PSU Repair - Tevion DVD Recorder

[Paul C]

Hi all, currently puzzling over why this PSU is outputting nothing, I can not find a schematic for the PSU or even it's transformer a BCK02, the transformer has 4 inputs on the hot side & they are getting 78VAC each across the 2 central pins & the 2 outer pins, without a pin out/schematic that's how I'm guessing it's fed, is 78VAC enough?

The main 68uf 400V cap could be suspect but I do not have an ESR meter nor does my Fluke 10 have a capacitor test function, looking at it the cap top is slightly domed but not to excess, when powered the cap is fed 230VDC which drops to 8VDC the instant the juice is turned off, IMO far more residual charge should be held for longer in this cap, what are you're thoughts?

Here are some pics of the offending PSU for you're viewing pleasure



Any assistance fixing this greatly appreciated

 

[debe]

What are the numbers on the Chip U2 as cant read them in the picture.

 

[Paul C]

The chip is a viper53

 

[debe]

This circuit is from the chip specs & is a typical type. Your voltage on the 400V capacitor sounds too low if your mains is 240v Ac. I would replace that capacitor & the little one which goes to pin7 VDD on the chip as this is important for the chips powersupply which is around 12v dc.

 

[Paul C]

Just checked pin 7 input against pin 3 & 4 source which are tied together in this instance (ground ref) & the volts are 0VDC! I guess we have found the problem, now to trace where it's fed from, would the small 22uf cap cause no volts to be present?

Thanks

 

[debe]

The small capacitor is critical in the startup phase, & it must be the same value that's in your circuit. Good capacitors are essential in a switch mode powersupply.

 

[Paul C]

Absolutely, & I always replace like for like, maybe with a higher temp if available

 

[Paul C]

68uf & 22uf caps replaced, now getting 330VDC at the 68uf cap with a nice steady drop off of volts over a few mins when powered down, the old one was defiantly knackered! But still no power on the cold side, also still no power on pin 7 of the VIPER53 any more thoughts? Could transistor Q5 be at fault? It's a 2N5550 (NPN) http://www.onsemi.com/pub_link/Collateral/2N5550-D.PDF

I'm not sure how to test transistors so I googled & used this method http://www.vetco.net/blog/?p=184 & it passed all tests apart from test 5 which showed as failed, my meter says 1.405 positive on collector negative on emitter & 0.488 positive on emitter negative on collector, test says meter should be out of range! Is that method suitable for a 2N5550? & do those readings sound like it's failed?

Thanks

 

[debe]

Check the diode that feeds the 12v to pin7, is it ok? Not sure about the test on the transistor. If in doubt I just replace them as they are so cheep.

 

[Paul C]

Does the main transformer supply 12V to power the chip? I'm beginning to think the transformer is dead.

Could do with a pin out/schematic for the transformer but I cannot find one, it says it's a BCK02 on the side also has numbers - 1-1220-0079-0000-R

 

[KrisBlueNZ]

You have a chicken-and-egg situation because of how the VIPer53 starts up.

Referring to the diagram in post #4 on this thread.

When power is applied, the drain terminal of the switching MOSFET in the VIPer53 is pulled up to the positive supply voltage by the transformer primary. An internal charge path in the VIPer53 feeds a small amount of current out the VDD pin, which charges up C4.

Once the voltage on C4 gets high enough, the VIPer54 starts up, and starts switching. Assuming the MOSFET and transfomer are OK, and none of the diodes is short, pulses coming back through D3 and R4 keep C4 charged up, and normal operation begins.

You said there's no voltage on C4. If there's the expected voltage on the drain pin, and no external path that could be holding C4's voltage at zero (e.g. faulty C4, faulty D3), then I would assume that the VIPer53 is faulty - either the charge path isn't working, or VCC is internally shorted to GND.

Measure on diode test range with red probe to VCC and black probe to GND on the VIPer53. If there's a short, isolate the VIPer53's VCC pin and check whether the short is internal or external.

 

[Paul C]

Thanks, I'll check it out later, day job getting in the way of PSU tinkering

 

[Paul C]

Did you mean VDD? Pin 7 to GND/source measure 1.410 on diode test.

All diodes appear to measure ok 0.490 on diode test & OL when reversed polarity.

 

[KrisBlueNZ]

Yes, pin 7, VDD, connected to C4 (or whatever number it has on your board).

Power up the board and check for ~340VDC on pin 5 (drain of internal MOSFET) relative to pin 3 or 4.

If that voltage is not present, there's a fault around the transformer primary.

Measure voltage on pin 7. If it's less than a few volts, there's a fault either inside the VIPer53 or in the external components connected to pin 7.

Power off and wait for the input cap to discharge (or discharge it through a big resistor).

So you measure 1.4V on diode range, between pin 7 and pin 3/4? That sounds too low to me. Isolate pin 7 of the VIPer53 IC from the board, and see whether you get that reading from the IC pin (in which case the VIPer53 is probably stuffed) or from the track on the board (in which case it could be the diode feeding it (D3 in the diagram in post #4).

 

[Paul C]

Thanks, I'll check later after work & report back

 

[Paul C]

Right, parts were sourced (New Viper53 & 2n5551) & board reassembled & it fires up perfectly now

The failed main C2 68uf 400V cap must have stressed & taken out the Viper53 I`m assuming!

All working now! Thanks

 

[KrisBlueNZ]

Good work!