Fender 4-way footswitch - how does this work??

[luap77]

Hi all,

I have a Fender Power chorus amp (130W, solid state) with a 4-way footswitch (channel select, reverb, chorus and boost). Each of the four switches has a led above it, which illuminates when that mode is active. The switch allows all four effects/modes to be switched on individually, or in any combination, including all together.

Inside the switch are 4 Carling PCB mount SPDT foot switches, 4 leds and 4 small diodes (2 mounted near the channel select switch, the other two near the reverb switch. That's all. The board is marked copyright 1989, O892, P/N 033259 C.

The footswitch plugs into the amp via a conventional 1/4 inch MONO jack. How does this work? I'm very curious as to how they can switch 4 different modes, have 4 leds which illuminate independently etc all through a single mono plug! 

Could anybody here explain this please, with a schematic, concept or otherwise.

Thanks

[Mark Hammer]

Here's a schematic for the Princeton Chorus that appears to use a similar method: http://www.fender.com/support/amp_schematics/pdfs/Princeton_Chorus_DSP_Sc9A4.pdf
The specific segment is in sector C-8

Truly odd.  I can't quite figure out how the decoding is done over that mono line, except that specific voltage drops are "interpreted".

[Gus]

looks like r119 powers the foot switch with ac voltage

looks like D18 C75(sample hold),R120(discharge)  act as a sample and hold or very simple integrator that charges to about the max neg voltage allowed by CH select diode drop.  that then goes to the comparator circuit

The other+ side side has a D24, C77(sample hold) ,R126(discharge) then voltage comparators triggered by the different diode drop voltages.

Top of switch
Ch  red led about 1.7V,  diode drop .7V
-.7V + -1.7V = -2.4V  then to the sample hold -.7 diode drop for -1.7 channel on

bottom of switch + section
both off .7 - .7v diode drop to sample hold  =0
+.7 + 2.7 + 1.7 + + 5.1  - .7 sample  diode drop  +4.4V RVB/DLY on
+.7 + .7 +1.7 = + 3.1 - .7 sample diode drop = +2.4 V CHR/MOD on
+6.8V both on

[luap77]

Thank you Mark & Gus.

This confirms what I've been thinking.

It sure is an interesting circuit. The 4 modes are switched pop-free and efficiently by this method.

I'm very curious as to how this could be implemented in stompbox design - i.e. a simple self-contained comparator schematic is what i'm after that could be used with such a footswitch. Perhaps the concept could be useful for switching between multiple effects via centrally located comparator box that then connects to the footswitch (placed appropriately on stage) via what can be quite a long standard mono guitar cable (doesn't introduce noise as not switching signal). I run a 6m cable with this footswitch with no problems.

Paul E