Electronics > Repair

would like some help with this dvd/vcr combo psu

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mj148:
thank you to anyone taking time to read / help!

i'm not all that familiar with modern type power supplies.. mainly older linear supplies and TTL logic 

the current symptom is the unit powers up then after about 10-30 seconds it displays  "shutting down" and switches itself off.. not sure if it is a problem with the control board or the power supply board so decided to investigate the power board first.

perhaps its nothing but some of the voltages appear to be too high? for example there is a rail labelled +17v but it reads around 26v

there is another rail +5v_0 which reads 0v... although the +5v_1 and +5v_2 read 5v.

the fact the unit powers on before cutting out some time later makes me think some protection is kicking it? id just like to verify that the power board is either ok / faulty and go from there..

I've tested some of the diodes and transistors to the best of my ability but not being familiar with how this type of power supply is supposed to functions im not sure if there is even anything wrong with it.. the 0v on a rail labelled +5v is curious though and hopefully i can learn something by trying to fix it.

inse:
Did you check the DVD and VCR mechanics already?
Can you hear motors running, or trying to?
I can hardly imagine that the power supply makes the set shut down.
Either there is a mechanical blockade or a belt has come off, or a position switch has failed, thats what comes to my mind.

abdulbadii:
any cross check its manual or its alike family on googling, the closer to be uploaded?

mj148:
Yes, been through all the obvious things like position sensors / mechanical switches..

unit will load a vhs and play it for around 30 seconds.. same with a dvd, both will eject fine too providing you are quick before it shuts down.. or you can eject them after repowering it up.

either playing a dvd or vhs or just leaving the unit powered up it will just shut itself down after 30 seconds or so.

belts in it are ok too... i assume the sensors are ok as it shows correctly when a tape or dvd is loaded or not.

will have to try and find a service manual for a similar model and hopefully find out what the voltages should be out of the power supply.. strange problem for sure!

mj148:
its fixed now.

the unit has a small fan connected... it spun freely by hand but the fan was open circuit...  only discovered by accident that the fan was no good!

the psu must monitor current draw from the fan or something similar because if you disconnect the fan (or if the fan is connected but open circuit) then the unit shuts down after about 30 seconds from power on.

replaced the fan with a working part and it spools up when the unit is powered on for 30 seconds or so then the fan stops until the unit gets warmer when it kicks on and off as needed.. had it running for a whole day no problems since.

disconnecting the fan brings back the original problem of it cutting out after 30 seconds or so.




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