Switching supply problem

[misisoso]

Hi everyone! I would like to ask you one question about problem I´ve been having recently... I am repairing Logitech speakers and I found out, that the fuse was blown out in the power supply... After replacing the fuse it still blows out... no matter how many Amp fuse I give there... I checked the circuit for some short circuits, but nothing... I think it is in the input part... I measured the resistance between input pins (230 V) and it shown 10 ohms, but nearer to the decouple transformer only 1,7 ohm (i know, this is the resistance, which is given to the direct current, but when it comes to alternating current, it is much more, because impedance from inductance comes along...)... I also experienced, that some blue cap (like ceramic, or so, not electrolythic) was blown, near some kind of TO220 package (in the part of circuit, that was decoupled by the input transformer)... what could be the problem please? I soldered out that cap and replaced fuse... thats all (and measured the voltage on big electrolytic cap behind decouple transformer and there were 0 Volt, so I think, the voltage never made it through decouple transformer....input part consists of : Fuse, Varistor, Thermistor, yellow bricked capacitor and finally, the decouple transformer).
The input decouple transformer is some kind of ferrite named UU16 HGX

[kripton2035]

hi, would be easier if you can post a picture of the supply !
thanks.

[SeanB]

From the desription this is a SMPS, and you are likely to find the TO220 device is most likely some Topswitch device. There will be  a bridge rectifier just before it, and most likely a current limiting resistor as well. Check the diodes are not open circuit, or short circuit, though most commonly the Topswitch is shorted out.  Normally caused from spikes on the input, or from the input capacitor being open circuit, and from the snubber capacitor being blown ( blue capacitor) I would guess it had a near lightning strike that killed it.

[Psi]

My Logitech Z5500 blew its fuse pretty soon after i got it.
I put a new one in (same current rating) and it blew that too, used a slightly higher current fuse and again it blew.

It then occurred to me that it does have a huge subwoofer with quite a bit of power supply capacitance, so the peak current at turn on is very big.
I replaced the fuse again, this time with the correct current rating but a slowblow type and it's never blown since.

[ptricks]

An old troubleshooting technique is to place a standard 60 watt light bulb in series with the power connection of the device you are troubleshooting. In your case you could connect the light bulb across the fuse terminals.  The light bulb will  glow brighter when something is shorted and this will allow you to probe around without constantly replacing fuses and hopefully find the culprit.

[misisoso]

Hello. Thank you for your responses... I will try to upload photo of that SMPS.