without killing yourself.richie00boy said:Glad you got it fixed in the end
Why did the fuse not blow?
Because the safety earth is not connected.
Is a double insulated power amp allowed to have a mains connection (the thermal switch) to the chassis?
Is this allowed by the Class11 regulations?
Really just the thermal switch would be sold as Class-II rated, if it isn't then assume it lacks the necessary insulation. Also even if it was rated to Class-II, the connections made (flying leads?) would also have to be insulated to Class-II standards. This would entail using specific insulation in a specific way.
The appliance could very easily be converted to Class-I simply by exchanging the mains lead for a 3 wire one and putting the earth wire to the chassis by a bolt of at least M4 size and with shakeproof washer each side of the tag soldered or securely crimped onto the wire. Obviously any paint around the area would need to be taken back to bare metal. This may aggravate hum due to earth loops in some circumstances, but could be worth it from a safety point of view given what has happened so far with the thermal switch.
I do wonder if the thermal switch had not been replaced at this point and simply left, if after time it would have broken down more and allowed dangerous currents to flow through anybody who touched the chassis, killing them.
The appliance could very easily be converted to Class-I simply by exchanging the mains lead for a 3 wire one and putting the earth wire to the chassis by a bolt of at least M4 size and with shakeproof washer each side of the tag soldered or securely crimped onto the wire. Obviously any paint around the area would need to be taken back to bare metal. This may aggravate hum due to earth loops in some circumstances, but could be worth it from a safety point of view given what has happened so far with the thermal switch.
I do wonder if the thermal switch had not been replaced at this point and simply left, if after time it would have broken down more and allowed dangerous currents to flow through anybody who touched the chassis, killing them.
usually components that break down partially do not improve with further breaking down. an ohmmeter check from the transformer primary to the chassis should have revealed this failure. usually what happens when these thermal button switches fail is that they just go open. shorting to the case is rare, but it does happen.
you should be fine bypassing the switch to test it. you may even get away with running the amp that way as long as the amp is properly cooled. the reason for the thermal switch is that subwoofer amps often are placed in places where cooling is difficult, such as under furniture, inside speaker boxes, etc.....
you should be fine bypassing the switch to test it. you may even get away with running the amp that way as long as the amp is properly cooled. the reason for the thermal switch is that subwoofer amps often are placed in places where cooling is difficult, such as under furniture, inside speaker boxes, etc.....
If the back panel is metal, and it's got a thermal switch mounted to it that isnt double insulated, why the hell isnt it earthed ? sounds very dodgy to me 
I would suggest replacing the 2-core cable with a 3-core, but you would have to go to the extra effort of completely isolating the RCA jacks from the panel. IMO, they should be isolated anyway!
Another possible solution - use the "sandwich toaster" type of thermal fuse. Crimp the wires to it. Put the whole lot inside some heat shrinkable sleeving. Fashion a small clamp from a bit of 1mm aluminium that holds the fuse to the back panel... because of the heatshrink, it will be totally isolated.
I would suggest replacing the 2-core cable with a 3-core, but you would have to go to the extra effort of completely isolating the RCA jacks from the panel. IMO, they should be isolated anyway!
Another possible solution - use the "sandwich toaster" type of thermal fuse. Crimp the wires to it. Put the whole lot inside some heat shrinkable sleeving. Fashion a small clamp from a bit of 1mm aluminium that holds the fuse to the back panel... because of the heatshrink, it will be totally isolated.
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