Medion MD41527 PS b/d hissing/crackling noise

[notallbad]

Re: Medion MD41527 PS b/d hissing/crackling noise

In my quest to beat this thing I am looking at replacing the Optocouplers marked P421F just out of curiosity. Once I come across a Viper that will be next on my list to change.
Can these be replaced with either a Cosmo 1010 817 or a PC123?

The PC123 looks like a good match comparing the spec sheets unless I'm missing something.

Thanks

[notallbad]

Re: Medion MD41527 PS b/d hissing/crackling noise

Well replacing the Opto couplers made no difference.

Perhaps try a 0.022uF 630v cap next? If I can find one. R75 MKP.

[tom66]

Re: Medion MD41527 PS b/d hissing/crackling noise

http://www.ebay.co.uk/itm/Panasonic-...item20c4ce85a4

[notallbad]

Re: Medion MD41527 PS b/d hissing/crackling noise

Just dug out a 60" plasma PS b/d and found a cap with 223K630 printed on it so guessing this is a 22000pf 630v cap, suitable?
The only problem I may have is the lead spacing.

[tom66]

Re: Medion MD41527 PS b/d hissing/crackling noise

Yeah it should be OK.

[notallbad]

Re: Medion MD41527 PS b/d hissing/crackling noise

Shall have to keep guessing, still no luck, exchanged cap with same results.

[notallbad]

Re: Medion MD41527 PS b/d hissing/crackling noise

How can I eliminate the main board being the cause? i.e test the PS b/d standalone

[tom66]

Re: Medion MD41527 PS b/d hissing/crackling noise

I wouldn't consider a main board to cause this fault.

But if you want, you would use some car lamps to provide a load, probably two 65W headlamps in series on the 24V (65W total load), and one brake light on 12V. Then jumper PS ON to turn power supply on (using a 1kohm resistor to 5V.)

[notallbad]

Re: Medion MD41527 PS b/d hissing/crackling noise

First reading in stby, second reading when on

CN904
1 - 0.15 4.4
2 - 0 0
3 - 0 0
4 - 0.22 4.95
5 - 0.22 4.95
6 - 0.22 4.95
7 - 4.9 4.87
8 - 0 0
9 - 0 0
10 - 0 11.68
11 - 0 11.68

CN903
1 - 0 0
2 - 0 0
3 - 0 11.68
4 - 0 11.68

CN902
1 - 0 0
2 - 0 0
3 - 0 0
4 - 0 0
5 - 0 0
6 - 0 23.2
7 - 0 23.2
8 - 0 23.2
9 - 0 23.2
10 - 0 23.2

[notallbad]

Re: Medion MD41527 PS b/d hissing/crackling noise

What reading should I be getting on the Viper22A, pins 5 to 8? The Pcb has a darkened brown area around pins 5 to 8.
Don't seem to get a steady reading when using either mains neutral or the main cap neg leg. Or should the com probe be on the bridge rectifier dc output neg leg?

[tom66]

Re: Medion MD41527 PS b/d hissing/crackling noise

Pins 5 to 8 are probably the drain, in which case stable reading not expected. Viper chip is for 5V standby only. If the standby transformer is not source of hiss, I'd rule it out for now.

[notallbad]

Re: Medion MD41527 PS b/d hissing/crackling noise

There is a hiss when just on standby which isn't that audible but I'm sure its not normal.

[tom66]

Re: Medion MD41527 PS b/d hissing/crackling noise

A light hissing may be considered normal if it is inaudible from operating distance.

[notallbad]

Re: Medion MD41527 PS b/d hissing/crackling noise

On Pin4 of Viper theres 21.2v when tv in stby and 20.2v when tv is on.
Measuring between main cap neg leg and viper pin 4, Vdd.

[tom66]

Re: Medion MD41527 PS b/d hissing/crackling noise

That is fine.

[notallbad]

Re: Medion MD41527 PS b/d hissing/crackling noise

A light hissing may be considered normal if it is inaudible from operating distance.

Not sure this is normal though, but appreciate it is sometimes 'normal'.

[notallbad]

Re: Medion MD41527 PS b/d hissing/crackling noise

Heres an audio clip. Hopefully it will be clearer to you what the noise actually is, perhaps my description hissing/crackling is not very accurate.
First when switching on mains power, then goes quiet but still has that 'normal' hissing then power on from stby.

[notallbad]

Re: Medion MD41527 PS b/d hissing/crackling noise

Would it be possible to lift the pri side legs on the 2 opto couplers (IC12 and 13, located under the small xformer and next to C72) and replace with a diode just to eliminate the sec side of this part of the cct? If so what size diode would be needed?
Thanks

[tom66]

Re: Medion MD41527 PS b/d hissing/crackling noise

If you did that... the power supply would either not start or go out of control and try to output a high voltage due to the lack of feedback.

[notallbad]

Re: Medion MD41527 PS b/d hissing/crackling noise

OK, so going back I mentioned the on/off rocker was replaced due to crackling/arcing when turning it on and off, what would have caused this?
A power surge/spike?
If this was the case what could be damaged thats now causing the current fault? Varistor, ok not blown but is it possible? Can I just temporarily bypass the varistor to eliminate it? Replace with a jumper.