Repairing a Gridcase 1520 Power Supply

A few years ago the power supply for my Gridcase 1520 portable computer died. It emitted a cloud of bad-smelling smoke and wouldn't turn on again. I placed it to one side and forgot about it for a few years. Now it's time to get it working again!

The Grid 15xx-series of portable computers use a removable power supply. It's shaped a bit like like a brick with one edge rounded off. Once the power supply is removed it can be replaced with a battery pack and the power supply used as an external charger. I have a couple of battery packs but they are long since dead and will not hold a charge. Maybe I'll open one up one day and see if they're fixable…

Opening up the power supply

Directions in this text assume that the power supply is oriented as if you were facing the rear of the computer about to insert it; i.e. with the IEC (kettle lead) socket facing you as shown in the image above. Obviously the power supply deals with mains voltage (240V here) so it should be unplugged while you are working on it.

Left side — First, remove the two outer screws on the left-hand side.

Right side — Second, remove the two screws from the right-hand side.

Top side — Finally remove the two screws at the front of the top side.

Top cover removed — Now the top cover should lift off. To remove the PCB you'll need to unscrew the nut on the inner-left side to release the earthing wire. Also remove the screw next to it that attaches the heatsink to the outer case.

Front cover removed — With the top removed, the front panel just tilts forward slightly and unclips from the bottom of the case.

PCB removed — The PCB can now be removed by sliding it forwards and out the front of the case.

Replacing the capacitors

Capacitor C1 — Closeup of failed capacitor C1. You can see the crack across it's top.

Capacitor C4 — (Blurry) Closeup of capacitor C4 hidden amongst other components.

With the cover removed it was very obvious what the problem was: two Rifa-style capacitors had expelled all their magic smoke. These are metalized film capacitors used as filters across the mains power supply, so they've had the full 240V ripping them apart. Because these are in a power supply across the mains input they're also X2 safety capacitors designed to fail safely. Replacement parts must have an equivalent rating in order to be safe. Fortunately enough of the outer capacitor was intact that I could read the part number: PME 271 M 40/100/56/B X2-type at 0.15µF 275V. A bit of searching found a datasheet with a compatible equivalent from Kemet: PME271M615KR30 from £1.44 each. Not cheap but worth it to get a near-identical replacement.

Unfortunately for me not only is it very cramped in there with the capacitors crammed up against a couple of inductors but Grid have also covered the contents of the power supply with copious amounts of resin. This is intended to reduce noise by preventing components (particularly inductors) from vibrating and can also help with thermal transfer.

I tried scraping and cutting the resin but with very little effect -- it's not soft like putty but hard like plastic. Time for plan B: I bought some acetone hoping it would dissolve the resin. It didn't, but it did cause the resin to soften and become incredibly sticky, almost like warm toffee.

I didn't think I had space to work on the capacitors with the inductor sandwiched between them, so I used a solder sucker on the reverse of the PCB to free all three components leaving just the resin holding them down. After removing the IEC socket to get a better angle of attack, I used the acetone along with a stanley knife to remove the capacitors and the inductor.

Removed components — The failed capacitors. Some of the dark material is resin but a lot is burnt up foil. You can't see the inductor but I nicked the enamelled wire and sliced through the leads connecting it to the PCB :-(

Inductor refitted — I used a little clear nail varnish to recoat the nicked inductor wires and the large capacitor which got scraped while removing the resin. I used old component lead cutoffs to extend the inductor's leads back to their pads and covered with some heatshrink tubing.

Capacitors replaced — Finally after all that I could fit the new capacitors. They're not polarised but it doesn't hurt to fit them in the same orientation…

A quick test went OK with nothing exploding. Let's see if it works!

Final result

Multimeter testing — Testing with a multimeter showed a touch over 16V output. Very stable too, considering it's running with no load!

Gridcase booting — With the power supply fitted the Gridcase starts booting…

Booting Windows 3.0 — …all the way into Windows 3.0! Check out that lovely plasma screen!

I'm going to call that a success!