repairing switch power supply in samsung plasma

[MAGNETRON]

Switched mode power supplies are notorious for going bang for a second time,when I worked on them,you just did a standard complete change of known failures,electrolytic capacitors,diodes and control I.C,opto-coupler,high value resistors in control circuit,ie 100k upwards-go O/C,switching transistors,also small ceramic caps hanging off switching transformer(tuning),the list is endless,sometimes you started off with more Power supplies blown rather than repaired,.....all great fun...not!!!

Have fun

 

[unclejed613]

like my rifle, they go bang every time, unlike my rifle (with rifles that's a sign of good reliability) switching supplies tend to get unreliable as they get older. and that's when they start going bang

i used to repair a lot of "Samtron" VGA monitors (made by Tatung with a lot of circuitry "lifted" from Samsung and IBM monitors), and 90% of them all had the same 3 small electrolytics in the primary side of the supply dried out. i replaced about 5-10 sets of them per day. i got to calling them the "3 Stooges". the same 3 caps would go bad in IBM 8514 monitors and Samsung VGA monitors, but not as often, because IBM and Samsung used 105 C capacitors rather than the standard 80 C type. once in a while the switching transistor or a diode on the secondary side would be shorted, but the vast majority of failures in those monitors was bad caps. another common failure was the pincushion coupling cap, usually a 10uf/100V bipolar cap.

 

[MAGNETRON]

Hi UncleJed

Same here...used to work for T.V rental company and then Monitors,POS terminals etc...all Switched mode P/Supplies.

If the caps are near to heatsinks,this is when the problem starts,high temps dry them out,high internal resistance/impedance.........................................and then all hell lets loose......BANG!!

Prefer the old Mains Droppers Myself...good old day's

 

[unclejed613]

i guess that a complex SMPS is less expensive than 10lbs of iron and 5lbs of copper wire
it sure isn't nearly as reliable. i see a LOT more SMPS supplies dying for various reasons (bad caps, shorted switching transistors and high side bridges, shorted high speed diodes, shorted TVS diodes, etc...) than i do transformers, bridges and big caps (occasionally i see an open primary from the thermal fuse opening up, or maybe a shorted bridge, and very rarely a shorted turn in the transformer).

btw, a word of warning... if a switching supply isn't working at all (not even ticking or chirping), before removing the power supply, check the DC voltage across the primary side caps. there can be anywhere from 150-300V or more across those caps if the switcher isn't running, and they can hold that charge for days or weeks. those caps discharging through your fingertips can cause severe, deep pinpoint burns. if you aren't in the habit of using the one hand rule (only use one hand in contact with anything in the equipment being repaired) that charge CAN KILL YOU.
if the caps are charged, use a 10k-33k resistor and clip leads to safely discharge the caps, and keep the discharger connected if you keep the board on the shelf for any length of time (like a day or more while waiting for parts), or you may find out the hard way what Dielectric Absorption is (electrolytics can, after having a charge on them, "recharge" themselves)

 

[Don Thompson]

Hello,
My first post. I too am attempting a repair of a Samsung SMPS in a 50" plasma TV. Failure caused by a 2 year old continuously turning the TV on and off until it no longer worked. A quick check of the SMPS showed the Vs and Va outputs pulse up to voltage (209 & 55 V respectively) but then drop rather quickly down to the mV range. The 15V and 5.3V seem to work properly. So the TV flickers and gives the "power on chime" when you turn it on but the display stays dark.
I am now studying the info at the link above. It doesn't seem like it should be too difficult to fix. Replacement SMPS is $157 + shipping which I would go ahead and pay if I knew there weren't any more costly failures downstream.

 

[unclejed613]

if you get the correct voltage for a short time, then the supply shuts down you may have a short "downstream" from the SMPS... in other words something else may be causing the supply to go into shutdown... it would be something fed by the two high voltage supplies (55V or 209V). the overcurrent sensing is causing the shutdown. you most likely have a bad panel, which is the most expensive thing in the TV.

you could also have a problem in the driver boards, but with Samsung, i think those come as part of the panel (they definitely do with LCD tv's, but i'm not absolutely sure about plasma....)

 

[Don Thompson]

Hello,
Well it does it even with the output connectors unplugged from the supply so it has to be something on the SMPS. I was reading that link posted above which gave me some ideas. It seems to pulse current into the caps then it drains off. It's using the 3BR2565 controller which has a built in soft start which may account for it but when the startup period is over it doesn't go into std operating mode.
I don't want to spring for a new PS if there are problems downstream but I need a working PS to find out. I think I will tinker a bit more.

Any and all ideas are greatly appreciated. SMPS's are not my forte' and tackling it without a schematic makes it slow. I have to study as much as diagnose.

 

[tvtech]

i guess that a complex SMPS is less expensive than 10lbs of iron and 5lbs of copper wire
it sure isn't nearly as reliable. i see a LOT more SMPS supplies dying for various reasons (bad caps, shorted switching transistors and high side bridges, shorted high speed diodes, shorted TVS diodes, etc...) than i do transformers, bridges and big caps (occasionally i see an open primary from the thermal fuse opening up, or maybe a shorted bridge, and very rarely a shorted turn in the transformer).

btw, a word of warning... if a switching supply isn't working at all (not even ticking or chirping), before removing the power supply, check the DC voltage across the primary side caps. there can be anywhere from 150-300V or more across those caps if the switcher isn't running, and they can hold that charge for days or weeks. those caps discharging through your fingertips can cause severe, deep pinpoint burns. if you aren't in the habit of using the one hand rule (only use one hand in contact with anything in the equipment being repaired) that charge CAN KILL YOU.
if the caps are charged, use a 10k-33k resistor and clip leads to safely discharge the caps, and keep the discharger connected if you keep the board on the shelf for any length of time (like a day or more while waiting for parts), or you may find out the hard way what Dielectric Absorption is (electrolytics can, after having a charge on them, "recharge" themselves)

Well said unclejed

SMPS faults are probably 70% of our repair revenue with modern CRT (probably the last of CRT being dumped here from you know who).
They all use the same parts and all blow up the same way. Boring repetitive work fixing this rubbish.

Oh how I still love my Linear PSU's.

Regards,
tvtech