi guys,

I often find help on these forums without needing to ask any questions. I thought I would post how I brought my Compact Portable back to life. I received the unit from a friend at work, he had two and only wanted to keep one. I didn't see any other post pointing to this specific failure so here it goes.

My debug process

First thing I did was to try and power the unit up. No luck, the fan spun up but nothing on the display. Since the fan is run directly from 120V AC, it makes sense that it would work even if the power supply wasn't working.

I opened the case up and noticed no red LED on the motherboard.

I unplugged all expansion cards and the hard drive / floppy. Still no red LED.

I tested the +5V and +12V floppy power cable and they both read 0V.

I disconnected power to the CRT, still nothing.

Disconnected motherboard power, still read 0V on +5V and +12V rails.

At this point only the power supply was connected, so obviously something in it had gone. I followed the service manual to remove the power supply board.


Debugging the power supply

I checked the resistance to ground from the +5V and +12V rails, the 12V had a short! (0.1 Ohms or something).

I did some research here and saw a number of people had the tantalum capacitors go bad on them. I tried using an ESR meter on the caps, but I really have no idea what I am doing with that thing yet. I removed the two 225 tantalum capacitors and the short was still there.

There are also two 3300uF and one 470uF electrolytic caps, removed them and saw they had bulged but the short was still present.

At this point there was not much left on the 12V output, some kind of coil and a TO-220 diode with a heatsink marked USD835.

I removed the diode, bingo! The USD835 had gone bad. Not the 12V rail had about 70 ohm resistance to ground, which I figured was OK since there is a coil there.

IMG_20190709_004944.jpg


Repair

At this point I enlisted help from another co-worker and ordered what we think is a compatible part (USD835 was not available anymore). Here is the replacement part I used for the USD835: MBR1635G (ON Semiconductor).

I also decided to order new 225 tantalum caps, and all new electrolytic. Pretty much all of the electrolytic caps had bulged to some degree, so replacing them all made sense.

Replaced all caps and the bad diode, connected 120V to the power supply and now I had +5V and +12V.

I re-installed the power supply into the case and connected the motherboard / video card. Success! I had the little green cursor display.

Ther hard drive spins, and the floppy does attempt to seek but I don't have a 5 1/4" boot disk and the hard drive seemed to time out.

I ended up using a Gotek with FlashFloppy to get booted into dos 3.20 and that is where I am today.

IMG_20190714_105054.jpg
 
Hi guys,


There are also two 3300uF and one 470uF electrolytic caps, removed them and saw they had bulged but the short was still present.

...Here is the replacement part I used for the USD835: MBR1635G (ON Semiconductor).

I also decided to order new 225 tantalum caps, and all new electrolytic. Pretty much all of the electrolytic caps had bulged to some degree, so replacing them all made sense.

I've had similar issues in getting mine working; I've replaced the 225 tantalums, but the voltage drops from what it should be almost immediately after flipping the switch on the power supply (with nothing else connected).

Do you have a recommendation or record of which electrolytic caps you replaced? This is not my area of expertise and there are a lot of options online. I'm putting in a Mouser order for the diode you mention, and figured I might as well get the electrolytic caps as well.

Thanks!
 
I'm trying to follow this to repair my own Compaq portable power supply. However, the Diode in question has 3 legs on the board, the the replacement from mouser (from part number above) has only 2 legs. Did you ignore the middle hole on yours? It looks like it's connected to stuff...
 
Hi,

Really sorry for not getting back to this sooner.

The USD835 I took out only had two legs as does the replacement part I put in. I can confirm I replaced it with the MBR1635G that also has two legs.

Replacement I used: https://www.mouser.ca/datasheet/2/308/MBR1635-D-76328.pdf
Original part (I believe) http://njsemi.com/datasheets/USD835.pdf


IMG_20191125_214643.jpg

IMG_20191125_215002.jpg


As for the electrolytic replacements, I have an old order with digikey or mouser that I think has the correct ones, but I did make a few changes.

Basically most of the larger capacitors I just tried to find a close match in terms of capacitance (uF), I may have used slightly larger capacitance ones for things like 3300uF, as they are larger filtering capacitors and I think using a 3500uF would be fine. As for the voltage, I made sure to get equal or greater voltage rating. So if the stock part was 10V, I got at least a 10V or maybe even 16 or 25 Volt replacement.

The smaller tantalum ones (1uF or 10uF?) I tried to again just find an equivalent, they should be available but are more expensive usually than electrolytics.


I actually just repaired the CRT on one, I will make another post about that process (was working, but the horizontal controls did not work and image was offset to the right).

Update: I think I found my Mouser order with the following parts:

6800uF 35V - LGU1V682MELZ
1200uF 100V - LGU2A122MELZ
1500uF 100V - LGU2A152MELZ
680uF 160V - LGU2C681MELZ

Some of these may have been for other projects. Double-check the capacitance/voltage of course.
 
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