Panasonic SA-PT470 keeps blowing internal 4A fuse

[SuspectZero]

Hi all,

I purchased this surround sound system only 2 months ago, which is a 2/3 year old model, but had been unused and was in a sealed box, (so pretty much new).

It's been fine up until 2 weeks ago, when it didn't turn on.

Checked all the usual things, which resulted in me finding the internal 4A fuse blown.

Bought a pack of 10 fuses and 90% thinking this isn't going to work, plugged it all back in and - POP.

It's only when the on/off switch is activated on the front of the unit, does the internal fuse blow.

I'm pretty savvy with electronics/fine soldering, but not too good at diagnosis. I've checked all the obvious things I can with a torch, looking for blown caps/coils etc, but nothing is standing out (I've not removed entire boards yet - in the hope it could be something simple).

Also, it seems that this model suffers from this problem a fair bit searching the internet, but I can't seem to find a conclusive diagnosis and fix. Similar problem also here - //www.diynot.com/forums/viewtopic.php?t=271870

Any help appreciated.

PS - it only cost me £60, so I'm not too willing on spending too much to get it repaired - i.e. - http://sparedparts.com/parts/details/panasonic_sa-pt470_power_pcb_repx0728b

Thanks kind people

Mike

 

[Sam Gangee]

Replacing a protection fuse without first diagnosing and fixing the cause is like repeatedly driving your car into a wall to see if the new brake shoes fixed the hydraulic fluid leak. It can make the damage a lot worse.

It's unlikely that anyone can diagnose the fault cause at a distance. You need visual inspection (look for damaged components) and measurement.

Replace anything that is visibly faulty (e.g. bulging capacitors, cracked semiconductors) and anything you can't measure.

Try http://www.The-Cool-Book-shop.co.uk/electronics.htm#repair

 

[SuspectZero]

Thanks for the reply Sam.

When it was dead, I have only replaced the internal fuse once after finding it had blown. Wasn't sure if it was a surge that the 5A plug fuse may have passed, but the 4A didn't, hence why I tried it.

So I have 9 internal fuses left, (knowing further damage could be caused).

As above, can't see anything obvious, but will take the boards out to see if I can spot something.

I was hoping as this seems to be a common issue with this model that someone might find this post and know what the issue is.

Cheers,

Mike

 

[Sam Gangee]

I have a DVD player in for repair. Fuse blown. Main switching I.C. obviously blown (half of the plastic blown away). All diodes measure OK. All Electrolytics measure OK, except for three, which indicate rather high ESR. Considering that the unit was bought at Christmas and used only twice, this is rather poor and I reckon the electrolytics were bad to begin with.

As I can't easily measure the TL431 voltage reference or the opto-isolator, I shall simply replace them, together with the I.C. and the three suspect capacitors. I know, from past experience, that these items may have been damaged and could cause instant failure again.

Original fault was probably caused by a voltage surge from a nearby lightning strike. The PSU has absolutely no protection against such surges so I've advised the owner to buy a plug-in surge protector.