Fender Footswitch Oddity?

[bnwitt]

I happen to have two Fender amps in for repair this weekend. One of them is a Red Knob Twin and the other is a Deluxe 112 plus. Neither of them had a footswitch and I am fabricating them from generic two button Marshall style footswitch boxes since everyone was out of stock on the Fender unit.

Both amps use the Channel Select/Reverb type Fender footswitch. However, they differ in the polarity of the diodes in the footswitch (and inside the amps' switching sections) that are assigned to either function according to their individual schematics. It would seem that there would have to be two different Fender footswitch models (one for each amp) in order for each particulary labeled button to operate the properly related amp function, and that these two pedals would not be interchangeable.

I have attached both schematics. For the reverb switch on the deluxe 112 plus, the cathodes of the led and diode are assigned to the tip of the plug. For the Red Knob Twin's reverb switch, the anodes of the led and diode are assigned to the tip. This diode polarity carries through to the circuits in each amp as well according to the schematics.

So my question is, would Fender really do this and not have one pedal work for both amps?

Edit: Now to make matters even stranger I find that on Fender's website they show one pedal part number for both the "The Twin" and for the Deluxe 112 Now, I'm wondering which layout to use to make these work. I guess I'll just go with a coin toss.

[metalangst]

It looks like the twin schematic is labeled backwards, otherwise it's the same circuit.

[bnwitt]

Yep, I'm starting to think one of the schematics is backwards. I'm going with the Deluxe 112 Plus schem and I'll try that in both amps. I'll post what I find out. They're not exactly the same circuit inside the amps. Definitely different but similar and the function is the same.

[bnwitt]

Now I'm really confused

Ok, I built the two button Fender footswitches today and I've got them working. However, now I'm really confused. Attached is a drawing of how I had to wire the components to get the switches to work with the amps properly. What you'll see is each LED and diode assembly for each switch is connected backards from the schematic.

What I mean by that, is the LED's anode is connected to the diode's anode in each switch circuit. In other words, they are connected head to head instead of head to tail like shown in the drawings.

I mean unless I'm identifying the LEDs' anodes incorrectly. The longer LED lead is the anode or positive lead right? When I put two volts DC across each LED it will light when I have the positive supply to the long lead and not light when I have the positive supply to the short lead.

[Enzo]

You got round LEDs? There should be a small flat side on them, and that indicates the cathode.

And they are after all diodes, so by testing with your meter or a current source, you should be able to determine polarity. I never think of LEDs having + and - ends. I think of them as diodes having anodes and cathodes.

[bnwitt]

Ok well that jives with the long lead being the anode. The flat side is the cathode lead or shorter lead. So the diode has it's cathode end marked with the stripe and the LED has it's cathode end indicated by the short lead and/or the flat. Now that makes sense. The pedal is connected like the schematic. I've fixed the drawing in the earlier post.

[bnwitt]

The bottom Line

Well the bottom line on the two schematics for these two amps is that the schematic for the the Deluxe 112 Plus footswitch is incorrect. Wiring both pedals with the cathodes on the tip for the reverb switch as in the drawing posted earlier in this thread works correctly in both amplifiers. I used 1N4148 diodes and standard 3mm red diodes if anyone is interested.

[outoftheaircrash]

reasonably related question..

Hey.. just discovered this board. Great stuff!

I have a quick question.. I'm about to hop off and build this footswitch (thanks for the schematic), but I also want to modify an early 90's blues Deville to make the effects loop foot switchable..

In my head, I would imagine that this couldn't be that complicated.. But has anyone tried anything like this before? Any thoughts?

Cheers

Kris