Magnavox DRV power supply SMPS

[johnfin]

Re: Magnavox DRV power supply SMPS

In or out of the circuit?

[budm]

Re: Magnavox DRV power supply SMPS

In circuit right now, we need it to be in the condition that every thing are still connected on the board, the removed and retest if the reading does not look right.

[johnfin]

Re: Magnavox DRV power supply SMPS

.5 ohms in circuit and that cap is fine. Tested 6 ways to Sunday. Remember, transformer removed, no short so intro parts in the primary are ok.

[momaka]

Re: Magnavox DRV power supply SMPS

.5 ohms in circuit and that cap is fine. Tested 6 ways to Sunday. Remember, transformer removed, no short so intro parts in the primary are ok.

The transformer is NOT connected in parallel / across the primary cap, though. So that means you have a short-circuit somewhere else.

There are only several paths you can get a short across the primary cap like that:
- bridge rectifier
- main FET
- MOV (if there is one across the primary cap)

Have a look at the 5VSB circuit schematic of an ATX PSU I drew here (Powmax LP-6100d):
https://www.badcaps.net/forum/attach...0&d=1451693563
It doesn't show the bridge rectifier and the main cap, but it shows the rest of the circuit. The "Pri. DC (+) BUS" on the schematic is connected to the (+) of the primary cap. And anything connected to a downwards-pointing triangle on the left side of the transformer is primary-side "ground" (i.e. connected to the (-) of the primary cap).

This is as basic as 2-transistor self-oscillating circuits get. If you need help understanding the operation, let me know and I will put up a post for that.

[budm]

Re: Magnavox DRV power supply SMPS

.5 ohms in circuit and that cap is fine. Tested 6 ways to Sunday. Remember, transformer removed, no short so intro parts in the primary are ok.

Nope, it is not OK to have 0.5 Ohms reading between the two legs of that main filter cap.
Are you sure the rectifier diodes and anything else that are connected in parallel with the two legs of the main filter cap tested OK?
Looks like you need to test 6 more ways till Sunday.

[johnfin]

Re: Magnavox DRV power supply SMPS

If you lived closer we would be poppin down some $20 bills cuz I dont believe you. All parts are fine on top especially in the primary side, no short when tranny is removed . I bet when the tranny is removed the resistance on that cap is fine. All caps were double checked with VOM, tel-ohmike, and ESR.

[budm]

Re: Magnavox DRV power supply SMPS

If you lived closer we would be poppin down some $20 bills cuz I dont believe you. All parts are fine on top especially in the primary side, no short when tranny is removed . I bet when the tranny is removed the resistance on that cap is fine. All caps were double checked with VOM, tel-ohmike, and ESR.

Well, if you how to read the circuit you will see that when the transformer is in place, one leg of the primary winding of the transformer is connected to the + leg of the main filter cap, the other leg of the transformer si connected to the Drain pin of the MOSFET, the Source pin is then connected to 0.68 Ohm to primary circuit GND to complete the circuit.
Wrong Data = Goose chase.
I am not going to bet with you because I do not want to make you $20 poorer.
You can ask any one here what they think about that 0.5 Ohms reading between the two legs of the cap means.

My post #42 "In circuit right now, we need it to be in the condition that every thing are still connected on the board, the removed and retest if the reading does not look right." It means all the components are back in place = transformer NOT REMOVED.
Since you do not believe me then I am out. What do I know about electronics compared to you? I am just a Knowledge Seeker.

[johnfin]

Re: Magnavox DRV power supply SMPS

Are you looking at schematics that someone posted? If so, my machine is different.

[budm]

Re: Magnavox DRV power supply SMPS

Post 15 is really close to what you have, especially the connection of the power MOSFET, transformer, main cap. All you have to do is to just trace out the connections.

[budm]

Re: Magnavox DRV power supply SMPS

Post #15 Schematic and PCB of the power supply.
c2109 snubber cap is also connected in parallel with the power MOSFET, that cap can also short out too..

[johnfin]

Re: Magnavox DRV power supply SMPS

So I have been taking a look the mosfet. Initially my mosfet machine said it was ok but I checked it with a meter on diode test, neg to drain and pos to source. ZERO This is an N-MOSFET so I assume my mosfet is trashed. Right?

[johnfin]

Re: Magnavox DRV power supply SMPS

p.s. I actually pulled c2109 last night and checked it. What else could take out the mosfet?

[budm]

Re: Magnavox DRV power supply SMPS

So I have been taking a look the mosfet. Initially my mosfet machine said it was ok but I checked it with a meter on diode test, neg to drain and pos to source. ZERO This is an N-MOSFET so I assume my mosfet is trashed. Right?

Can we see this MOSFET machine and the picture of the readout that shows it is OK?
When I test the device for short circuit, I use Ohm meter. When semiconductor shorted out or has bad leakage resistance then it will no longer work like semiconductor any more, using Ohm meter will tell you what the resistance of that device is.
Yes, the MOSFET is bad that is why you see 0.5 Ohms between the two legs of the main filter cap.
How about that $20?

[budm]

Re: Magnavox DRV power supply SMPS

p.s. I actually pulled c2109 last night and checked it. What else could take out the mosfet?

Crappy circuit design with minimum or just about 0 protection for over load.
Two-Transistor switching power supply is about as cheap as you can make it.

[johnfin]

Re: Magnavox DRV power supply SMPS

Iam going to put it on the b&k 520 tonight.

[budm]

Re: Magnavox DRV power supply SMPS

B & K Model 520B
Read carefully about the how and the expected test result for each type FET (JEFT, DMOS FET, EMOS FET)

[johnfin]

Re: Magnavox DRV power supply SMPS

I know how to operate the 520. I used a new N-Mosfet as a test. You start off by connecting the leads up to the any of the mosfet leads. Run thru the 6 settings and if it beeps it shows which lead is the base (gate). The one on my circuit board failed. Ok with VOM but I trust the 520. So the big question now is what is taking out the MOSFET?

[budm]

Re: Magnavox DRV power supply SMPS

So why did the test show as OK?
My Post 5 'Are you sure that the new MOSFET is not shorted out again?' Simple Ohm meter would have shown that it is shorted out.
All these time you have shorted MOSFET but you did not believe me when I old you that with 0.5 Ohms reading means you have shorted circuit, you should get rid of that tester, bad data = waste of troubleshooting time.
Did you use the 520 to begin with?

"So the big question now is what is taking out the MOSFET?" Post 54. Parts do fail too.
Philips uses this same circuit topology in lots of TV's that we have lots of experience with at Badcaps.

[johnfin]

Re: Magnavox DRV power supply SMPS

I used this method with the vom and it passed.
http://electronicsbeliever.com/how-t...-is-defective/
The 520 failed it and i trust it now over the vom. I used new fets as a comarison test. Now on to figure out whats destroying the fet.

[budm]

Re: Magnavox DRV power supply SMPS

Like I said, all you have to do is to put meter in Ohm mode to find out if the MOSFET is shorted out or not, simple as that.
I had you test the resistance between the two legs of the main filter cap and it shows 0.5 Ohms, that information alone was enough to find out what shorted out.
So at this point you better retest all those parts in the primary side again since you already made mistake about the MOSFET.
Wrong DATA = Wasted times.
Using Ohm meter works just fine:
https://www.badcaps.net/forum/showth...799#post782799