Need help emulating Fender amplifier footswitch

Thread Starter

djs317

Joined Oct 29, 2012
4
Hi all,

I'm new to the forum and new to electronics in general. This is a great site and I've spend many hours pouring through the learning material posted here. Thanks to all of you for pulling this together.

I have a project I'm hoping I can get some help with. In short, I'd like to be able to control my guitar amp via a MIDI foot pedal. The amplifier has a foot switch that allows you to toggle the channel, clean channel gain, and reverb. My goal is to build a small box that takes the logical switches available on the MIDI foot pedal and control the amplifier like the foot switch does.

The schematic for the foot switch and foot switch port on the amp are attached. The full schematic can be found here: http://support.fender.com/schematics/guitar_amplifiers/94_Twin-Amp_schematic.pdf (Kudos to Fender for publishing these!)

I bought all of the parts and mocked up the foot switch on a breadboard. 2 caveats: 1) the LEDs in the foot pedal have two lights and three legs and look heavy duty. I replaced these with plain-old LEDs. 2) I left out the reverb switch and associated diodes. Using my mock-up I can switch the channel and the LEDs appear to work, but I cannot get the gain on/off to work.

I measured the foot switch voltage in all positions and here's what I have (1 = on; 0 = off):

The reference voltage is 5V (without the foot switch connected).

Gain Channel Reverb Voltage
0 0 0 10V
1 0 0 9.2V
0 1 0 8.1V
0 0 1 -320mV
1 1 0 7.5V
1 0 1 -1.3V
0 1 1 -2.4V
1 1 1 -3.2

So my questions: is this a really hard project? Seems like it would be simple, but I have no idea what is causing the voltage to jump from 5 to 10 to -3.2. Could I just add a battery and a bunch of resisters and mock up the voltage levels? Does the amperage matter?

Thanks for any advice.

-David
 

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MrChips

Joined Oct 2, 2009
21,281
That is an amazing circuit in the footswitch.
I have not studied it carefully but I cannot imaging the REVERB GREEN LED LD3 ever coming on,
or the CHANNEL GREEN LED LD2.
 
Last edited:

MrChips

Joined Oct 2, 2009
21,281
I don't think your voltage readings are accurate.
The footswitch jack is being fed with 27.5VAC.

The operation of the footswitch is rather intricate and the amp uses three comparators to sense the various voltages.

Emulating the footswitch with three switches is one thing.
Using a MIDI foot pedal is another thing all together.

Don't use a battery.

You can try some diodes, LEDs and resistors to try and match what the footswitch is doing just to get a feel for how the circuit works.
 

MrChips

Joined Oct 2, 2009
21,281
The other point is the circuit uses both halves of the 60Hz AC cycle.

The negative half cycle selects the REVERB ON/OFF

The positive half selects one of three options using three different voltage thresholds:

CH 1
CH 1 GAIN
CH 2
 

Thread Starter

djs317

Joined Oct 29, 2012
4
I don't think your voltage readings are accurate.
The footswitch jack is being fed with 27.5VAC.
The other point is the circuit uses both halves of the 60Hz AC cycle.

The negative half cycle selects the REVERB ON/OFF
Well, there's one mystery unraveled. I assumed it was DC. I checked the reference voltage again (AC, tip to sleeve on the cable that feeds the foot switch) and it reads 20.6V. Do you think leaving out the reverb switch, LD3 and CR3 is causing problems? Are all LEDs the same?
 
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