Debug Help: Vox AC15 Footswitch

[mattwells]

I am trying to make a footswitch for my Vox AC15.  Two stomps with LEDs.

I found this build guide:  http://i.imgur.com/ElbZtNo.jpg
Only changes:  I am using standard stomp switches (upper left and middle left pins) and smaller resistors (appropriate to my LEDs).
It works fine as long as I am not using an LED - as soon as I introduce an LED + resistor, the effect (whether Tremolo or Reverb) is "always on" regardless of the switch.  The switch only ends up operating the LED without changing the effect at all.  Thoughts?

[Mike Burgundy]

Is the effect always on without the switch? What have you measured? (both switch behavior without it hooked up to the amp AND voltages coming from the amp)

[mattwells]

Did some more testing after posting this - going off memory here is what I know.

20v coming from the amp.  I had two switches, one with the LED/Resistor one without.  Whichever effect the non-LED switch was hooked to worked as intended.  Switch with the LED just ended up turning the LED on and off.  Changed out the switch and had the same behavior (trying to eliminate the switch as a variable).

Then I took everything apart.  Just a wire.  If the wire goes from Tip/Ring to ground the effect is off.  If it goes nowhere the effect is on.  (Again, going from memory here - one is on one is off, and I'm fairly certain it is that order)

Did the same with the LED/Resistor - going to ground the LED comes on, but it didn't switch off the effect.

Here is a couple of shots of where I soldered the resistor/LED to a vero scrap (this is what i have been using for my LED/Resistor wire).  http://imgur.com/a/vp4u5

[mattwells]

I also can't figure out how the LEDs are supposed to come on when the effect is engaged if that requires not sending it to ground (assuming my memory is correct and going to ground is what switches it off - that is also what switches the LEDs on).  Maybe the diagram I found (that I made my handwritten copy of) was wrong and I need to do a little more with the switch.

I found this diagram and will try this organization of things next even though I can't quite wrap my head around how it works:  http://s5.postimage.org/edr7o9z1z/VOX_VFS2_A_Wiring_Diagram.jpg

[PRR]

Use the right-value resistors (100K smells much too large), and RED LEDs.

If the voltage-drop in resistor and LED is too large, the brain inside the amp ignores it. 100K is maybe too much. Yellow, Blue, White LEDs have larger drop than Red LEDs.

If the new LED ends up too bright, scuff it with sandpaper and swap some dilute black paint on the end.

[mattwells]

I'll try the red LEDs.  I used a 15k (I believe) resistor instead of the 100k with the Blue LED.

Still can't figure out how to make the LED come on when the effect is on, though.  It only comes on when the effect is off.

[PRR]

Need to see the circuit INside this "AC15". I'm not finding one with a footswitch. Can you find one showing how the footswitch jack senses the switch state and also powers the LEDs?

[mattwells]

Google turned up this link to a pdf.  I don't know if the schematic is 100% the same as I am not positive of the year of my amp, but the footswitch has not changed and is the same as the AC30 footswitch.

[PRR]

> same as the AC30 footswitch.

I did not know that. (Suspected it, but that's a lot more trawling.)

That's a very messy drawing. I can't trace the Reverb side because it goes to a mystery chip, though before that it is similar to the Tremolo side so probably similar.

The Trem side has 30 Volts through a 3.3K resistor to the jack. This flows up-to 9mA, plenty to light an LED.

The jack terminal also goes thru 10K to a transistor, which makes/breaks power to the Trem Oscillator.

If the jack terminal is below 0.6V, the Trem is off. If the jack terminal is above 0.6V, the Trem is ON.



So if you wire an LED to the jack (right polarity!), the 9mA flowing in the LED will bring it up to 1.6V-4V, which is above 0.6V, so the transistor turns-on, the Trem turns-on, meanwhile the LED glows.

And if you throw a short (say a switch) across the jack, the LED is shorted-out (dark), the transistor goes to 0.0V (less than 0.6V, off), the Trem Oscillator does not have a complete power-path and quits working. (*)

Your other link shows a 1.5K resistor. This "may" help protect the switch-box LED against wrong-hole accidents (the output of a 100 Watt power-amp, a 9V supply). I doubt it, because if the switch is in the shorted position, *something* is going to cry or fry.... wire, supply, or switch. You can't put the 1.5K where it would "protect the switch" because then it would not pull-down enough to turn-off the transistor.

(*) Yeah, there's a 10K leak R25. This is not bypassed, so oscillator gain goes to 100K/10K= 10. This type oscillator needs a gain over ~~20 to sing, so it goes mute.

[PRR]

me> goes to a mystery chip

Probably M5201, a Misubishi AV switch chip. Someone could trace it out and figure if high/low off AC15/30's TR2 results in A or B and which is/isn't reverberated. Not me. http://www.audiolabga.com/pdf/M5201.pdf
Since the assignment A/B reverb/dry was arbitrary, I ass-ume they made it work like the Trem switch/LED. But you never know what the factory was smoking.

[mattwells]

Thank you so much for the helpful explanation - I'm trying to understand more than just "put parts together and it works [or doesn't]" so that was just what I was looking for.  I'll give it another shot this weekend and see whether I can't get it working correctly.

Once I finish this I can get back to troubleshooting my actual pedal projects...

[mattwells]

Ugh.  So I had it working correctly with alligator clips, etc. running all over, so I started boxing it up. 

Have one side done (reverb side) and...the switch portion is working correctly but it is not lighting the LED.  Nothing has changed from when I had it un-boxed except I trimmed wires and put it in the box.

Album of pics here, but I am not sure it helps at all:  http://imgur.com/a/7eAnd

The only thing I can figure is maybe the LED is in backwards - but since an LED is a diode it wouldn't pass any current which means that the jack terminal would be 0V and the effect (reverb in this instance) would be off I thought.  In my instance, the reverb kicks on but no LED lights.

Edit:  now that I pull up that album and look at the pics, it looks like there may be a solder error/bridge between the top two rows bypassing the LED (that is Ground [top row/yellow] and from the input [middle row/orange]).  I'll check that tomorrow.

[PRR]

A backward LED won't light up, which matches your symptom.

[mattwells]

I had the 'offboard' wiring backwards.  Seriously need to write down which color I intend to have going where instead of thinking "I'll just remember that."

I'm sure it went something like this:

"Orange is ground, Yellow is live."
"Orange is ground..."
"Dang, burnt my finger on the soldering iron."
"Yellow is ground, Orange is live."

[PRR]

> thinking "I'll just remember that."

I've been wrasslin with the same problem. Red wire or black wire? Since this is for my heating furnace fan, it's somewhat important to get it right.

Now I have to sort-out my scribbled notes and document it for the next fool who works on it. (Me, when I've had even more time to forget.)