Roland KC-550 Power Supply Blows Fuse 1

[jayceekeys]

Hey guys,

I have a couple of Roland KC-550’s which I haven’t used in a few years. One of them is faulty and I decided to take a look at repairing it today.

The main fuse (fuse 1) in the top/rear of the amp blows as soon as it is powered. I traced the cables down (after the fuse) into the main body of the amp where the power supply board is. The first component is the main transformer. I unplugged the output connector from the transformer (CN16) from the power supply board and the fuse stopped blowing which meant the transformer was fine and the fault was further on. I then disconnected all outputs from the power supply (main amp feed etc) and the fuse still blew - so the fault is on the power supply PCB itself. Removing fuses 2 & 3 isolated a large section of the PSU and left just the bridge rectifier (BD1), a small cap (C2) and two large 3300uF caps (C1 & C3) that would have fed the amplifier board via WP25. With these limited number of components in line the fuse still blew so I think I’ve narrowed it down.

I’m going to isolate the caps by lifting a leg on the bridge rectifier tomorrow to see which component has failed. The rectifier is obsolete now so I’m thinking of going for something like a KBPC808 as an alternative if it does need replacing. Has anyone else swapped one of these out before and was there any other damage resulting from the failure? Also just in case I do need to change the large caps, would it make a lot of difference what brand of cap I use - a generic cap or do they have to be audio ones?

Best wishes and many thanks,

Jay

[nickb]

Don't get sucked into the "Audio" cap vortex. It's just a sales gimmick IMHO. For PSU apps go for, in order of importance
1) Min Voltage rating
2) Capacitance
3) Physical compatibility
4) high ripple current rating
5) highest temp rating
6) longest life

[jayceekeys]

Thanks Nick

[Enzo]

Use your ohm meter, leave the caps wired for now. There is nothing in a circuit that can make a shorted cap appear not to be. And use the meter on the bridge, see if one or more sides is shorted.

A bridge is just a bridge, nothing special, any rated enough bridge will work. Same with caps, just meet or exceed the existing specs.

[jayceekeys]

I’ve just realised.... the fuse that was in the amp that originally blew was a F3.15A fast blow fuse. That’s what I replaced it with. However the schematic says it should be a 2.5A antisurge / slow blow fuse. I’m wondering if the power up / inrush current to the two large capacitors is what is tripping the fuses and whether the correct anti surge ones would fix the problem.

Just ordered some antisurge slow blow fuses to arrive tomorrow so here’s hoping !

Cheers,

Jay