Question for electronics gurus

[Monkeyleg]

31 days ago I bought a slightly used Fender Super Reverb amp. I know it was 31 days ago, because Guitar Center, the place I bought it from, told me that the 30 day warranty expired on Thursday.

Anywho, the amp comes with a pedal with two foot switches. One turns the vibrato setting on and off. The vibrato only works if the pedal is plugged in. The plug is a standard 1/4" stereo  plug.

The other switch is supposed to turn the reverb on or off. Without the foot pedal plugged in, the reverb is on, but the control can be set to zero. With the foot pedal plugged in, reverb is supposed to be off or on, with the option to set the dial from 0 to 10.

The vibrato switch wouldn't turn off the vibrato, so I checked the switch and found the wiring was all messed up. I ordered a replacement switch, soldered everything up properly, and the switches work fine. Everything on the pedal and the plug works as it should.

The problem is that I can't turn off the reverb. I can have it set to 0, but I can't have it set to anything higher and then turn it off with the foot switch.

I know that various electronic devices will have circuits that are cut when a plug of some type is inserted. For example, stereos usually cut the speaker circuits when headphones are plugged in.

I'm looking at the schematics, but can't figure out where or how the reverb circuit would be interrupted by insertion of the plug and then controlled by the switch.

Any ideas where to look?

Thanks for any replies.

[Gewehr98]

The 1/4" plug should have either Normally Open or Normally Closed contacts controlling that reverb circuit, which should show up on the schematic.

[Monkeyleg]

Thanks, Gewehr98. I'm assuming you mean the socket for the 1/4" plug, and not the plug itself. I'll see if I can tell from the schematic what type of socket it is. Then I have to decide whether it's worth opening up the amp to fix it, as I don't know what's involved.

[Brad Johnson]

Two possibilities come to mind...

Possibility 1...
Are you plugging in the cable all the way?  I bet there the jack is switched.  If so, not getting the plug properly seated could cause all manner of weirdness.

Possibility 2 (an elaboration on what G98 suggested)...

I'm betting the re-wire somehow shorted between circuits in the switch box, shorting between the rings on the plug (effectively putting the reverb switch into a permanent NC state).  Take a multimeter and check continuity between the two rings, then between each ring and the tip, as you press and depress the switches.

Brad

[Brad Johnson]

Does the interior of your switch look like this?  If so, and without seeing the amp schematic, looks to be just a couple of simple ground circuits.  Not a lot to go wrong unless you have the wrong kind of switch, have the polarities or NC/NO sides swapped, or the ground is flaky.

Brad

[Monkeyleg]

That's what the interior looks like. I replaced both switches. I also ordered a new plug, so there's no issues with that.

I've pushed the plug in as far as it will go. I've tested the foot switches six ways from Sunday, and there's no reason they shouldn't work.

From looking around inside the amp, it looks like the speakers would need to be removed, as well as the speaker mounting board and grille, and probably some other parts, too, before I could drop the electronics box far enough to see the jack inside. I have to decide if it's worth it. If there was a song where I need a mix of reverb and no reverb then, yes, it would be worth it. If not, then the hell with it. It was the vibrato I wanted to fix, and that's done.

[Brad Johnson]

That's what the interior looks like. I replaced both switches. I also ordered a new plug, so there's no issues with that.

Two or three conductor plug, and did you make sure to get the right tip polarity?

Brad

[Monkeyleg]

The cable has two white wires: one for the reverb, and one for the vibrato. The reverb wire has a metal sheath around it, which serves as the common. The sheath is the wire that's soldered to the jumper wire that attaches to both switches and to ground in the photo above. There's no mistaking that.

If everything in the amp was working properly, and I got the polarity wrong on one circuit, I don't think either of them would work.

[Brad Johnson]

Meter out everything and make sure you have two discrete circuits.  There could be a problem in the wire or factory plug connections.

Brad

[Monkeyleg]

If you're talking about metering the foot switch and plug, I've already done that. There's two completely separate circuits, now. Before the wire sheath was touching the positive reverb post in the plug housing, which was why the vibrato wouldn't shut off.

If you're talking about the jack on the amp itself, I don't know how I could meter that without getting inside. I've looked all over online for a service manual or even some detail photos of a dismantled amp to figure out how to get the guts out, but can't find anything.

[Brad Johnson]

Two things...

1. Is your amp an original or a reissue?

2. Which contacts on the plug do you have corresponding to ground, vib, and reverb?

Brad

[Monkeyleg]

The amp is a reissue, and the problem is solved. There was a loose wire on the foot switch jack. Everything now works perfectly (for the moment).

My soldering skills definitely need improvement.

[Brad Johnson]

Shoulda used more duct tape.   

If you haven't already, you might want to check out some of the Fender Amp forums.  When I was searching for a schematic I came across a couple pages where people were doing some basic mods to customize the sound.

Brad