SWR workingman's 12 - blows main fuse

[bluelang]

Hello everyone,

Is there anyone out there who can send me a quick picture of what's behind (underneath/attched to the fuse holder) the mains fuse on their SWR WM12? A while ago I pulled a dead fuse from there, but I can't recall what it was. In other words, there is a 3A sloblo between hot and the power switch - but between that fuse and the IEC plug, there was ANOTHER fuse. (I think). I have bypassed it with a wire for now. I moved across country between debugging and getting back on the bench, so I've lost recollection of what was in there.

Oddly, even when I removed the bridge rectifier, it was still shorting out. I assumed a power supply problem, but removing all of the power leads from the main board results in no longer blowing the fuse.

I have the schematics and am competent at reading them. Unfortunately I've never found one that showed the AC coming in and the switch/fuse combinations.

There are 2 pics on google images but both are low rez.

Mine is a 1995 and I love it and want to keep it working.

It blew when our bassist was playing at full volume with the limiter defeated. (for great rock.)

Thanks!

[g1]

The 2004 schematics show a surgistor before the fuse, was it a board mounted part that you bypassed with your wire?

How many wires are there from the transformer to the circuit board? Can you post some pictures?

[bluelang]

The 2004 schematics show a surgistor before the fuse, was it a board mounted part that you bypassed with your wire?

How many wires are there from the transformer to the circuit board? Can you post some pictures?

Surgistor! That's what I need! Did it give any values? It wasn't board mounted - IIRC it was in a pigtail fuse holder. Unfortunately I have lost it in the move.

Xformer has H, H, CT. It's the same board posted in the other "blowing fuses" thread. I just replaced the BR.

Man I've gone over that schematic a dozen times, which one did you use?

Thanks!

[bluelang]

Well let's see if we can share pics from Picasa.

This is where it went - between H side of the IEC and the fuse holder.

[g1]

The surgistor would be mounted on the board. If it was a pigtail fuse, or inline fuseholder, then it was a back-up fuse. It would be a little larger than the user-accessable fuse, maybe 1/2 to 1 amp larger. They use these back-up fuses in case a user replaces the main fuse with too large a fuse or tin-foil. If there is a fault with the amp and the main fuse gets tin-foiled the back-up fuse will blow. You can leave it out or replace if you want.
If there are only the three wires coming out of the transformer, the only place they can go is the bridge and maybe a cap as shown in the schematic below (.1uf 250V). So if the fuse blows with the bridge out, but not if the transformer leads are disconnected, check the cap and check for any wiring or traces from the transformer that may be shorting to ground.

[bluelang]

All righty - you've confirmed my fears, basically. I'll put the hardwire back in and start working down the stack until I run out of fuses.

Thanks!

[g1]

Look up light bulb limiter and make one, will save on fuses and save the amp from further damage.
Can you take another pic that shows the labelling on the bridge (+,-,~,~), then remove the bridge and another pic of the underside of board where the bridge mounts.

[bluelang]

The + for the bridge indicator is on the top of the board. I got em lined up. I replaced the bridge and it's still poppin'. I got several megohms between legs on all the transistors.

One thing I did notice is only 4 ohms between the mains on the power transformer. That seems loooow. Any idea if that's in the right range?

At this point I'm either going to get some perfboard and replace it with a simple mono amp or use it as an extension cab - for another used WM12 when I find one.

Thanks for your help!

[bluelang]

Lemmy rules btw.

[bluelang]

Here are the original pics.. bigger than it shows here.

https://plus.google.com/photos/10660...249?banner=pwa

[J M Fahey]

4 ohms DC resistance in a transformer primary looks reasonable.
Donīt give up, retrace your steps:
1) transformer with all 3 secondary wires in the air ... blows?
2) 3 wires properly soldered, .1 x 250V cap connected, bridge out of the circuit ... blows?
3) same but now bridge connected ... blows?

The laundry list is much longer but letīs check this first.

Meanwhile, build a lamp bulb current limiter , youīll need it:
Light Bulb Limiter