Samsung laser printer power supply repair.

[Refrigerator]

Hey, guys.
So i had this printer laying around, it's a dumpster find.
It has a soft power on and seems like it won't turn on.
Upon further investigation i discovered that there are absolutely no voltage on the low voltage secondary, all the fuses are ok and the main AC filter cap is charging up, yet the standby circuit does not operate. The PSU has all the spaces with various tests masked ( burn in, ICT OK etc. ) this must mean that the printer was once in a working order.
Othervise the printer itself looks to be barely used, almost like new, i guess it failed shortly after being purchased.
The main reason why i'm repairing this printer is that my old one started acting up on me, the ink cassette is damaged, the print quality is crap and it would be about time to retire it.
Pics:
Topside.

Bottomside.

ICs:
U2 - STRW6251
Datasheet:
http://www.datasheets360.com/pdf/-73504389237973938?query=STRW6251
U1 - MA166S
Datasheet:
http://www.datasheetarchive.com/dl/Datasheets-IS62/DSAH0058010.pdf
U103 - KIA7818A  +18V linear regulator.
U102 - KIA324F
Datasheet:
http://pdf1.alldatasheet.com/datasheet-pdf/view/69346/KEC/KIA324F.html
U104 -  IN74LS07D
Datasheet:
http://pdf.datasheetcatalog.com/datasheets2/10/100917_1.pdf
Thanks for helping.

[tautech]

Others have mentioned powering the controller alone with the appropriate DC voltage and check that the switching is working.

Any power up after repair should be with a dim bulb tester to keep the magic smoke in.

[Refrigerator]

Others have mentioned powering the controller alone with the appropriate DC voltage and check that the switching is working.

Any power up after repair should be with a dim bulb tester to keep the magic smoke in.

So instead of repairing the standby, i could temporarily wire in a power supply to check if the processor and main power supply operate ?

[Refrigerator]

Well, i made some progress.
On the processor board i located where the 24V supply connects and found an unpopulated footprint of a capacitor that was connected to the 24V rail, the voltage regulators are also connected to the same 24V rail.
So i went in and soldered two leads on that pad, also putting a diode in series with the positive lead just in case the 24V rail turned on.
While it should be 24V i didn't have a supply that would give me that voltage thus i went for a 9V supply instead.
After connecting the 9V i started touching the ICs with my finger to see if anythin's heating up and i noticed that one of the regulators was heating up quite alot.
So i immeadiatly disconnected the 9V and started beeping around with my DMM, i found out that the first regulator was feeding pretty much everything on the board including the second regulator, which only supplies power to the CPU.
The first regulator is a fixed 3.3V and the second regulator is a fixed 1.8V, atleast that's what they output.
All the sudden this got quite interesting, too bad i don't have the proper equipment to do a serious repair.
Well, the fact that the first regulator is the one heating up and that it supplies power to the whole CPU board means that something on that board is pulling all that power out, it can't just go nowhere.
Actually when i think about it this might be the reason why the standby supply is dead.

[poot36]

If possible try disconnecting the output of the 3.3V regulator from the rest of the circuit and see if it still gets warm.  Try this with the CPU board disconnected from the rest of the printer if possible.  Also you can use your multimeter on ohms range to try and find the short on the board.  If you are lucky it will be a electrolyte cap or a smd ceramic cap for filtering that is shorted out.

[Refrigerator]

If possible try disconnecting the output of the 3.3V regulator from the rest of the circuit and see if it still gets warm.  Try this with the CPU board disconnected from the rest of the printer if possible.  Also you can use your multimeter on ohms range to try and find the short on the board.  If you are lucky it will be a electrolyte cap or a smd ceramic cap for filtering that is shorted out.

I checked it and there's 450 Ohms between the output on the first regulator and ground, i measured the current and it was pulling 0,570 A out, since it's  linear regulator that 5.7 V drop @ 0,57 A = 3.249 W of power to dissipate, which for a SMD regulator is way too much.
It's weird how the regulator is the only thing heating up, i guess it's a good idea to take it out and test it off the board.

[poot36]

I think that there is a semiconductor somewhere on that 3V rail that only shorts out when power is applyed.  Try the diode function on your meter and see what number you get.  I am going to guess around 0.5V to 1V.  You could also try powering that 3V rail from a external power supply (PC power supply with a current limiting resistor?) and see if any other components get hot.  If you do remove the 3V regulator try powering the printer again and see if the power supply starts working again.  If not the power supply may be expecting a signal from the CPU board to turn on.  I guess the good thing is if the 3V regulator is still working then there is less chance that other components on the 3V rail (and maybe the 1.8V rail) have not been damaged from receiveing 24V.

[Refrigerator]

I think that there is a semiconductor somewhere on that 3V rail that only shorts out when power is applyed.  Try the diode function on your meter and see what number you get.  I am going to guess around 0.5V to 1V.  You could also try powering that 3V rail from a external power supply (PC power supply with a current limiting resistor?) and see if any other components get hot.  If you do remove the 3V regulator try powering the printer again and see if the power supply starts working again.  If not the power supply may be expecting a signal from the CPU board to turn on.  I guess the good thing is if the 3V regulator is still working then there is less chance that other components on the 3V rail (and maybe the 1.8V rail) have not been damaged from receiveing 24V.

I lifted the first regulator to see if it pulls any power and the regulator and everything else on the 24V rail are only pulling 5mA while the regulator is still regulating at 3.298V, i also removed the cap after the first reg and then lifted the second reg.
While measuring the current i shorted the first reg output to the pad and it started pulling 0,5 A, so there must be something wrong on the circuitry connected to that regulator.
Some pics:

[simplifyiyuk]

thats an ML-1910 or similar yes? this is a "soft power" power supply. I can say with some certainty that the fault is with the SMPS and not the main board. these suffered issues with the 24v dropping out and also the soft power. if the soft power button is held down some ticking or attempts at life may occur.
the part was always cheap so we never investigated the cause in any great detail

[simplifyiyuk]

a bit more background for you. the SMPS outputs 5V and 24V. the thyristor listed is likely for the fuser lamp control (halogen). there is some mention of FDC642P (FET) in my scribbled old notes from when I was curiously looking into this. as I remember the service documentation wasn't very detailed 
hope that gets you somewhere

[Refrigerator]

Well, some semiconductor definetly failed, i tested the output on the first reg and there was a 0,5 V drop going to ground.
I have ordered myself a hot air station some time ago and it should arrive in a couple days, so that i can start removing parts one-by-one.

[simplifyiyuk]

anything is possible with trash finds I suppose, when we had these in the field we only saw them shortly after failure, not so much when all options were exhausted and they ended up in the trash. I'll see if we have any hanging around "out back" and have a poke with DMM time permitting.

[Refrigerator]

I got my hot air station just yesterday, played with it a little to get the hang of it and today i started removing things off the processor board, i kept checking the current draw after removing each of the smaller ICs and after i removed one of the bigger chips i noticed that the current draw went down to 80mA. 
The chip is A3983SLPT.
Datasheet:
http://datasheet.octopart.com/A3983SLP-T-Allegro-datasheet-11972016.pdf
The chip has a heatsinking pad on the bottom to dissipate the heat to the copper beneath it, that must have been the main reason why i couldn't feel it getting hot.
The chip is not too cheap it's 10$ a pop ( that's shipping included ) but it's certeinly cheaper than buying a new printer.
Pic of the chip.

What you don't see in the pic is that the chip has a tiny "L" shaped black spot on it and i think that i could be damage.

[tautech]

What you don't see in the pic is that the chip has a tiny "L" shaped black spot on it and i think that i could be damage.

Thats probably where the magic smoke leaked out. 

Welcome to the world of hot air rework. Enjoy.

[Refrigerator]

What you don't see in the pic is that the chip has a tiny "L" shaped black spot on it and i think that i could be damage.

Thats probably where the magic smoke leaked out. 

Welcome to the world of hot air rework. Enjoy.

Yes, but now i have to start saving up money for that chip because i spent all of my savings on that hot air station ( that station is worth every cent, though ).

[tautech]

Well if it repairs the printer for ten bucks and you've learnt a bit in the process, who will be the winner....
And you've got a rework station for the next project. 

BTW is the printer anything special? colour? duplex? multi tray?

[Refrigerator]

Well if it repairs the printer for ten bucks and you've learnt a bit in the process, who will be the winner....
And you've got a rework station for the next project. 

BTW is the printer anything special? colour? duplex? multi tray?

Not much, just a regular monochrome laser printer, but this one looks to be barely used, the +24V is still dead, i might take another PSU from an inkjet that has a 24V output on it and connect instead of the not working 24V rail on the laser printer.
These lazer printers aren't cheap here where i live, so repairing this one could save me quite some money. 

[paulhm81]

I just repaired a scx-4623 with the same power supply. Mine power cycled when I kept the power button pressed and in the end it was a 4R7 smd resistor on the FB of the STRW6251. It was over 10Mohm. If you still have the printer you should check it. It's on the bottom part of the supply board.

[Refrigerator]

I just repaired a scx-4623 with the same power supply. Mine power cycled when I kept the power button pressed and in the end it was a 4R7 smd resistor on the FB of the STRW6251. It was over 10Mohm. If you still have the printer you should check it. It's on the bottom part of the supply board.

Thanks, i'll check mine, i just replaced the faulty motor rdiver IC and gave it the supply voltage, the processor board powers up but it gives me an error which would suggest that the main PSU is dead.