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Old 04-26-2021, 11:42 AM   #1
reessi
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Default JSK4228-050 Power Supply Standby voltage changing! (Possible from Wharfedale LTF37K1)

Hello,

I am trying to re-purpose this power supply that I saved years ago from a tv with a cracked screen for a monitor but that is not really that relevent at the moment. I do not recall which tv but a google search looks like it is from Wharfedale LTF37K1.

You may notice I made some alterations moving some components to the top and some wires which are from a 5v regulator I fitted to the 12v output as the monitor requires 5v. The standby was already at fault from the beginning, it has nothing to do with the alterations...

Years ago it had some bulging caps etc on the output section so I re-capped it.

Connecting power to it with nothing else connected the standby voltage was jumping around from 3v to 4v all the time.

Since I had recapped it years ago I decided to check the caps on the standby with the esr, while messing around I noticed that if I removed one of the 2 caps (Should be both 16v/680uF) but had 2 x 35v/680uF then the voltage was restored to 5v. I didn't know why !

Since it worked I preceeded with the other alterations as explained above to test in the monitor.

First I tested forcing the PSU on with a resistor from standby to PSU_On.

It works all voltages come up.

Then I connect it all to the monitor and now the standby is back to floating around. It cannot work with any load on it or both caps in place...

But everything else works.

Please see attached photos to see the circuit and caps in question. It seams like a simple circuit, a single diode from the transformer on the output section etc.

But can the fault be the output section or the hot side feeding the transformer for it to fail under load...

Just jumping the PSU on with a resistor does not make the standby drop only once connected to a load like the monitor...

The monitor works fine from this PSU if I feed it 5v from my bench PSU to the monitors 5v standby and force the PSU on with the resistor, all other voltages are working...

An explanation of the circuit would be great... I am still learning alot about switched mode power supplies:

As far as I understand it is doing half wave rectification with the diode and 2 large filter caps, is the inductor acting as a choke to stop ac passing? What is the purpose of CS31 and RS1 connected around the diode? And also RS32? Please see the ringed section in the picture of the schematic I edited...

The 2 blue caps tacked on are 2 caps I was testing in the 5v standby section...

cheers.
Attached Images
File Type: jpg IMG_3258[1].jpg (825.8 KB, 5 views)
File Type: jpg IMG_3259[1].jpg (767.7 KB, 4 views)
File Type: jpg IMG_3260[1].jpg (587.9 KB, 8 views)
File Type: jpg IMG_3261[1].jpg (304.3 KB, 6 views)
File Type: jpg 5v standby.jpg (44.9 KB, 8 views)
Attached Files
File Type: pdf HBTV-3201HD-JSK4228-50.pdf (457.8 KB, 7 views)

Last edited by reessi; 04-26-2021 at 11:52 AM..
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Old 04-26-2021, 02:46 PM   #2
R_J
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Default Re: JSK4228-050 Power Supply Standby voltage changing! (Possible from Wharfedale LTF3

If the standby voltage is erratic, ty replacing C22. The ic uses a internal startup cell to provide the initial VCC to pin 7 But the transformer winding (2) and circuit (R55,D16) must charge C22 to maintain this voltage. If C22 is weak the power supply will not operate properly

Last edited by R_J; 04-26-2021 at 02:53 PM..
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Old 04-27-2021, 03:04 AM   #3
reessi
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Default Re: JSK4228-050 Power Supply Standby voltage changing! (Possible from Wharfedale LTF3

Perfect, many thanks.. The cap measured 110 ohms on my ESR meter!! Totally kaput!

In hindsight I should have checked all the caps on the primary side as well. I just wasn't sure what causes that type of issue..

Power supply works great in the monitor I moved it to...
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Old 04-30-2021, 10:33 PM   #4
momaka
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Default Re: JSK4228-050 Power Supply Standby voltage changing! (Possible from Wharfedale LTF3

Yup, those small "startup" caps on the primary can often cause this issue with the standby supply "bouncing" around. Many new techs in the field also often don't suspect them, because they almost never bulge. But remember this: an electrolytic capacitor does not have to be bulging to be bad.

Quote:
Originally Posted by reessi View Post
Since I had recapped it years ago I decided to check the caps on the standby with the esr, while messing around I noticed that if I removed one of the 2 caps (Should be both 16v/680uF) but had 2 x 35v/680uF then the voltage was restored to 5v. I didn't know why !

Since it worked I preceeded with the other alterations as explained above to test in the monitor.
Generally speaking, you should avoid doing this with switch-mode power supplies - that is, remove output filter caps and/or test the PSU with less caps than it has intended in the design. Doing so can sometimes have detrimental results in other parts of the PSU. So if you're going to remove a component, make sure to replace it with something functionally-equivalent.
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Old 05-01-2021, 12:56 PM   #5
reessi
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Default Re: JSK4228-050 Power Supply Standby voltage changing! (Possible from Wharfedale LTF3

Thanks, I learnt some new stuff on this one.
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