ONN 100012588 Has fluctuating Voltage on PSU 12v

[triplefour]

Re: ONN 100012588 Has fluctuating Voltage on PSU 12v

Did you replace R355 with a 2.2Ω? or just leave the one that checked 20.1k

i havent replaced that resistor. i guess thats the next logical step. i guess i was thinking to look around for other bad components. but oh well its just a 2.2ohm resistor...if i lose one no big deal. can i just replace it with any regular resistor or do i have to use a surface mount?

[R_J]

Re: ONN 100012588 Has fluctuating Voltage on PSU 12v

Yes you can replace it with a regular resistor, anything from 1~10Ω should be fine, That is why the circuit is not working. The transformer aux winding supplies the high freq. a/c to D107 anode, it provides about 25vdc to charge EC103 to keep the circuit operating.

[triplefour]

Re: ONN 100012588 Has fluctuating Voltage on PSU 12v

Yes you can replace it with a regular resistor, anything from 1~10Ω should be fine, That is why the circuit is not working. The transformer aux winding supplies the high freq. a/c to D107 anode, it provides about 25vdc to charge EC103 to keep the circuit operating.

Dang I replaced that resistor with another 2.2ohm. Still have the fluctuating voltage on ec103. Between 14 and 20v...what else could it be?

[R_J]

Re: ONN 100012588 Has fluctuating Voltage on PSU 12v

If it made no difference, then check the resistance of the aux winding and check the diode D107, also check D109 and D109.
Alre you still getting a voltage on the secondary side of the power supply?

[triplefour]

Re: ONN 100012588 Has fluctuating Voltage on PSU 12v

So I remembered that when I got the replacement board, I tested it's voltages before hooking up to the mainboard and they were fluctuating. I thought oh no the guy sent me a broken power supply with the same issue. But just to be sure I went ahead and plugged it into the main board and tried it again. The TV came on and the voltages were stable. TV ran for a couple weeks then did the same thing. When I got there, voltages were unstable with or without mainboard. Anyway I just did the same with the repaired power supply and plugged it into the main and plugged the power button into the main. The power supply voltages are stable now and the main seems to respond to button board and powers on and off appropriately. Seems we have identified two things here. A common failure on these power supplies, and also an established baseline that when not connected to a main board, we should EXPECT to see a fluctuating voltage on the psu. Seems strange to me but that's what it is. Anyone have any insight here? Seen other psu that are like this when unloaded?

But whats really got me worried is if there isn't something else that's causing the power supply to eventually blow this resistor again and I'll be back at dudes house in two weeks.....is there anything else I can do or check before plugging this board back into dudes TV and crossing my fingers?

[R_J]

Re: ONN 100012588 Has fluctuating Voltage on PSU 12v

It is possible that the VCC voltage will fluctuate when the power supply is in a standby state, They put the main ic in burst mode so it only supplies a burst of pulses to the transformer in standby because the tv does not require much current in that state.
It is odd for R355 to open, It might be that it's wattage is not high enough and after a couple years running it started to go high in resistance. The vcc line to the ic does not draw that much current.

[triplefour]

Re: ONN 100012588 Has fluctuating Voltage on PSU 12v

It is possible that the VCC voltage will fluctuate when the power supply is in a standby state, They put the main ic in burst mode so it only supplies a burst of pulses to the transformer in standby because the tv does not require much current in that state.
It is odd for R355 to open, It might be that it's wattage is not high enough and after a couple years running it started to go high in resistance. The vcc line to the ic does not draw that much current.

Personally I've never seen a power supply that did this before but we're always learning so I'm happy to add this to my list of anomalies!

I also had that thought about why the resistor burnt out and thought to myself I hope I put a resistor in there that at least has a higher wattage rating than the one before it. So I looked up what the wattage ratings on SMD resistors are and it seems like the biggest ones have a rating of about a quarter of a watt. The resistor I replaced this one with has a rating of a half Watt. So hopefully it should be fine right??

[triplefour]

Re: ONN 100012588 Has fluctuating Voltage on PSU 12v

ok well i went and installed the power supply in the guys tv and everything works as expected. hopefully it lasts a bit longer than two weeks! will be back here if it blows again to ask more questions why!

[nomoresonys]

Re: ONN 100012588 Has fluctuating Voltage on PSU 12v

so the fix was replace that 2.2 ohm resistor, that's all that was wrong?

[triplefour]

Re: ONN 100012588 Has fluctuating Voltage on PSU 12v

so the fix was replace that 2.2 ohm resistor, that's all that was wrong?

Yep that's all I changed