Amp question for the techs/experts

[famous beagle]

Hey y'all,

I made a stupid mistake while trying to fix my tweed Champ clone amp. I was hoping some of the experts could chime in here and let me know the extent of the damage I may have caused.

I built a 5F1 kit from Weber about 5 or 6 years ago, and it worked great right away. After a few months I think, I opened it back up to try and clean things up a bit because it was pretty messy in there. I think I added a ground buss bar and tried to dress the wires better. When I sewed it back up, I got no power at all. It was completely dead. I was busy with things at the time, and I had other amps, so I never got back around to it. Life happened, and years passed.

A few days ago, I decided to finally open it back up and see if I could troubleshoot it. I noticed that I had the two wires on the switch--from the PT and the fuse--backwards compared to the layout. (See two layout images: one original and one "wiring error")

I also thought I noticed another issue, but it was just me being an idiot. Without looking closely at the layout (and reading the fine print), I wired both the Blue and Brown wires from the PT to the fuse. I know ... I know ... I'm an idiot!!! It had been a long time, and I was out of practice. I should have taken more time, but I didn't.

I sewed it back up, forgot to use my current limiter (again ... it had been a while, and I had forgotten about this stuff), and fired it up. The pilot light came on for a split second (which I thought was a good sign, because before it was completely dead), but then I immediately saw a spark in the volume knob/switch, and the amp went dead. The switch is fried I guess, because it no longer clicks at all.

After examining the layout and realizing my mistake, I've corrected the PT wiring, attaching the brown wire to the fuse and tying off the blue. I also examined everything and didn't see any visible damage (burn marks, etc.).

So, I have two questions:

1. How badly did I screw things up? Did I just fry the switch, or is something else more likely damaged/trashed as well?

2. Assuming everything else in the amp was wired correctly, would swapping the two wires on the switch (as in my "wiring error" layout image) like I had done cause the amp not to work? I didn't think polarity mattered with switches.

Thanks much to anyone who can help!

 

[arcaxis]

Did the fuse blow when it went totally dead. I'm thinking once the switch was closed a short circuit would have been protected by the fuse and the switch 'should' have been OK, presuming the fuse is the correct value and wired as shown.. I think I'd start by checking point to point that the wiring from the AC cord is exactly as shown in the correct diagram. Does the 'hot' side of the AC cord go to the switch? Do you have a schematic you could post?

 

[famous beagle]

Thanks for the response. The fuse did not blow; it's still good. (I checked for continuity to be sure.) It's a 2amp slo blo, which is the correct fuse for the circuit.

Attached is the schematic.

 

[arcaxis]

I think when the Blue and Brown wires of the PT were tied together the difference of the winding voltage which is 5 v (125v-120v) between those leads created a short circuit. Unfortunately the slo blo was too slow to save the switch. With some luck the transformer may be OK. No burned odor from the PT???

If you haven't repowered it after fixing the transformer lead error, pull all the tubes, but leave the pilot lamp in, jumper the wires on the switch (if you haven't replaced it yet). Put a quick blow type fuse of maybe 1 amp or less in the fuse holder. If your 'current limiter'(?) device has a power switch use that to apply power. If you have a voltmeter and feel comfortable poking around in the chassis, power it up and see if you have about the 6.3vac across the pilot lamp and just be aware that the 600vac(center tapped) at the WY3GT (pins 4 & 6) will not feel particularly good if you accidentally poke your finger into it. From chassis ground to either pin 4 or 6 on the WY3GT you should get about 300 to 330vac. If the fuse blows and there is no pilot lamp or no secondary voltages, the PT is likely toast. Maybe let it set a while energized before putting the tubes back in.

Evidently the transformer is available W022772 Transformer

 

[Steenamaroo]

I'm trying to get my head around what would happen here.
Would the transformer not simply operate as if you had connected brown only?

Either way, which one were you meant to be using? I think I'd disconnect the secondaries and connect them up to a meter, then test the transformer.

Test using Blue/Black and Brown/Black separately and check all secondary combinations, just to rule out transformer damage.
If there is/was any dodgy smell from it, I'm not sure I'd risk proceeding with that transformer but I'm thinking there's a good chance it's fine?

Given that the amplifier was an unknown quantity prior to this, I'd exclude any part of it from your troubleshooting, for now.

If you conclude the transformer to be healthy, snip, fold, and tape the unused primary.

Be safe - Use one hand - if you're not sure, don't - etc....

 

[famous beagle]

I think when the Blue and Brown wires of the PT were tied together the difference of the winding voltage which is 5 v (125v-120v) between those leads created a short circuit. Unfortunately the slo blo was too slow to save the switch. With some luck the transformer may be OK. No burned odor from the PT???

If you haven't repowered it after fixing the transformer lead error, pull all the tubes, but leave the pilot lamp in, jumper the wires on the switch (if you haven't replaced it yet). Put a quick blow type fuse of maybe 1 amp or less in the fuse holder. If your 'current limiter'(?) device has a power switch use that to apply power. If you have a voltmeter and feel comfortable poking around in the chassis, power it up and see if you have about the 6.3vac across the pilot lamp and just be aware that the 600vac(center tapped) at the WY3GT (pins 4 & 6) will not feel particularly good if you accidentally poke your finger into it. From chassis ground to either pin 4 or 6 on the WY3GT you should get about 300 to 330vac. If the fuse blows and there is no pilot lamp or no secondary voltages, the PT is likely toast. Maybe let it set a while energized before putting the tubes back in.

Evidently the transformer is available W022772 Transformer

Thanks very much for the detailed info.

I don't smell any odor from the PT at all, so maybe it's ok. I've ordered a replacement switch, but it hasn't arrived yet.

We're leaving for a two-week vacation in Scotland tomorrow, so this will have to wait until I get back, but I'll get on it then.

 

[famous beagle]

I'm trying to get my head around what would happen here.
Would the transformer not simply operate as if you had connected brown only?

Either way, which one were you meant to be using? I think I'd disconnect the secondaries and connect them up to a meter, then test the transformer.

Test using Blue/Black and Brown/Black separately and check all secondary combinations, just to rule out transformer damage.
If there is/was any dodgy smell from it, I'm not sure I'd risk proceeding with that transformer but I'm thinking there's a good chance it's fine?

Given that the amplifier was an unknown quantity prior to this, I'd exclude any part of it from your troubleshooting, for now.

If you conclude the transformer to be healthy, snip, fold, and tape the unused primary.

Be safe - Use one hand - if you're not sure, don't - etc....

I was meant to be using the brown wire. I know this for a fact because the amp worked great when I first built it. Whatever I did to screw it up while redressing the wires and all had nothing to do with using the wrong PT wire. When I reopened it a few days ago, the blue wire was still taped up with a wire nut, and I'm 100% positive it was that way when I originally built it because I can see that in the picture I took after I completed the wiring.

I can't see too much detail in the picture, because it was really messy (there are few components, but the chassis is really small and cramped!), but I can see that.

 

[arcaxis]

I'm trying to get my head around what would happen here.
Would the transformer not simply operate as if you had connected brown only?

The blue and brown wire give the option to select either 120vac(brown) or 125vac(blue) operation. Only one should be connected depending on what the voltage at the mains outlet is thought to be.

Either way, which one were you meant to be using? I think I'd disconnect the secondaries and connect them up to a meter, then test the transformer.

Pulling the tubes 'should' achieve the same thing (but leaving the pilot lamp in the circuit just for quick indication when the power is applied) and the PT wires won't be loose and possibly touch something they shouldn't or each other. The caveat is the the location of the mysterious 'ground buss bar' and whether that is where it could cause a problem.

Be safe - Use one hand - if you're not sure, don't - etc....

Sometimes not easy to do, but good advice.

 

[Steenamaroo]

The blue and brown wire give the option to select either 120vac(brown) or 125vac(blue) operation. Only one should be connected depending on what the voltage at the mains outlet is thought to be.

Yeah, I'm aware of that.
I'm trying to understand what would happen through his error.
There should be no current flow between brown and blue as brown to black would be lower resistance, right?

Beagle, you don't really need to wait for a switch, as arcaxis says.
If you put in a safer test fuse, manually bridge the switch, and use the mains switch to operate, you can get on with testing.

 

[arcaxis]

Yeah, I'm aware of that.
I'm trying to understand what would happen through his error.
There should be no current flow between brown and blue as brown to black would be lower resistance, right?

The winding between the brown and blue is like an auto-transformer winding that the magnetic flux in the core will develop a voltage across those leads from the voltage impressed on either the blue or brown wires to the black. It conceivably tried to develop 5vac (125vac-120vac) into what was essentially a short on the winding and produced whatever short circuit current it was capable of. Very important to ensure the lead that isn't used is properly taped up to keep it isolated.

 

[Steenamaroo]

Ah, I see! Thanks for the explanation, Mark.

 

[famous beagle]

Yeah, I'm aware of that.
I'm trying to understand what would happen through his error.
There should be no current flow between brown and blue as brown to black would be lower resistance, right?

Beagle, you don't really need to wait for a switch, as arcaxis says.
If you put in a safer test fuse, manually bridge the switch, and use the mains switch to operate, you can get on with testing.

I would, but I'm en route to Scotland right now for two weeks.

 

[Steenamaroo]

Hey man, if you look for excuses you'll always find them.
Have fun!

 

[famous beagle]

Well, good news. Back from the trip (which was brilliant), and I just replaced the switch last night. After verifying I had the tranny, fuse, and switch wired correctly, I felt confident to hook it up to the current limiter (just a DIY job -- a light bulb in series) and turn it on. Everything acted as it should. That was late last night, so I waited until this morning to fire it up officially.

Just got done running my Tele through it, and it's purring like a kitten now.

So ... the funny thing is, I really can't tell what the problem was in the first place. As far as I can tell, I've just put it back to the way it was before I started messing with it a few weeks ago, albeit with a new switch.

This makes me think that maybe the switch had already failed before I completely fried it by wiring the PT wrong. Or perhaps one of the solder joints on the fuse or switch was cold or something.

Regardless, it's all good now.

Thanks to everyone for the input!

 

[Steenamaroo]

Cool man! We don't always get an explanation, but sometimes we get results! haha.

Congrats and clips please.

 

[famous beagle]

Clips posted in the Guitar/bass forum.