Switch-mode power supply in Samsung DVD-V3500

[ThrushAudio]

I have FOUR of these that belong to an elememtary school that all have the same problem. The Power supply is essentually dead. ( P/N on the board is: DVD-V3500/XAA ) I can hear the "buzz" in the main transformer, but the run circuit won't kick in. They use an oscillator circuit instead of the more common power IC. I've tried swapping all the components in the start-up and run circuits with no success. I was wondering if anyone else out there have had any success solving this problem in the switching supply. Help would be MUCH appreciated!

 

[davenn]

the electrolytic capacitors would be the first things to check, see how many are bulging on their tops
take a clear sharp pix of the board so we may be able to help more

Dave

 

[ThrushAudio]

Thanks Dave.... There's only three electrolytics in the primary side. The bulk DC is good, and I've swapped out the non-polarized 22uf 16V, but not the 4.7uf 250V or the main filter cap (150 uf 200 V). I guess I should try them next. I've included some photos and schematics. Maybe these will give you a better idea of what's going on. THANKS for taking a look!

 

[davenn]

in pic 1, there's a brown electrolytic cap to the left of the white hi watt resistor ... towards the lower right corner of the board and just up from the heatsunk transistor.

altho the cap isnt bulging, the PCB looks browned in that area from heat, its possible the cap has dried out somewhat. I would replace it just as a matter of course because of that heating in that area.

from the overhead pix its difficult to tell for sure the bulge or no bulge of the electros, but I cant see any obvious signs.

there doesnt seem to be any startup feedback circuitry from off the board (other than the immediate feedback from L.Volt DC to primary side of the transformer via the opto coupler) that means you should be able to operate the PSU whilst it is disconnected from the rest of the electronics. It would be interesting to see if you could measure any of the DC rails in that condition ? (why? ... maybe its something in the main part of the system electronics causing the PSU to shut down)

cheers
Dave

 

[(*steve*)]

To me, the scary thing is that there doesn't appear to be anything that would shut the regulator down.

It's either grossly overloaded and delivering a very low voltage, the oscillator isn't oscillating, or a component has died.

Such a device should be operating at a frequency well above what is audible, if you can hear a buzz coming from the transformer then that doesn't sound particularly good to me.

Unfortunately because the whole thing (at least the input side) is operating at mains potential I could not advise doing measurements whilst the power is applied, and using an oscilloscope would require a somewhat hazardous technique.

I would check all the rectifier diodes to ensure that none are shorted.

If they pass (i.e. none are shorted or open) then tell me if there is any DC voltage on any of the outputs. Try it with the power supply both connected and disconnected from the rest of the DVD player.

The low voltage side is safe, but remember that you will have mains wiring and the hot side of the power supply exposed when making these measurements.

There is no indication in what you have written that the problem is *definitely* in the power supply, so we need to check that out first.

 

[davenn]

Such a device should be operating at a frequency well above what is audible, if you can hear a buzz coming from the transformer then that doesn't sound particularly good to me.

Thats what was making me think the PSU is being loaded down, either on the secondary side or somewhere else in the electronics

D

 

[ThrushAudio]

Swapping out the brown electrolytic solved the problem in two of them... THANKS for the good advice Davenn.... The origional was a 4.7uF @ 250V ... I had on hand a 4.7uF @ 200V. I put that in, and one of them came right up !! ... I have since ordered online the right voltage cap. I guess the heat generated by the surrounding higher wattage resistor dried that cap with no apparent physical damage. I checked the bad cap with an ohmeter, and it still reacts like a good cap, but must not work like one in the circuit. I have still to find the problems in the other two. One had a shorted power transistor that blew the main fuse, and has taken out some other components too, so I'll keep you posted... THANKS again for the good advice !

 

[ThrushAudio]

Oh... By the way, The "buzz" in the main transformer is a result of the startup ciruit. I layed a scope probe next to the transformers core, and it revealed about a 400 HZ repetative startup cycle. I know these switchers run at a much higher frequency ( above audible) when they're actually running.