Fender Mustang non valve guitar amp - owner connected 110v amp to 220v.

[t4796]

Hi all,

I recently picked up this amp as not working. I took it as is. Didn't notice until I got home it had foreign power plug and was marked 110v and our local voltage is 220v so then I knew what had happened to it.

Opened it up and was able to find the schematic online. In the schematic I have uploaded I have erased the parts for 220v operation to make it easier for me to visualise however I have since added the two additional diodes and remove the wire link from the power supply so I can test it easier on 220v

From the FAQ I learnt that for 110v the power supply doubles the voltage with the two diodes and caps. In 220v operation there are two additional diodes and a wire link is removed.

I see the mains filter capacitors but not a lot else. Fuse is blown. Is it safe to assume after seeing 220v the 200v capacitors are toast? Whilst paired one looks taller with less of its wrapper overlapping the top so I think it might have vented.

Is there anything else obvious I should check which would have been damaged by the wrong line voltage? Would some components down the line have seen 440v potentially for a short time or are these power supplies designed to blow the fuse before that happen?

Thanks for any input.

[stj]

that was destined to blow unless your editing removed more than you think.
post the full schematic.
and check the diodes and fet for shorts

[t4796]

With the circuit acting as a doubler but getting fed 220v instead of 110v the capacitors would have seen over 300volts dc each? I already replaced all diodes since I was already adding two more. I thought they tested ok, but it was late will try to find them. They are 4007 1000v diodes.

[stj]

thanks.
what i said about the fet and diodes still stands,
but those primary caps should have had resistors in parralel.
not just to discharge them, but to balance them.
if your going to 240v you could just replace the pair with a 450v cap though.

i would also check the resisters didnt change value!

[t4796]

Ok mate. Thanks for the reply. Will take some further measurements of the diodes, fet and resistors. That's a good tip about the cap since there is plenty of room and it might be easier to find a single/I might have one already.

[momaka]

Primary MOSFET most likely has survived, since it's rated for 700V, and also the primary caps very likely did not allow the voltage on the primary to double before they started conducting severely / shorted out.

I agree with STJ in regards to the omitted balancing resistors across the two primary caps - they really should have been there. If you do replace the two primary caps with 2x 200V ones again, it would be a good idea to add those resistors (one resistor across each cap). Commonly used values are 270k, 300k, 330k, 390k, and 470k (with 300 and 330k being the most common), 1/4 Watt.
Probably also a good idea to add two MOVs - one across each cap - to protect against over-voltage / surges. The MOVs should have actually been part of the design and added from the factory... but I suppose the manufacturer further cut costs with the design of this PSU. If they were there, they would have blown first, taking the fuse out immediately, before anything else got damaged.

Alternatively, do as STJ suggested and use 1x 400V (or 450V) cap. Capacitance-wise, you can get away with roughly half of the capacitance of what each 200V cap was rated for - i.e. if the primary caps were 470 uF, then you can use either 1x 220 uF or 270 uF or 330 uF as the primary cap.

[momaka]

By the way, forgot to mention... once you replace all parts and are ready to power up the amp, it may be a good idea to do so through an incandescent or halogen lamp in series with the AC line to limit input current, should something still be very wrong with the primary circuits. 100W incandescent or 72W halogen is usually OK for testing PSUs, IF their output is disconnected from the rest of the device. But if that's not possible or you want to test the rest of the device too, then a 100W incandescent may not be enough. In such case, going up to 150W or 300W halogen may be needed.

[t4796]

Here is the update, sorry about the delay I didn't have much time to get back to this one.

I could only get the fuse in a lot of 10 locally so I took a chance and powered it up with the diodes and link removed configured for 220v and it works.

I'm trying to understand the theory behind it though as to why fuse blew but nothing else appears damaged.