Input impedance tester

[randomair]

I put one of these together and was trying to test some input impedances of effects pedals. This is a link to the schematic and article:

http://www.keyboardmag.com/article/m...r/mar-07/25986

http://www.keyboardmag.com/pop-up.aspx?content_id=67466

Well, I have tried this little circuit but I cant seem to get it to work properly. It says to put meter leads across point 1 and 2 to test VAC in, but with the effect plugged in the out jack it does nothing but short the reading? Does anyone know the steps required to make this little setup work?
I measured the VAC across the input but once trying to measure it on the out jack, it just goes to zero. Anyone built one of these?
I used a 1MEG pot and ran 2 VAC from a signal generator in but not sure of the proper steps to make it work. Any help would be great!
Thanks!

[Steve Conner]

When you measure the impedance of an "Out" jack, you're measuring an output impedance. Only "In" jacks have input impedances.

Output impedances are much lower than input impedances (should be about a factor of 1000 in most modern audio electronics) so the "shorting out" effect you saw is exactly what I'd expect.

It doesn't actually say in the article that the device can only be used to measure input jacks, but that is the case.

[randomair]

I am trying to measure input impedances of the effect pedal. So I may have mislead you by saying "out" . To clear this up, I am referring to the output jack of the tester. Here is a small illustration:
impedance tester
signal generator-->(in)----pot-----(out)-->effect input

The (in) and (out) are the jacks of tester.

The article explains to test across points 1 and 2 while there is a cable running from output of illustration above to input of effect. All of the test points I used were on the tester, not the pedal. Thanks!

[Steve Conner]

Is the pedal you're measuring powered up? If not, the input protection diodes on ICs will more or less short the input to ground.

Do you get zero reading even with the pot turned to minimum resistance? (is minimum resistance anticlockwise or clockwise? depends which way you wired it)

With the device plugged into the pedal, can you hear the signal generator through the pedal by plugging the pedal output into an amp? Does the volume vary by turning the test pot?

If not, can you hear anything by plugging the signal generator straight into the pedal? How about plugging the "out" jack of your device into the guitar amp?

Are the cords you're using faulty?

Have you transposed the signal and ground wires somewhere? If this were the case, and your signal generator was referred to ground, and your effect was too (maybe by a power supply that's grounded, or an amp it's plugged into) your signal would get shorted to ground.

If you made a wiring mistake somewhere, these tests should help you track it down.

Lastly, is your multimeter set on AC volts?

[randomair]

Thanks for your help Steve, got it working, bad pot. The pedal (Ibanez standard wau fuzz)I was interested the most in testing, ended up reading 340k input impedance. My initial concern with the pedal is the amount of RF it is picking up, so i figured it would read well over 500k. I guess the transistors and FET at input must be picking up the RF. I tried shielding the box, but to avail. I thought of trying to change the input cap to a higher capacitance thinking this may change the impedance, right now it is 10uf.

[Steve Conner]

Changing the DC block cap at the input won't have any effect on RF. You need to install a small capacitor of around 100pF or so, across the input terminals. (ie between tip and sleeve terminals of the jack socket) A Fender-style 68k "grid stopper" before the capacitor may make it more effective.

You may also need a cap on the output: a jack that's an output at audio frequencies can easily become an input for RF.

[randomair]

Cool thanks Steve, I will try the caps across the input and output terminals. I was looking through "The Tube Amp Book" at the Fender schematics and noticed that 68k grid stopper on most of their schematics. If the caps dont do the trick I will try the 68k and report back. As of now, the pedal is picking up some serious RF, hopefully this will reduce the incoming "alien transmission"!