Howdy... I have a Sonos ZP120 (Connect:AMP) that accidentally got wet and it blew. I opened it up and started testing components and found everything intact except a 10 ohm 2W resistor and a thermistor. The resistor was blown, so I replaced it and the thermistor tested out fine (its a SCK054). I hooked it up to a dim bulb tester so I don't blow any more components and sure enough, the same resistor gets ultra hot. It would clearly blow if it was plugged in straight.

I went hunting with my DMM and tested the usual suspects... the big electrolytic caps tested out fine... in fact all caps tested out fine... the primary rectifier bridge tests fine. The thermistors and varistors tested fine. The transformers appeared fine as well. I have attached a photo of the Sonos' internals and the resistor that gets hot.

Does anyone have any recommendations or methods to help find the short or bad component?

 

A slight update... the choke filter in the upper left corner of that picture (the big one) has 4 wires. I think I have detected a short on it or something connected to it... between the primary and secondary, I get continuity with a resistance of 148 ohms. Could that be that the inductor has a short? Keep in mind it was DMM tested on board and I have not removed it.

The resistor that blows comes off the primary... so there seems to be a relation.

 

I pulled that inductor and its good.

 

Troubleshooting such a complex circuit without a schematic is usually a crapshoot.
About all you can do is follow where the output of the resistor goes and try to determine what on that path might be bad.

When you used the dim bulb tester, did anything else get hot?

 

Nothing else got hot except for the PWM (it got warm). The PWM is the thing in the TO-220 connected to the heat sink in the middle of the photo between the transformer below it and the 2 big caps above it. It was warm...not hot. I couldn't find anything that would be considered hot.

The PWM circuit is on the output of that hot resistor.

 

Ok... I let it sit for a while and one of the big 1000uf caps got pretty warm. I tested it and tan D and it seemed to be within spec.

 

Scratch that... the Cap is not worm... its the PWM...

 

Old post I know, but I may be able to help if you still need it.
I would remove the PWM and see if your hot resistor goes cool. That 2W 10ohm resistor it is a current limiter / damping resistor to the bias converter circuit. If its getting hot the bias converter is messed up. When you plug in the amp, describe the behavior of the dim bulb. It should just pulse one for about 1/2 second (if its a 100W bulb).