Frequency reading on DMM giving unexpected results.

[Locrian99]

Good evening. 

I recently built a Madbeans fraudhacker.   In the biasing directions it states to adjust the clock trimmer so at tp1 so that it reads 50kHz or just set it around the middle.    I've been wanting to upgrade my dmm so I used this as an excuse to get one.   Measured it and it's bouncing all over from 150khz to 800khz adjust clock trimmer don't really see any difference.   Checked pin 8 of one of the 4558's and got the 60hz I believe I should expect there.   

These are some mn3102/mn3207 I got off of eBay I've used them in a few circuits so far and I cannot hear anything to makes me think they are bad.   But this has me wondering a bit.   The circuit appears to be working fine to my ears.   I had already biased it and was fairly happy with just wanted to try fine tune I guess a bit.   Here's a link to the build docs with schematic
https://www.madbeanpedals.com/projects/_folders/FilterMod/pdf/Fraudhacker.pdf

Could this be a sign even though I'm not hearing anything there's something funky going on with the 3102, operator error (I'm not sure how but who know maybe), or anyone have an idea why I'd be getting these fluctuations.   I had read it could be from distortion in the signal, this isn't measuring audio... but is receiving power through a breadboard. 

Thanks.

[Rob Strand]

I've been wanting to upgrade my dmm so I used this as an excuse to get one.   Measured it and it's bouncing all over from 150khz to 800khz adjust clock trimmer don't really see any difference.   Checked pin 8 of one of the 4558's and got the 60hz I believe I should expect there.

You need to set the Depth to zero and the Manual to minimum ie.  both fully CCW.  It's mentioned on the schematic.   It's the normal thing to do as the modulated clock will mess up any frequency measurements.

For debugging you can also measure the DC voltage on pins 2 and pin 4 you would expect about VD/2 = 2.8V on each pin.

If you still have problems try adding a 1k resistor in series with the +ve meter probe.  That helps stop the meter input capacitance loading down the circuit.

[Locrian99]

Thank you, been a bit since I messed with this and just jumped straight to the frequency part spaced where I had my pots.   Worked like a charm.