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PSU Repair Tips

 
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markie76
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PostPosted: Mon Nov 26, 2007 11:38 am    Post subject: PSU Repair Tips Reply with quote

Hi,

I have a combo Video/DVD player Philips DVD757VR which went dead one day. I downloaded the service manual but no fault flowchart in it.

I have removed the PSU and no visible component faults.

I have started by measuring the voltages at the swiching transistor 2SK3566 but I am getting 135v at each pin - is this normal?

Any tips on how to trace the fault apprecited.

Thanks,

Mark.
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torbjorn



Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Mon Nov 26, 2007 3:44 pm    Post subject: Reply with quote

I suppose that this transistor is located on the primary side of the power supply and that you are measuring the voltages relative to the negative terminal of the mains rectifier bridge.
If the unit is supplied from a 110 Vac mains, then 135 V would be a fairly normal output voltage from the rectifier. If you have this voltage on all terminals of the transistor, then the transistor is probably shorted and the source resistor has went open circuit. Probably, some parts connected to the gate are also blown.

Make resistance / diode test measurements on all relevant parts and replace the suspect ones. If the gate drive comes from an IC, and there is no protection between the IC and the switch transistor's gate (i.e. zener diodes, fusible resistors etc), then the IC is probably blown.

Try to find out the reason why the transistor failed, it might be due to faulty components (resistors, capacitors, diodes) in a snubber network across the transformer primary or in parallel to the transistor. Another possibility is that it failed due to a lightning surge from the mains or due to overheating, for example if it did not make good thermal contact to its cooler.
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PostPosted: Sat Dec 01, 2007 6:10 pm    Post subject: Reply with quote

Hi torbjorn,

I should have mentioned this is a 240v supply and yes it is on the primary (hot) side of the circuit.

I will re-check the other components on the hot side again, perhaps removing them, but a visual check shows no abnormalities on any of the components.

Do you by any chance know of an alternative swiching transistor for the 2SK3566? as I am having trouble finding one in the UK.

Thanks for your help.

Mark.
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torbjorn



Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Mon Dec 03, 2007 11:32 am    Post subject: Reply with quote

Okay, in case of 240 Vac supply, you should have at least 300 Vdc after the bridge rectifier. I think it is in place to check all components in series with the supply voltage (resistors, filter coils etc) to see if there is a open circuit somewhere.

The 2SK3566 can be bought at Farnell under part no. 1300782, so it should not be a big problem finding that part. The datasheet can be found at
http://www.toshiba.com/taec/components/Datasheet/2SK3566.pdf .
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PostPosted: Sun Dec 16, 2007 6:07 pm    Post subject: Reply with quote

Replace all the small electrolytic capacitors, specially on the primary side of the transformer. Big chance one of them is faulty, preventing the psu from starting up. I have replaced a shitload of such, with success almost every time.
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