Re: Samsung LCD TV Power Supply BN44-00438B
replacing the TL431?
gruv2ths
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Last Activity: 01-27-2019, 08:32 PM
Joined: 02-08-2011
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Re: Samsung LCD TV Power Supply BN44-00438B
Thanks for the links, some great info..
So the zener in question has 2.4V on it's ref pin, and 2.5V on the Cathode. Seems like with Vout being 20% low the ref pin should be lower than 2.4V? Based on the design guide and data sheet, it should be 2.5 when it is right at 5Vout. Am I correct in my understanding??
Seems like the feedback network is making the converter think that 4Vout is correct.Leave a comment:
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Re: Samsung LCD TV Power Supply BN44-00438B
On the DMM the 4V and the 11V are stable. On the scope it is bouncing around a little. Not sure how obvious it is from the scope pic above, but the converter is in burst mode, it is not constantly switching.
So is the zener diode a small plastic package, 3 pin divice?
The opto that i changes is circled in the pic below, am i right in assuming that the zener in question is in the circle?
Thanks!
[ATTACH]87343[/ATTACH]...Leave a comment:
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Re: Samsung LCD TV Power Supply BN44-00438B
OK so I replaced the opto and no dice. The standby output is still 4V and the 13V is approx. 10V.
So I have replaced the PWM/FET IC, opto and the electrolytic on the output.
Could it be that the xformer has a shorted winding or 2?
Attached is a pic of the Output converters drain waveform.
(looks like a flyback converter min duty cycle? Am i right about that?)
[ATTACH]87334[/ATTACH]...Leave a comment:
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Re: Samsung LCD TV Power Supply BN44-00438B
Would a bad start up cap change the regulation point? Is it soft-start?
Thanks!
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[ATTACH]87246[/ATTACH]Re: Samsung LCD TV Power Supply BN44-00438B
Would a bad start up cap change the regulation point? Is it soft-start?
Thanks!
[ATTACH]87245[/ATTACH]
[ATTACH]87246[/ATTACH]...Leave a comment:
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Samsung LCD TV Power Supply BN44-00438B
All,
Been troubleshooting a Samsung 32" LCD TV.
Initially i noticed that the red LED was flashing twice and would not turn on.
Now there is no LED at all.
Observations so far:
-5V standby is 4V, 13V output is 10.5V (both off equally - 20%)
-If i disconnect the 5V standby from the PS(actually 4V), and inject my own 5V the led in the front now gives me the 2 blinks again and the PS_ON pulses momentarily enabling the PS including the power factor controller and it comes up at 400V.
-If i put a scope probe on the 5V/5V standby... -
Re: power supply board BN44-00438B repair
Good question, on the scope it looks oscillatory. (forgot to mention that)
The 13V at the 13V rectifier is 13V, but the 5V is the weird one at 4V. Will look at the 13V for a similar oscillation. Probably has it, the 5V and 13V come from the same converter.Leave a comment:
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Re: power supply board BN44-00438B repair
OK great info thanks!
Also, since my 5VA is only 4V, is that enough V for the logic to run on the main board? (it stays 4V with or without that main board connected)Leave a comment:
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Re: power supply board BN44-00438B repair
Hey guys,
Sorry about all the nube questions.
After re-reading the comments above, I have a few more questions.
Someone said that the PFC voltage would rise once the TV is turned on.
So i guess my understanding of the sequence of events is all wrong.
It appears that the PFC is kind of Or'ed with the main diode bridge. And the converter that generates the 5V and 13V are driven by that Or'ed voltage.
So if the PFC is not always running, can the 5V/13V converter generate enough...Leave a comment:
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Re: power supply board BN44-00438B repair
Thanks for that, so it appears in my case the PFC converter is not running.
There is no VCC at the VCC pin of the PFN controller PWM. So now i am digging through the traces to see what powers the chip. Just from a cursory look at looks like it runs through a few optos before it gets to the PWM.
Another question on the subject, I ordered the service manual for this set because i could not find it online. $15 later, i get a file with troubleshooting tips the tend to end in statements like 'Replace Main Board" LOL...Leave a comment:
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Re: power supply board BN44-00438B repair
The TV wont turn on. I only have a strange 5V standby, nothing else.
Need to go and look at the output of the PFC converter and see if it is a mess or not. Any idea what the output voltage of the PFC is?
Thanks!Leave a comment:
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Re: power supply board BN44-00438B repair
110 VAC
Thanks!...Leave a comment:
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Re: power supply board BN44-00438B repair
Hello,
I am working the same power supply. I noticed my standby voltage is only 4V, that sounds not close enough to 5V to me. Also on the scope it appears to be oscillating. I replaced the Fan7930B, it is a PWM with integrated switching fet, same 4V on the standby output. Is the big electrolytic supposed to be at 380V? Mine os only 155 to 160V.Leave a comment:
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