Re: Lexmark 544dn power supply buzzing sound model 40x5409

I did check the before pictures to make sure I had the caps in correctly. Those also matched the new supply and the replacement caps I put into that. So, it sounds like the voltage feedback was the issue. There was a small arc-burn and a blown up 0-ohm resistor under the board right in the area of the coil and rely near the large primary cap. That gap (to the back plate) is over 1/4” so that must have been a pretty high voltage indeed. The smell from steam of the secondary caps was awful.

Re: Lexmark 544dn power supply buzzing sound model 40x5409

Well, after I replaced all the suspect caps and powered the supply on (with no load), it powered up for a short moment. But the short moment wasn't to last. A strange click, a huge spark, and a hissing cloud of steam from the main 2200uf caps and the supply was dead. Obviously something other than the caps was horribly wrong.

No, the caps weren't backwards. I checked before and after the blow-up.

Fortunately, a new supply only cost $100. I ordered it and when I received it, I immediately...

Re: Lexmark 544dn power supply buzzing sound model 40x5409

I just bought a very good rework station (Hakko) and am now able to easily replace whatever caps I need to. Still waiting on the order to arrive from Digikey for new caps (All Nichicon and a few Panasonic). I've already tested a few (Most are still fine but one 10uF 50V cap right next to the big 450V cap had an ESR around 15ohms even though it tested at 9.2uF at 1kHz). I'm no ESR expert, but that seems pretty darn high so that's going to get replaced for sure.

Re: Lexmark 544dn power supply buzzing sound model 40x5409

That's a great guide. Thanks! I'll have to add that to my standard references.

And yes, it's a switching supply.

Re: Lexmark 544dn power supply buzzing sound model 40x5409

The lowest ESR I could find which would fit the space, yes. And the LCR meter is a Fluke 6303A (1kHz). That brings up a question... While I was looking for caps, the tall thin ones seem to have the higher ripple while short wide ones have lower ripple for the same specs otherwise. Any idea why that might be?

Re: Lexmark 544dn power supply buzzing sound model 40x5409

I just bought a smattering of good caps for my parts bin and will try. There's a lot of caps though and I'd like to aim at the ones that are likely to have failed (Which I'm not quite sure how to choose). If I had a good desoldering station I wouldn't be so wary of pulling caps, but I'm stuck with a solder pump and wick until I can afford to upgrade.

I have a Lexmark 544DN laser printer which I'd prefer not to throw away. I know I can buy a new power supply for about $150, but the printer isn't even worth that at this point.

The printer started acting up about a year ago. Sometimes it would power on normally, sometimes not. When if failed, it would make a single 'click', flash the LED and display backlight, then go completely dark. I got used to this but now it refuses to power on no matter how nicely I talk to it.

Having a workbench, o-scope, and all sorts of fancy tools, I figured I might try to find the issue myself....

Re: New Members - please post your introductions here

Just joining in after discovering this forum and wishing I had found it earlier.

I'm an active EE with a background in digital/analog electronics and embedded software. I have a fairly extensive home lab where I try (I'll repeat... TRY) to repair whatever comes my way. This mostly involves small power supplies with bad capacitors. Sound familiar?

I'm hoping I can learn a thing or two through this site and get these old PCs, printers, microwave ovens, and assorted devices running once again!