A „Vox-Wah“ project, some circuit analysis and measuring results.

[Helmholtz]

In principle, yes.

1) Are you able to measure frequency with your scope?

2) Is your DMM able to accurately measure a low (say 50mV) 2kHz voltage (depends on the meter's bandwidth)?

3) Do you have a working wah pedal?

[35L6]

1) Probably . I've never done it but my scope was calibrated in '96 . How accurate ? My signal generator is not continuously variable .
2) Amprobe AM-220 should be good .
3) It will work if I put it back together .

[Helmholtz]

1) My signal generator is not continuously variable ..

Does that mean regarding frequency or amplitude?
If frequency is not variable, this excludes methods based on measuring a resonant peak and leaves the impedance method.
For this you preferably want a 2kHz sine and continuously variable generator amplitude.

The problem with airgap-less ferrite core inductors is, that their inductance is a moving target as it depends on signal level and frequency.
So the measuring conditions have to be chosen close to those in the wah.

The impedance method is based on the relation Z= Vrms/Irms, where Z (= 2*pi*f*L) is the impedance of the inductor at the measuring frequency (e.g. 2kHz),
Vrms is the signal voltage across the inductor and Irms is the current through the inductor.
When Z is found, L is calculated from L= Z/(2*pi*f).

The voltage across the inductor can be measured with the scope as Vpp and dividing by 2.83 yields the required rms value.
Inductor current is found from the voltage drop (Vr) across a 100k resistor wired in series with the inductor, as Irms = Vr/100k

Preferred measuring conditions are a generator output of around 1Vrms@2kHz, connected to the inductor in series with a 100k resistor.
The generator output is adjusted to get 40mVrms = 113mVpp across the inductor.

2) Amprobe AM-220 should be good

Check the meter's manual for sufficient bandwidth and ACV input impedance.

[35L6]

G
enerator is a handheld Tenma with rotary switch for frequency and variable amplitude . 2k is one of the settings . Its not real powerful but I'm sure it will put out a volt . I couldn't find the manual for the DMM but I know it's here somewhere .
I'll see if I can get measurements and then post what problems I have . This should be interesting !

[35L6]

Alright , this was a bit of an adventure . To start with the inductor was glued to the board , so I lifted the legs of components connected to the traces . With the inductor isolated , I tacked on 2 wires , one with 100k resistor . Hooked up my scope across the inductor and applied a 1V 2K signal through the 100K .With the scope set for 2 mV / Division and 10X probes the trace crossed 6 divisions or 120 mV . The trace was bouncing up and down so I used my meter to check the voltage . The control on the generator was a little touchy and 40.5 mV was as good as I could get . At this current I measured 606 mV across the 100k ( which measured 98K9 ) .Using 100K I get Z = 6750 for 537 mH . Using 98K9 I get Z = 6607 for 526 mH . Not so big a difference . I now see why people like digital scopes that do the measurements for you . Never the less , I think it was good to go through the process and see what accuracy you can and can't get . Since it's a moving target I think this as good as could be expected . My meter has a10M input impedance .

[Helmholtz]

Alright , this was a bit of an adventure . To start with the inductor was glued to the board , so I lifted the legs of components connected to the traces . With the inductor isolated , I tacked on 2 wires , one with 100k resistor . Hooked up my scope across the inductor and applied a 1V 2K signal through the 100K .With the scope set for 2 mV / Division and 10X probes the trace crossed 6 divisions or 120 mV . The trace was bouncing up and down so I used my meter to check the voltage . The control on the generator was a little touchy and 40.5 mV was as good as I could get . At this current I measured 606 mV across the 100k ( which measured 98K9 ) .Using 100K I get Z = 6750 for 537 mH . Using 98K9 I get Z = 6607 for 526 mH . Not so big a difference . I now see why people like digital scopes that do the measurements for you . Never the less , I think it was good to go through the process and see what accuracy you can and can't get . Since it's a moving target I think this as good as could be expected . My meter has a10M input impedance .

Great!
That's as good as it gets.

The signal level across the inductor doesn't have to be exactly 40mVrms. I guess that with strong PUs it might get as high as 200mVrms in a wah.
Now as you know the method you may try different levels and frequencies to see how much L varies. Different inductors show different amounts of variation.

[35L6]

Oh, I forgot to mention , this is a really great thread . This is the kind of stuff that keeps me coming here . Hope you are having as much fun as I am .

[Helmholtz]

Here's the frequency response of an RMC 4 picture wah (actually an original RMC 4 replacement board built into a Vox V847 shell):




The upper peak frequency is rather low (at 1.83kHz) due to the high inductance of around 630mH (the inductor looks and measures identical to an aftermarket "Eleca" Halo inductor) and a 12nF res. cap.
The meager LF peak is due to the particularly high inductor DCR of 88R.
In addition a 100k "Q" resistor increases the height of the upper peak in relation to the LF peak.

The RMC 4 wah has an output buffer amp (LF351N). A gain trimpot allows to more than double the height of the treble peak compared to old Vox wahs, but the LF peak stays relatively small, like in the picture.