Lexmark 544dn power supply buzzing sound model 40x5409

[xl600]

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. As soon as I pulled the power supply and inspected the caps, I noticed two 2200uF 10V caps bulged out at the top. Once those were desoldered, they tested only 10uF on my LCR meter (Obviously bad). So I replaced them with much better (And identical size) 16V 2200uF caps thinking this would solve the problem.

Of course it didn't and the supply behaves the same way it was before replacing the caps. The only difference is now it makes a weird buzzing sound from somewhere.

I tried to locate the buzz using a shotgun mic. I've circled the area of the buzzing sound and think it's coming from a coil (Or very nearby). The buzzing has an f0 of about 300Hz.

Can anyone speculate what my next course of action should be? I have no real experience troubleshooting switching power supplies so I'm a bit lost. I will replace the bug 450V 120uF cap once I find a replacement (The one on the board tested bad even though it looks fine). Hoping that is all it will take.

I did find this post which talks about the power supply I'm having issues with at the moment. None of the issues in the post describe the problems I'm having but it's a good reference.

https://www.badcaps.net/forum/showth...hlight=40x5409

Thanks,

xl600

[Per Hansson]

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

I would say that noise could likely be that the main capacitor has failed.
The reason for the unit not powering up however is likely to be any of the small caps drying out.
Since you have an LCR meter why don't you just desolder all caps and test them?

[xl600]

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.

[budm]

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

Does your LCR meter perform ESR test?
So did you use low ESR caps?
Check and replace small lytics caps as suggested.

[xl600]

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?

[budm]

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?

https://cdn.badcaps-static.com/pdfs/...f6d3e3e891.pdf

[petehall347]

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

that noise is pretty high frequency .. saying that it is a switching supply .

[xl600]

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.

[momaka]

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

I see a lot of blue Ltec caps.
Ltec is a really shit brand. Best to replace all of them... including that big one if it tested bad. This PSU appears to have Active PFC, so it actually matters that the primary cap is a good brand and durable. On that note, if the big cap was rated for only 400V or 420V, upscale that to 450V.

And let's hope when the crappy Ltec primary cap failed, it didn't kill anything else on the APFC circuit.

[xl600]

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.

[momaka]

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

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.

Yeah, that's a bit too high even for a small cap.
And the thing is, those small caps are often used for the "start-up" circuitry, so the PSU not starting could be due to that as well. I've fixed a good deal of power supplies with those small start-up caps failed and responsible for the PSU to not start up.

[xl600]

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 replaced the big caps with extras I bought for the repair. That supply powered up fine and I'm printing once again. Hopefully, the better caps will keep it alive much longer that stock.

[momaka]

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.

Sorry to hear the repair didn't work out.

Perhaps when that primary cap went out, it could have taken out some part of the voltage feedback circuit on the secondary - hence the secondary caps venting (most likely due to over-voltage.) Otherwise, I don't see how the caps could vent without being backwards... but you already confirmed that's not the case, so that can't be it (unless the cap silkscreens are marked backwards, like on ASUS motherboards, perhaps?) If you've checked with the "before" pictures (i.e. before working on the PSU), then the failure probably did have something to do with the primary cap going bad in the first place.

Hopefully, the better caps will keep it alive much longer that stock.

If the PSU is designed normally and doesn't run excessively hot, you can expect good Japanese caps to provide at least 15 years of useful life.

[xl600]

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.