Magnavox DRV power supply SMPS

[johnfin]

Having a problem with this power supply I think I have narrowed it down to the transformer. When I connect just the primary side it blows the fuse. Removed its ok. Cant find any info on this Chinese Transformer. Is there any chance on repairing this power supply without a schematic or OEM parts?

[Quaddro]

Re: Magnavox DRV power supply SMPS

Are you sure that the transformer is the main cause..?
if it's true, this is mean disaster.
Because the hardest part to find the replacement is the transformer, except you pull out from the identical board.

Have you already check all component in primary side..?

Try with light bulb trick. not directly from the wall.

[stj]

Re: Magnavox DRV power supply SMPS

a shorted switching device will blow the fuse when the transformer is in.
lets see the whole board.

[johnfin]

Re: Magnavox DRV power supply SMPS

This is really a puzzler. All of the caps are ok including the polys. Diodes are fine including ones on bottom. The mosfet was bad and I replaced it. I checked the small to92 transistor on the primary side (I assume that controls the mosfet ) and its ok. Xfrm out its ok, primary side connected only it blows. There are 5 connections on the primary xfmr. 2 are connected and the other 3 are. So I was going to put a scope on those connections. I am guessing the signal would be a pseudo AC because its already rectified. Any thoughts?

[budm]

Re: Magnavox DRV power supply SMPS

If the power MOSFET that drives the primary winding is stuck fully on instead of switching and off 50~60,000 times a second, that will cause the fuse to blow.
Are you sure that the new MOSFET is not shorted out again?
This is one of those Philips two-Transistor switching power supply, so you have to check ALL the devices in both primary and secondary side really well.
What is the model number of this DVR unit?

[johnfin]

Re: Magnavox DRV power supply SMPS

Its a Magnavox mdr557h/F7. Its not that old. Do I have to unsolder the mosfet to really check it correctly. I used a mosfet machine to check it originally but I don't really want to remove it. So what could be taking out the mosfet if its bad. There really are not that many parts on top of the board. There are a bunch of micro components on the bottom that I have not checked. A schematic would be golden.

[johnfin]

Re: Magnavox DRV power supply SMPS

I checked the transformer and between certain pins measure complete continuity, digital and analog vom. This means the transformer is shot, right?

[budm]

Re: Magnavox DRV power supply SMPS

Transformer winding will show very low DC resistance, you need a transformer ringer tester to test it.

[johnfin]

Re: Magnavox DRV power supply SMPS

How about a flyback tester, I have one of those. The SMPS are basically flyback designs. When you say low, just how low. My DVOM is super sensitive with about 5 place marks.

[Quaddro]

Re: Magnavox DRV power supply SMPS

Yup, you cannot test smps transformer with traditional multimeter (analog or digital)
In many design, it's shows continuity. Seems shorted, but it's not.

Like budm said, you need ring tester to check it.

[johnfin]

Re: Magnavox DRV power supply SMPS

With the transformer removed, should I be able to see the PWM on the primary side with a scope?

[budm]

Re: Magnavox DRV power supply SMPS

With the transformer removed, should I be able to see the PWM on the primary side with a scope?

Nope, the circuit use one of the transformer winding to create the positive feeding so the circuit will oscillate.
Same topology as used in this circuit. PDF.

[goontron]

Re: Magnavox DRV power supply SMPS

Replace the fuse with a light bulb. It will limit the current, letting you run tests.

[johnfin]

Re: Magnavox DRV power supply SMPS

Can anyone tell me exactly what type of tests I can run while current limited? Without a schematic its like making love in a canoe. I also have a variac which I could use in conjunction with the bulb. Wish there was a SMPS step by step diagnostic chart.

[R_J]

Re: Magnavox DRV power supply SMPS

Try this schematic, it looks like the same power supply. If the original fet had a short from drain to gate I suspect parts in the drive circuit are damaged.

[johnfin]

Re: Magnavox DRV power supply SMPS

I saw that schematic, close but not exact. I need to check the 670meg ohm resistor in the primary side. Any SIMPLE way to measure it.

[johnfin]

Blown again

I used the 60watt bulb in place of the fuse and it blew the filament so fast I could not even see it light. Guess I need to use a variac.

[budm]

Re: Blown again

I used the 60watt bulb in place of the fuse and it blew the filament so fast I could not even see it light. Guess I need to use a variac.

The 120V 60W cannot blow the filament! What kind of 60W lamp did you use? It has to be 120V 60W incandescent lamp.

[johnfin]

Re: Magnavox DRV power supply SMPS

I used an incandescent 120v 60w. I will try it again. I know these Chinese bulbs are junkie nowadays.

[johnfin]

Re: Magnavox DRV power supply SMPS

in that older schematic attached in this thread under the problem flow chart it says if the fuse is blown check the "primary component". Like what would that be?