DOD FX-17 Wah / Volume / Voltage Controller

[hoerni]

I don't post hear too often, but often use it for reference.  I spent some time in the lab with an FX-17 today and wrote up a post for this Harmony Central thread.   Hopefully
someone will find it useful

The original poster asked about the function of the trimpots.  Looking at the schematic and taking some measurements, I got the following:
(copied from my HCEF post)


Okay, spent a few minutes with it in the lab, and I've determined that it isn't easy to describe what these pots do.

If you look at the schematic


Technical Explanation (you can skip this)

There are a few sections of the schematic that can be grouped by functionality.

First we have the Controller section.  This is comprised of the 4007 chip, the variable capacitor (created by the foot pedal) and the MPS4124 transistor. 

The next section is what I'll call the controller drive section.  This is built around the LM358 op amp.

Then you have the 2 parallel audio effects, the volume circuit based on the 3080 op amp and the wah circuit based on the 13600 chips.

From my rough understanding of the circuit, it looks like the 4007 is running as a ring oscillator (at about 1MHz).  I think that the variable cap is changing the cutoff frequency of the filter at the input to the MPS transistor.  Then the transistor and associated components act as an integrator to create a DC voltage in the 3.8 to 3.3 volt region (heel down to toe down)

Here's what the trim pots do

As I mentioned in the last post, the trim pots are in the controller and controller drive section, so they will affect both the wah and volume functions. As well as the 0 - 5 volt output - which is actually a good way to measure what you are doing with the pots.

First we'll look at P1 (the right hand trimpot if looking at the front of the unit).   With the trimpot in the center, you get full sweep on the voltage output from 0 to 4.75 (measured with a relatively new Duracell ). You will see 0 volts with the heel down and 4.75 with the toe down.

As you increase P1 clockwise from the middle, the maximum voltage (with the toe down) will decrease from 4.75 to 0 (while leaving the minimum unaffected).
If you then bring the pot back the the center and rotate it clockwise from there, you will raise the minimum voltage (with the heel down) from 0 to 4.75. (while leaving the max relatively unaffected)

This has the effect of changing the min or max frequency the wah is swept too or changing the min or max volume for the volume control.

The other pot P2 will narrow the range by increasing the minimum.   With the pot all the way counterclockwise, the minimum is 0 volts (and the maximum is slightly lowered).   As you increase the pot (rotating clockwise), the minimum voltage (heel down) will increase to about 1 volt. (when P1 is in the center).   

The fun thing is this pot works in conjunciont with P1 so you can lower your max freq with P1 and then reduce the range with P2.


Some quick numbers
(using a Fluke 37 volt meter and an old HP 3562A Dynamic Signal Analyzer)

Voltage -> peak freq
0 volts -> 237Hz (3dB BW = 56Hz Q~=4)
0.5 volts -> 459 Hz
1 volt -> 750Hz
1.5 volts -> 1.05 kHz
2 volts -> 1.33kHz
3 volts -> 1.88 kHz
4 volts -> 2.5 kHz
4.58 volts -> 2.90kHz
(battery must be dying - this should be 4.75 volts)


To answer the original question

I think you want to check out P1 and see where it is set.  Turn it more clockwise to reduce the maximum voltage and hopefully reduce that thin upper end sound.

If that doesn't work, try the mods.

Damn! This pedal is a lot cooler than you're average wah (from a geek perspective :cool: )

- Tim

[Marco Pancaldi]

Hi Tim,

very intresting. I have an FX-17 and I love it very much for his "sinthy" tone. The filter is a 2nd order bandpass and can sound very close to a synthesizer VCF.

I have two problems with it:

1 - when the wah is "on" I have a slight but noticeable loss of volume due to the filtering. Someone can suggest a simple way to add some dB gain (variable or fixed)?
2 - the bypass is very low fidelity, with lot of low order distortion. I suspect this is due to the wah/volume "returns" always connected to signal path even in bypass mode.
Some suggestion to obtain a better bypass tone is welcome.

Thanks all

Marco Pancaldi

[hoerni]

Hi Tim,

very intresting. I have an FX-17 and I love it very much for his "sinthy" tone. The filter is a 2nd order bandpass and can sound very close to a synthesizer VCF.
[/B]

Yeah, I love that synthy tone too.   I'm going to have to understand that filter so I can try to make an auto-wah with it some day.  It would be great in a bass synth pedal.

1 - when the wah is "on" I have a slight but noticeable loss of volume due to the filtering. Someone can suggest a simple way to add some dB gain (variable or fixed)?
[/B]

I would guess you could get some extra gain out of the filter stage with the right changes.  The filter is already providing some amount of gain relative to bypass, but I guess it's not enough for  you.  I'm not to good with active analog filters, so I can't guess what to change right now.

2 - the bypass is very low fidelity, with lot of low order distortion. I suspect this is due to the wah/volume "returns" always connected to signal path even in bypass mode.
Some suggestion to obtain a better bypass tone is welcome.
[/B]

I would think you could cut the traces between the wah/volume swith and the output signal path.  Then insert another J113 FET connected to the A source.  That should isolate both sides of the wah when in bypass.

Thanks all

Marco Pancaldi
[/quote]

[Marco Pancaldi]

Hi Tim,

you can make a search on the net for the Penfold Auto-wah schematics. I think you can find it maybe on the Mark Hammer site.
This can be a good starting point for an "auto" mod.
A low pass/band pass switch can be another idea to test.

last: I read about a possible "pedal phaser" mod connecting the "filtering" caps in a different way. Have you some details about it?

thanks,

m.p.

[hoerni]

Hi Tim,

you can make a search on the net for the Penfold Auto-wah schematics. I think you can find it maybe on the Mark Hammer site.
This can be a good starting point for an "auto" mod.
A low pass/band pass switch can be another idea to test.

last: I read about a possible "pedal phaser" mod connecting the "filtering" caps in a different way. Have you some details about it?

thanks,

m.p.

Do you have a link to the Mark Hammer site?   When I googled for the Penfold wah stuff I only found references to his book (which I should probably get)

I like the low-pass switch.  I'll have to look at that.

I'm not super comfortable with phasers (I'm more of a digital / DSP guy).  From what I know though, I don't think you can get enough phase shift with only the 2 op amps in the FX-17.

[Dream suite Music]

Hi There, I am trying to get a schematic for a DOD FX -17 pedal I have just bought, it needs fixing

I found a link through this site but when I use the link it comes up with the following message...

Forbidden
You don't have permission to access /Schematics/FX17-Schem.pdf on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
:icon_confused:Apache/1.3.41 Server at www.diyguitarist.com Port 80

[Paul Marossy]

Hi There, I am trying to get a schematic for a DOD FX -17 pedal I have just bought, it needs fixing

I found a link through this site but when I use the link it comes up with the following message...

Forbidden
You don't have permission to access /Schematics/FX17-Schem.pdf on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
:icon_confused:Apache/1.3.41 Server at www.diyguitarist.com Port 80

Send me an email and I can send you the schematic that is on my website.