Samsung PSU BN44-00370A - need help for repair

[sebr]

Hello,

my Samsung LE-22C456E2W LCD TV quit working. The main fuse in the PSU was blown and the rectifier also. After replacing these, the fuse blew again, so there must be some more damage. Unfortunately no schematic available for the PSU
I'm familiar with electronics but never repaired a SMPS so any help would be appreciated. As the main fuse FS801S was faulty and rectifier also but the fuse for the inverter part on the board (J812) not leads me to the conclusion that I should take a closer look at the primary circuit. So i checked the zeners, diodes and caps (ceramic disc + MKP) for a short but they are OK. Pin 1 (drain) of theFSQ0765RQ isn't shorted to GND either. Primary cap CM801S is also good (btw it's missing on the pic so don't wonder). At this point i'm stuck now. My next try would be to unsolder the transformer TM801S and check it + run the circuit without transformer to check if the primary part is working. Is that a good idea?

[vinceroger69]

Re: Samsung PSU BN44-00370A - need help for repair

can you straighten the bridge rectifier up a bit and retake the pics so we can see what items its hiding .have you tested dm821/824 as well for shorts?

[multimeter]

Re: Samsung PSU BN44-00370A - need help for repair

unlikley that the smps transformer is faulty,check the surge suspressor (green disc)next to the glass fuse for a short,should show "1" or "0L"

[sebr]

Re: Samsung PSU BN44-00370A - need help for repair

Thanks for your reply. The Diodes DM821 and DM824 aren't shorted. The "green disc" (VX801S) isn't shorted either. Looking into the terminals is high impedance above 1M. So problem must be behind rectifier.
Under the rectifier I discovered a blown up cap (CM806) - thanks for the hint. It is located between GND and DRAIN of UM801S. Probably some overvoltage there because of the damaged rectifier before. Likely that UM801S is faulty too, or?
Too bad the side with the marking of the cap is burnt.

[vinceroger69]

Re: Samsung PSU BN44-00370A - need help for repair

have you checked the diodes by where you removed the bad cap from?
also someone may reconise the size of the blown cap and tell you what its value is.
is the bridge recifier still testing ok?
since you have removed that blown cap are you still getting a short circuit reading on the psu? just wondering if its just the cap you removed thats shorted what is causing your issues

[budm]

Re: Samsung PSU BN44-00370A - need help for repair

What is the P/N of the UM801S?

[awyatt2]

Re: Samsung PSU BN44-00370A - need help for repair

cm806 is 47k 1ku if you pull up the pic on shop jimmy you can see it. talk about luck it is tthe only cap you can read the value on. hope it helps.

[sebr]

Re: Samsung PSU BN44-00370A - need help for repair

The diodes in this area are fine, just doublechecked. Actually the cap died as disruption, not a short. The rectifier is still fine. But I still wonder what has blown the fuse again. As I had desoldered the transformer I measured:

1-3 0R
4-5 0R
6-10 0R
7-8=9 0R

I will solder it back and test

@budm: see 1st post.

@awyatt2: Thanks very much! I will order it then. But I think its not critical for operation, at least for testing purposes?!

[vinceroger69]

Re: Samsung PSU BN44-00370A - need help for repair

Fsq0765rq um801s

[sebr]

Re: Samsung PSU BN44-00370A - need help for repair

Update: the fuse doesn't blow anymore. the 5V look good. But the 13V are only 7,7V.
Output caps are fine.

[budm]

Re: Samsung PSU BN44-00370A - need help for repair

So the cap is 47pF 10% 1000V or is it .047uF 1000V? I think it is 47pF. I looked at shojimmy also when I first read the thread but I could not see the cap well enough.

[jetadm123]

Re: Samsung PSU BN44-00370A - need help for repair

Where are you measuring the 13v at? Try measuring the voltage of the middle leg of DM821 and DM824. One outputs 5V and the other 13V.

[sebr]

Re: Samsung PSU BN44-00370A - need help for repair

@budy: its 47pF +- 10%

@jetadm123: yes on middle leg.

[jetadm123]

Re: Samsung PSU BN44-00370A - need help for repair

Since DM821 and 824 are fed directly off the secondary of the main transformer, then there still must be issues at the primary input. Was the 47pf cap also connected to the primary winding of the main transformer?

[budm]

Re: Samsung PSU BN44-00370A - need help for repair

It does not make sense that the 5Vdc is OK but the 13 Vdc is not since both secondary winidngs for the 5 and 13 are generated by the same transformer, if there is problem in primary, BOTH output voltages will be wrong. I would recheck the rectifier circuits of the 13v supply section.

[sebr]

Re: Samsung PSU BN44-00370A - need help for repair

Argh! My DMM was fooling me! Tried another one and now the voltage is rock solid 13V
The TV ist starting and working fine. Thanks everybody for your quick help!
I ordered a 47pf 2kV cap and will install it next week

[budm]

Re: Samsung PSU BN44-00370A - need help for repair

Do not run the TV too long without that cap in place, it is part of damping circuits or you may damage the IC.

[sebr]

Re: Samsung PSU BN44-00370A - need help for repair

Don't worry it's set aside already. Btw I wonder as I couldn't find this cap in the Datasheet or AppNotes for the FSQ0765RQ