Fixing a Samsung SCX-4725FN laser printer

My friend called me about a Samsung multi-function laser printer that didn't wanted to properly power on anymore. I said that I don't know anything about printers, but that it sounded like a PSU fault (as always...) and that he could try to google it. After just a few minutes he sent me this link, apparently describing more or less the exact same problem that he had. So I said, sure, if it's only a cap I can always take a look at it.

Said and done, I opened it up and here's what I found

The PSU at the back of the printer. The faulty cap in red square.

The faulty swollen cap in red square.

 It was identical to the one pictured at Flickr. The PCB was brownish around the cap indicating heat from the resistors. As this wasn't enough, the poor cap must also tolerate heat from the heatsink right next to it. No wonder it can't hold up for too long (this one about 5 years). On the other hand, it wasn't the worst cap model available that is normally rated for 85 degrees Celsius, but a slightly more tolerant, rated for 105 degrees C and 3000h.

The cap was easily replaced with a new one, this time with one that should withstand 105 degrees C for 10000h. I think something else will brake before that next time the printer fails ;)
Below is the result.

New better cap in place.

Printer up and running again.

Fixing a Canon CA-910 battery charger

A friend of mine (and also neighbor) came by the other day. He had borrowed a MiniDV and with it, of course, a battery charger. Now, when he connected the charger to the outlet, it just "poffed" and didn't of course work after that. That's when he called me ;-)

The Canon CA-910 battery charger

What I first noticed was that there seemed to be something loose inside it, because when I shooked it I could clearly hear something rattling inside. If this something was of some kind of conducting material, then that could very well have caused the failure.

Well, nothing else to do but to open it up I have a look. And this is what I found.

The loose objects inside the charger (above the pcb).  Note the missing ferrite core on one of the coils.

It was the ferrit core of one of the coils/chokes that had split into three pieces and fallen out of the coil housing. Now, those ferrit parts are indeed conducting, and if you take a closer look at the resistor to the right of the fuse, you see that it's been burned. So this is probably what's happen.

The loose piece that short the charger.

The good news was that nothing was really damaged, because the current went straight through the fuse and blew it. However, after taking a look at the other side of the pcb, I saw this.

Track burned off due to the short circuit.

The fix was i.e. quite easy. With some quick glue the pieces was replaced where they belonged, and the PCB track gap was bridged with a piece of cable.

New fuse and the ferrite core glued into place.

A piece of cable to bridge the track gap.

After those fixes, the charger was working again! Job done =)

Reparing an Airam Cooler 262T cool box

We bought this "active" cool box to keep the food in good shape on longer car trips. The box can be used on both 12V DC and from a 230V AC outlet. The 12V side sports a voltage guard, keeping the ~60W peltier from draining the car's battery.

The Airam Cooler 262 T

The box haven't been used that much the 2 years we have owned it, but there already seemed to be something fishy with the voltage guard this summer. I didn't mind taking a look at it before our planned trip to Lapland was closing in in September.

Verifying the fault

I begun by testing it in the car, with the engine running, and as I feared the box didn't start but only complained about low battery level. I then took it inside and hooked it up to a wall outlet, and to my big surprise the same "low battery level" LED lit up. What?! Nothing else to do but to open it up and see what's going on. However, that was easier said than done, because I couldn't find any screws that kept the top part of the lid in place. I almost broke it trying to figure out how to open in, when I realized that the screws must be underneath the lid's foam gasket...

Lid screw position under the foam gasket. One screw per side.

Reverse engineering the PCBs

Inside the lid I found two PCBs (actually three, but the third one is the 230VAC/12VDC transformer). One with a couple of relays, seemingly some kind of power board, and the other with some finer electronic components.

Relay and heat-or-cooling switch PCB

Under-voltage protection and LED indication PCB

I figured that it's best to just trace the tracks on the PCBs and sketch a schema over the circuits.

Sketched schema over the power PCB

Sketched schema over the voltage sensing/illumination PCB

Nice, huh? ;)


So, I realized that relay 1 (REL1) is a switch for battery or outlet power and REL2 is the safety switch for preventing the battery from draining. The thing is that I could hear a *click* when the outlet power was connected, which meant that the REL1 worked but REL2 didn't or wasn't just energized.

Next I tried adding 12V directly on the relay coil, and it clicked, which meant that the relay was working. This also meant that some part of the under-voltage protection sensor electronics was broken.

Finding the faulty component

By looking at my schema I quickly found a transistor grounding the relay coil on signal from the comparator (IC1). By measuring the voltage on the transistor's (Q1) base I saw that it sure did get the signal comparator, but it didn't lead current through to the emitter. Ahaa! There's the problem. I then tried to short circuit the collector and emitter legs, and voila! the fans started spinning!

Faulty transistor.

I replaced the transistor with one that I found in my drawer, which had kind of the same specs, but the collector and emitter legs where switched, so I just had to turn it 180 degree compared to the original one before soldering.
Tested with both 12V DC and 230V AC power source and both now works again!

Green light and a spinning fan, the thing is working again!

Repairing a Philips Plasma TV

I was visiting my sister some weeks ago and their TV had given up. It was an almost 10 years old, first generation plasma TV which was state of the art when it was new. Now it was about to be part of the gigantic e-waste mountain the "planned obsolescence" industry and our crazy I-always-want-the-latest-gadget society generates every year. To my luck, they asked me if I wanted to have it, in case I could repair it and hopefully make use of it for still some more time. Said and done, I stuffed it into our car and brought it home.

The TV set comes with the plasma screen and a separate tuner/input box. The two are linked with a vga cable with integrated stereo audio cables with cinch connectors.

The problem they have had was that the TV had begun to take some time to start-up and after a few weeks it got worse and they had to turn it off on the power button on the side of the TV a couple of times before it kind of got warm enough to come to light up.

Fixing the tuner box

When I got home and connected everything together, it seemed to me that the tuner box didn't want to start, because the power LED did some blinking, both green and red and the ended up in an amber colour. A quick google search, and I found this thread on FixYa. Seemed that some PIP (Picture In Picture) board had a faulty fuse. One of the solutions was to disconnect the board completely, and to my big surprise the power LED did now stay green! However, now the SCART inputs didn't work. Only sound came through...hmm...however, replacing the fuse 1800 (MP40, 400mA) and re-connecting the board did the trick.

Tuner box's PIP board with "replaced" MP40 fuse.

Fixing the plasma screen

Next problem was the plasma itself. After removing a fairly large amount of screws, the back plate did finally came loose.

TV's back side.

TV with back plate removed. PSU in the middle.

The TV did take a couple of on-off cycles before it finally showed any picture. But google to my rescue! I was certainly not the only one trying to fix a similar TV. Turns out its a quite common error on these models (if not almost all plasmas) due to the high temperature they generate and is most often caused by some dried out capacitors in the PSU. On this thread at avforums.com (22 pages long!). As I read the first pages I found out that I should try to change what seemed to be the most typical caps to fail, i.e. 2662 (25V 1000 uF) 2663 and 2664 (both 50V 100uF), but without any luck.

The PSU with the caps that I canged marked.

However, on page 10, I found someone that had figured out the most fragile capacitors on the exact same model as mine. The post is found here. I marked all those caps with a marker pen and found out that the caps were in smaller groups. By using the old hair-dryer trick I quickly found out that about 5 caps in one group was failing, because when pre-heated, the TV fired up instantly! These caps (2465,2508,2540,2541) were surface mounted (except one, 2510), so it was a bit tricky to remove without destroying the PCB. Actually one of the tracks came loose, and I had to replace it with a small jumper wire. The new caps were for hole mounts, so I had to bend their pins to fit the pads on the PCB.

New caps in place.

Surface mounted caps replaced with standard ones.

When the caps were changed, the TV lit up instantly, the fix had worked!

TV stand

Next thing was to get a stand for it, because I didn't want to mount it on the wall. So a bit of carpentering, and voila! a nice TV stand. Sorry for the bad picture quality (taken with my mobile's cam in a dark room...)

TV on stand, back side.

TV on stand, front side.