Fender Rumble 100 mystery

[4cx10000]

Hi!

I am currently repairing a Fender Rumble 100 bass amplifier. It was completely dead when it came in and what I found was a shorted diode - minus side - in the mains rectifier bridge and a blown 1206 zero ohm on the bottom side of the pcb. Now … this zero ohm resistor, that was blown, was connected from the heat sink of the switching transistor, see schematic, and to ground. My humble question: why would you do that? 

[TheMG]

Probably for EMC reasons. They used the zero ohm resistor so they decide whether or not to connect it, depending on results of pre-compliance testing most likely.

It's actually very common in a lot of SMPS for the heatsink to be connected to the negative of the bulk capacitor (primary side circuit gnd reference). Even with the switching device isolated from the heatsink, at operating frequency the capacitance between the device and heatsink is significant enough to cause the heatsink to become a source of EMI.

I would guess either something shorted against the heatsink, or the insulation of the thermal interface broke down.

[4cx10000]

Thank you TheMG for taking your time to explain this circuit!

When I read your post it all fall in place, thank you!  Either one of the things you mentioned or that the short on the minus side of the bridge rectifier caused, if I would guess, a high potential relative switching transistor to the mains ac voltage and the result, an insulation breakdown. I will have to check the insulation for breakdown and if that happened - most probably as you also stated - I will post it here. 

[4cx10000]

Well well ... under the microscope it looks like this!