You've already replaced the caps (which would usually be the next step), right? Do you have access to an oscillosocope?

Yes but its analog Textronix 50 MHz.

 

Hmmm...okay. Without a schematic, this is a little bit of a long-shot. Can you post designations ("Q12" or "D6" for example) of the upright components on the heatsinks, along with the part numbers from their casing?

I have to get back to work...somebody else want to step up??

 

Hmmm...okay. Without a schematic, this is a little bit of a long-shot. Can you post designations ("Q12" or "D6" for example) of the upright components on the heatsinks, along with the part numbers from their casing?

I have to get back to work...somebody else want to step up??

Don't worry about following through...I can hack through it.

I have identified all the chips connected to heat sinks. The two switching pair of transistors are mdf5n50 ( 500v, 5 a, 1.4 ohms) channel MOSFETs. There is two schottky rectifiers one mbrf20100ctl, the second rectifier mbrf2045ct. One for the 12v the other for 24v. Mdf18n50 (500v, 18a,.27 ohms) MOSFET connected on the same heat sink as the diode ( looks like a transistor case) erf806f. Finally a MOSFET that sits alone, not connected to a heat sink, free air but encased (isolated tab). Mdf2n60

 

Don't worry about following through...I can hack through it.

I have identified all the chips connected to heat sinks. The two switching pair of transistors are mdf5n50 ( 500v, 5 a, 1.4 ohms) channel MOSFETs. There is two schottky rectifiers one mbrf20100ctl, the second rectifier mbrf2045ct. One for the 12v the other for 24v. Mdf18n50 (500v, 18a,.27 ohms) MOSFET connected on the same heat sink as the diode ( looks like a transistor case) erf806f. Finally a MOSFET that sits alone, not connected to a heat sink, free air but encased (isolated tab). Mdf2n60

M1=mdf18n50
M3=mdf2n60
M4/M5=mdf5n50
D2=er806f
D6=mbrf2045ct
D8=mbrf20100ctl

 

Can you check the MOSFETS and make sure they are okay. These have been known to let go on other units. You should probably remove them from the board to perform the test. If you are not sure on how to test them, there are several Youtube videos that will help.

 

Can you check the MOSFETS and make sure they are okay. These have been known to let go on other units. You should probably remove them from the board to perform the test. If you are not sure on how to test them, there are several Youtube videos that will help.

bwilliams60, I removed all the transistors and tested each one out of ckt. They all show the ability to switch the source on/off and none of them have resistance values from the gate to either the drain or source leads.

Since, I was unable to find a faulty transistor. I decided to put all the original capacitors back in and remove the "Jackson" brand caps. They seemed to make the problem worse. At least with the original capacitors the power supply would stay on for 15 minutes or more the first time, and once power cycled after losing the outputs the power supply outputs stay on for 3-5 minutes. As apposed to the 4+ min time, and 10-20 seconds after that with the "jackson" brand. At the time that's all the local electronic supply house had.
Now i am asking this question. By simple deduction if palcing the origianl capacitors back in the problem changes as in the amount of time it takes for the output to shutdown (with a load on them all). Do I spend $ on low ESR capacitors? OR Is there something wrong with the flyback circuit? I have not checked any of the IC's on the bottom of the board. All SMD's.

 

Can you post any pics of the board itself, both sides? Also, what are you using to test the new caps, old caps or are you just replacing them for something to try? The reason I ask is that you may have received some faulty components and if they weren't tested before they went on it, well.....

 

I am unable to post pictures from my mobile. I will post the pics later today. I did not test the new capacitors...oops. I don't have an ESR meter so I have been testing them with my extech DVM in caps mode. Replacing the caps is just a thought, for something to try and rule out. Although after some research and suggestions from a TV repair forum. I think that I might have a problem with the VFC. I was told the filter cap needs to have 360-400 vdc while operating ( loaded down). But mine reads 166 vdc in stby mode and when loaded with dummy loads. Additionally the bridge which should be before the vfc is 166 vdc. I don't think the frequency ckt is working properly.

 

I am unable to post pictures from my mobile. I will post the pics later today. I did not test the new capacitors...oops. I don't have an ESR meter so I have been testing them with my extech DVM in caps mode. Replacing the caps is just a thought, for something to try and rule out. Although after some research and suggestions from a TV repair forum. I think that I might have a problem with the VFC. I was told the filter cap needs to have 360-400 vdc while operating ( loaded down). But mine reads 166 vdc in stby mode and when loaded with dummy loads. Additionally the bridge which should be before the vfc is 166 vdc. I don't think the frequency ckt is working properly.

I have made it to a point where I can safely post my findings and be confident that I am close to resolving the problem. I have tracked the problem back to the power factor circuit IC chip. It dawned on me that when the outputs when off, the filter cap and bridge would measure the same VDC (160-170). This opposed to the 360-400 VDC measure on the filter capacitor with the outputs working, 160-170VDC on the bridge. the outputs shutdown becasue the PFC shutdown on the high voltage side. It was suggested that to confirm the problem is PFC, warm up the PFC chip first and then plug in the power supply with the dummy loads connected. The supply should shutdown the outputs faster than normal. And as such it did. When I warmed up the PFC IC area on the board, the board had been off all night, normally the power supply would run for ~15 minutes before shutting down, but with it warm it took less than 60 seconds. To further test the theory I measured the PFC IC at VSS, GND, and the Mosfet driver current draw. VSS=12.5-13 VDC, Mosfet driver is 60-80mA (measured in series, removing a jumper to take the measurement), the voltage of the driver with the jumper back in place is 2.5-3 VDC. I have also checked the surrounding SMD resistors and capacitors and have not found any to stand out as problems. I will be ordering a new PFC IC, along with an equivalent Mosfet transistor (just in case). All the mosfest tested good and working out of circuit but this mosfet is connected directly to the PFC and as such will be replaced to be safe.

 

To bring this thread to a close and hopefully help someone else. The problem is an smd capacitor labeled c404. Removing the smd cap and adding a shunt in its location solved the problem. I hope this makes someone else's repairs easier. It was not the output caps the transistors or anything else...

 

To bring this thread to a close and hopefully help someone else. The problem is an smd capacitor labeled c404. Removing the smd cap and adding a shunt in its location solved the problem. I hope this makes someone else's repairs easier. It was not the output caps the transistors or anything else...