Some smoke coming from my power switch, SWR SM400S

DannDubblewe · #1 04-09-2011, 07:55 PM

So, just about 20 minutes ago I went about my playing routine as normal, looping some bass lines and drumming along. Lately i've noticed a small blue spark inside the small gap of the power switch whenever I powered on but was never worried (STUPID). So, I was just playing along when I noticed the loop was going in an out and basically crackling. To my horror, I went over to the amp and smoke was rising from the power switch. POWER OFF. I just took off the lid of the SM400S to scope it out, everything seemed 'ok' but what do I know? I plugged it in with the lid off and powered it for about 1 second and it smoked more out of the back of just the power switch. Here's a photo of what the power switch looks like now:

It is run bridged into a 4ohm Eden D410XLT, there is a furman PL-Plus and a DBX160A in my rack, I had a boss NS-2 running in the effects loop of the amp.

Has anyone experienced this problem or know what it may be? Am I best off taking this to an amp repair place before possibly (If it already hasn't happened) destroying the amp? I am simply horrified right now.

will33 · #2 04-09-2011, 08:04 PM

From your description, it may just be the power switch and nothing else. Simple test would be (with it unplugged of course) to twist together the 2 big red wires that connect to the back of the switch. Should wrap them in some tape and make sure they're not touching anything else. That's the same as having the switch on all the time. It'll turn on as soon as you plug it in and the only way to turn if off would be to unplug it. If it works normally, it's just the switch, if not there are other problems. If this makes you nervous, have somebody else do it.

If the switch shorted all the way, it should've popped the fuse at the back of the amp. It may not have completely shorted yet but is well on its way.

DannDubblewe · #3 04-09-2011, 08:16 PM

To clarify, I would snip them off where they are connected and twist them together completely separate of the switch, correct? I have no problem doing that, I just have to figure out how to remove the switch.

I will be attempting any serious troubleshooting tomorrow evening. I am exhausted and a little emotionally stressed currently, so, yeah.

I really REALLY appreciate your fast response, will33.

will33 · #4 04-09-2011, 08:24 PM

Yeah, you can snip them at the back of the switch, just make sure there's enough slack left to solder in the new switch. That's really all the switch is doing. When your amp is plugged in but the switch is shut off, juice can flow up the power cord and as far as the switch. When you flip the switch on, it's the same as connecting those 2 wires allowing power to flow to the rest of the amp.

I have my SM400S sitting here with the cover off due to other problems with it, let me go see how to remove the switch.

will33 · #5 04-09-2011, 08:29 PM

The switch just snaps in. There's 2 plastic tabs on the top and 2 on the bottom. It's kinda hard to get your hand in there but you squeeze the top and bottom together and pop it out the front. May need something to help pry it with if you got fat fingers like me.

DannDubblewe · #6 04-09-2011, 08:39 PM

Okay, awesome. I saw those tabs and thought that was it, I just didn't know if it went out the back or through the front and it was definitely a tight fit. And yes, i've got quite the sausage fingers so that helps greatly

. This is extraordinarily helpful, I will be sure to report back my findings.

Downunderwonder · #7 04-09-2011, 10:13 PM

Wow, a very rare case of garden variety smoke. More commonly it's the magic kind and you're shopping for a new amp.

Or there isn't a fuse and there's something else wrong, causing the switch to "fuse".

Jim Campbell · #8 04-09-2011, 10:22 PM

Quote:

Originally Posted by will33

From your description, it may just be the power switch and nothing else. Simple test would be (with it unplugged of course) to twist together the 2 big red wires that connect to the back of the switch. Should wrap them in some tape and make sure they're not touching anything else. That's the same as having the switch on all the time. It'll turn on as soon as you plug it in and the only way to turn if off would be to unplug it. If it works normally, it's just the switch, if not there are other problems. If this makes you nervous, have somebody else do it.

If the switch shorted all the way, it should've popped the fuse at the back of the amp. It may not have completely shorted yet but is well on its way.

i'd pop the switch out of the harness(unplugged of course) first and leave the wires on while i sniffed, tested with a meter,etc.....you can sometimes feel a soft switch,and smell if something is burning....

will33 · #9 04-09-2011, 10:39 PM

There is a little fuse soldered on the terminals behind the transformer that should go if there's a real problem. After the sloblo on the mains and before the switch/rest of amp.

Jim Campbell · #10 04-09-2011, 10:43 PM

Quote:

Originally Posted by will33

There is a little fuse soldered on the terminals behind the transformer that should go if there's a real problem. After the sloblo on the mains and before the switch/rest of amp.

not always....if a wire starts to fray it can build up heat without tripping a breaker

will33 · #11 04-09-2011, 10:46 PM

True.

Jim Campbell · #12 04-09-2011, 10:50 PM

Quote:

Originally Posted by will33

True.

i'm betting that the switch gave up the ghost.....those wires look pretty good from here...i've had switches smoke like that before,and it's only a matter of time before it fails completely

DannDubblewe · #13 04-09-2011, 10:50 PM

The smell was definitely readily apparent. I have not replaced a fuse so I do not know how to tell if they are blown, though they all appeared unharmed to my untrained eye. Hopefully it is just as simple as a switch replacement.

Quote:

Originally Posted by Jim Campbell

i'm betting that the switch gave up the ghost.....those wires look pretty good from here...i've had switches smoke like that before,and it's only a matter of time before it fails completely

Ooooohhhh Lordy I hope this is not the case *fingers crossed*

Jim Campbell · #14 04-09-2011, 10:53 PM

Quote:

Originally Posted by DannDubblewe

The smell was definitely readily apparent. I have not replaced a fuse so I do not know how to tell if they are blown, though they all appeared unharmed to my untrained eye. Hopefully it is just as simple as a switch replacement.

if you have access to a multimeter you can check fuses for continuity....fuses are so cheap that if you even think it may be a weak link,pop in a new one ....

Jim Campbell · #15 04-09-2011, 10:55 PM

Quote:

Originally Posted by DannDubblewe

The smell was definitely readily apparent. I have not replaced a fuse so I do not know how to tell if they are blown, though they all appeared unharmed to my untrained eye. Hopefully it is just as simple as a switch replacement.

Ooooohhhh Lordy I hope this is not the case *fingers crossed*

a bad switch is probably the best case.....better that than a problem that requires a tech,or worse yet fails when you're at a gig...

T-Bird · #16 04-09-2011, 11:03 PM

Hi.

I don't know whether it's a common problem with older SWR amps with that exact switch, but back in the day I had to replace mine as well.

It was a SM400 though, so it can't bee just a bad batch of switchs.
IIRC the current carrying capability was a bit on the low side for ~110V operation, so it should've been perfectly ok for 220V.

Regards
Sam

Jim Campbell · #17 04-09-2011, 11:07 PM

Quote:

Originally Posted by T-Bird

Hi.

I don't know whether it's a common problem with older SWR amps with that exact switch, but back in the day I had to replace mine as well.

It was a SM400 though, so it can't bee just a bad batch of switchs.
IIRC the current carrying capability was a bit on the low side for ~110V operation, so it should've been perfectly ok for 220V.

Regards
Sam

any mechanical device can wear out.....fortunately for the op ,if it turns out to be a bad switch,the smoke gave him a heads up and time to deal with it off stage....

Munjibunga · #18 04-09-2011, 11:11 PM

I think it's the seal around the switch. When the rubber gets old, it starts to let the smoke out.

Jim C · #19 04-10-2011, 08:41 AM

I have an ancient SM400 and the switch has never been a problem.
Are you certain the smoke come from the switch and did not just drift out around the panel opening?
Think I'd test the switch before I did anything else.
The only deffective item that has been documented on older SM400s (in my memory) was the rectifier that Steve used to meet either a UL or California spec. The replacement has been in place for over 20 years for me.

DannDubblewe · #20 04-10-2011, 08:55 AM

After it initially started smoking I took off the lid and powered it again - smoke came from the switch only. Certainly so.