Medion MD41527 PS b/d hissing/crackling noise

[tom66]

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

Dirty contacts on the switch would cause arcing (not really a problem though), I do not think they would cause the thing to hiss/crackle.

[notallbad]

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

Well I replaced the rocker and it definately was a broken contact inside, contacts checked and clean. I didn't actually mean that it was the switch causing the fault or the cause of the fault but a possible spike/surge.

[notallbad]

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

Between the mains inlet and the ac-dc bridge rectifier are a couple of CD332M caps, 3300pF 14kV.
Is it really necessary for it to be such a high voltage rating? Will 1kV or 2kV caps in series or parallel be adequate replacements temporarily?

I have 332M, 222M volt ratings not specified
and
102 2kV caps

[tom66]

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

They are probably 1.4kV not 14kV.
And for class Y, typically blue or orange caps, yes. They are safety capacitors designed to -never- fail short even under extreme conditions. If they failed short it is plausible the chassis could be energised creating a shock hazard.

[notallbad]

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

OK so not had any bolts yet so not likely to have shorted.

Did you get to listen to the audio clip? Any thoughts?

[tom66]

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

I don't have working computer speakers at the moment, so no.

[notallbad]

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

OK, now I'm puzzled!

Every attempt to prevent the hissing and crackling noise by applying pressure to various points safely by using an insulated screwdriver and all it took (took a deep breath and held it whilst gritting my teeth and holding my eyes tightly shut) was a finger applying pressure to the Mosfet and diode heatsink at the top of the board well actually I don't even need to apply pressure, just touch it. But of course I cannot indefinately hold my finger there so what is it, some kind of grounding issue?

[tom66]

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

Tightened the screw on the heatsink?

[notallbad]

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

As tight as they will go.
The noise also stops if I connect a grounding strap between the chassis and the heatsink. Or at least reduces it considerably to an acceptable level, probably inaudible with the casing put back on.

[tom66]

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

Well attaching a strap from chassis to heatsink could make the chassis live, so you can't do that. Is the earth in your house or plug OK? Check for cracked solder around mains connection too.

[notallbad]

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

We're on a TT system here but everything else electrical works fine.

I have used different kettle leads and different sockets.

Besides trying these it did make this noise at the original owners home on a TN-C-S system so think these can be eliminated.

Having put the heatsink plate (bridges heatsinks top and bottom) and casing back on the noise is still there and still audible but not as much.

[Th3_uN1Qu3]

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

Well attaching a strap from chassis to heatsink could make the chassis live, so you can't do that. Is the earth in your house or plug OK? Check for cracked solder around mains connection too.

Since it didn't shock him when he touched it, it's safe to assume it's not live. The switching transistor is likely insulated from the heatsink.

This sounds like an EMI issue, interference from the power devices getting into the feedback path and disturbing the control circuit. Careful inspection of the PCB will likely confirm that. Possible causes would be bypass caps too far from the chips, or a trace carrying a sensitive, low-level signal being run too close to the noisy power section.

[tom66]

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

I think the risk is the single mica layer for the power transistor is not sufficient for a doubly-insulated electrical appliance.

[notallbad]

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

Its a lot noisier with the heatsink plate off than when its on.

With the heatsink plate on and strap to chassis the consumer unit breaker tripped so won't be doing that again nor shall I be touching it.

[ben7]

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

Its a lot noisier with the heatsink plate off than when its on.

With the heatsink plate on and strap to chassis the consumer unit breaker tripped so won't be doing that again nor shall I be touching it.

Yeah your heatsink is live, don't connect to the chassis again, like you said.

Be careful, with no heatsink on the transistors, they can overheat quickly and burn out!

I think the risk is the single mica layer for the power transistor is not sufficient for a doubly-insulated electrical appliance.

There is no mica insulator. The transistors are the TO-220F style, with a plastic tab instead of metal.

My guess is either what Th3_uN1Qu3 said about the interference, OR that one of the transistors or the diode is slightly leaky, and arcing inside making those noises. I know in power amplifiers, a failed transistor sometimes makes a lot of hiss and crackly noises in the sound output.

[notallbad]

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

Thanks for your comments, appreciate it.

I know you weren't suggesting it but thought I should state that the noise is not coming from the speakers, just for clarification in future reference.

Well I have just about covered nearly everything with glue and minimised the noise, not an ideal solution I know but it has somewhat drastically cut down. If you ever come across this one you will most certainly remember it! No kidding, off to buy more glue sticks. Hope it doesn't create overheating issues.

If its even possible to unsolder anything or even identify those components mentioned I may replace if I find suitable alternatives just to see.

Thanks again. Off to test the tv fully functions, fingers crossed.

[notallbad]

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

Well after just 12 mins it switched itself off.

Odd thing is when I power it up by rocker switch the backlights come on briefly before turning off then remains in standby until its switched on.
After about 10 mins the whole tv shuts down and after a few seconds the red standby light comes on, then the backlights(blue screen) comes on then off and tv remains in standby.
Turn the tv on again staright afterwards it reboots alot sooner than 10 mins.

It does this with or without a source input.

I am sure the TV stayed on before and the guy it came from just said the noise was the issue, no mention of switching off, not that you can always believe everything your told I know.

[notallbad]

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

No blink codes just completely powers off so assuming it must be a PS b/d issue.

Removed all the glue so back to horrendous noise.

The main filter cap measures around 321-6v when in stby and 376/7v when on, at the point off shut down it drops back to 321-6v and remains until the set powers back into stby, stby led comes back on.

The 5v stby also drops completely to 0v during the shut down.

Any thoughts?

[tom66]

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

Maybe your glue is conductive causing problems. Or stuff is getting hotter and you have an intermittent component or cracked solder joint.

[notallbad]

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

Well come to think of it I believe it was shutting down before applying glue, just put it down to the ' no signal detect' power off without realising what exactly was happening.

Of course this still doesn't rule out poor solder joint(s) but I have looked around and reflowed most just to be sure.

What does the FB and Isense legs actually do on the Viper22A?