I live in a neighbourhood where people throw out good things. A few days ago, I found a Hartke A70 bass amp in a pile of rubbish waiting for collection outside a nearby house.
I carried the 21kg amplifier home, plugged it in and switched it on, but it did not power on. I unscrewed and removed the electronics from the speaker cabinet and looked inside. There was no obvious sign of anything wrong – no bulged capacitors, or scorched power resistors, etc.
I discovered that the fuse inside the power plug was missing. Writing on the back of the amplifier said 115V 2A / 230V 1A. I live in Australia, which has 230V 50Hz mains power, so I bought a replacement 1A fuse. I bought a few F1AL250V fuses (F = fast acting, L = low breaking capacity).
When I inserted the new fuse and switched it on again, the fuse blew, and I suddenly had an insight as to why the original owner was throwing out their amplifier.
I am comfortable with simple low-voltage DC electronics, but mains-powered AC is outside of my experience. I have tried a few things already, but I am feeling a bit stuck, and I would really appreciate any suggestions.
Here is what I have tried so far…
The removable fuse holder in the chassis mount AC plug can be inserted two ways, which selects 110-120V input or 220-240V input, with the label for the selected voltage upright (and the other voltage label upside-down).
I thought that I might have inserted it the wrong way but, just to be sure, I followed the wiring from the plug to the switch to the transformer, and unplugged the orange wire between the switch and the 110V input on the transformer. I inserted a new fuse and powered it on, but the fuse blew again.
Next, I thought that something on the circuit board might be shorting out, so I unplugged connector 7, which connects the output of the transformer to the bridge rectifier and voltage regulators, etc. This means that the only components being powered are: the input plug (with in-jack fuse), a power switch (with an LED indicator), a thermistor and fuse, and the toroidal transformer. I powered it on, but the in-jack fuse blew again. I guess this means that the problem is isolated to the first part of the input power circuit (see attached circuit diagram).
I have run out of ideas, and my knowledge of AC electronics is weak. Could the 1A fuse be blowing because of the inrush current as the transformer is initially receiving power? Would this be fixed with a slow-acting 1A fuse? Could there be a fault with the transformer, even though there are no signs of physical damage or scorching? How would I test this before buying a new transformer? Have I missed something that is obvious to everyone except me?
I carried the 21kg amplifier home, plugged it in and switched it on, but it did not power on. I unscrewed and removed the electronics from the speaker cabinet and looked inside. There was no obvious sign of anything wrong – no bulged capacitors, or scorched power resistors, etc.
I discovered that the fuse inside the power plug was missing. Writing on the back of the amplifier said 115V 2A / 230V 1A. I live in Australia, which has 230V 50Hz mains power, so I bought a replacement 1A fuse. I bought a few F1AL250V fuses (F = fast acting, L = low breaking capacity).
When I inserted the new fuse and switched it on again, the fuse blew, and I suddenly had an insight as to why the original owner was throwing out their amplifier.
I am comfortable with simple low-voltage DC electronics, but mains-powered AC is outside of my experience. I have tried a few things already, but I am feeling a bit stuck, and I would really appreciate any suggestions.
Here is what I have tried so far…
The removable fuse holder in the chassis mount AC plug can be inserted two ways, which selects 110-120V input or 220-240V input, with the label for the selected voltage upright (and the other voltage label upside-down).
I thought that I might have inserted it the wrong way but, just to be sure, I followed the wiring from the plug to the switch to the transformer, and unplugged the orange wire between the switch and the 110V input on the transformer. I inserted a new fuse and powered it on, but the fuse blew again.
Next, I thought that something on the circuit board might be shorting out, so I unplugged connector 7, which connects the output of the transformer to the bridge rectifier and voltage regulators, etc. This means that the only components being powered are: the input plug (with in-jack fuse), a power switch (with an LED indicator), a thermistor and fuse, and the toroidal transformer. I powered it on, but the in-jack fuse blew again. I guess this means that the problem is isolated to the first part of the input power circuit (see attached circuit diagram).
I have run out of ideas, and my knowledge of AC electronics is weak. Could the 1A fuse be blowing because of the inrush current as the transformer is initially receiving power? Would this be fixed with a slow-acting 1A fuse? Could there be a fault with the transformer, even though there are no signs of physical damage or scorching? How would I test this before buying a new transformer? Have I missed something that is obvious to everyone except me?
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