4PM Build nearly done but filter sounds like it has FM / Noise modulation on it

Hi, I thought I’ld start a new thread rather than adding this to my build log.

I’ve built a 4pm from the boards and eprom supplied by Mutable Instruments. The interface seems fine but I am having problems now I have connected the output expansion from board to board.

I’ve selected 4pm for the type of filter in the settings menu. If I go through the different filters I can hear a change and the HPF does sound a bit like HPF etc, but there is something wrong. The cutoff control does something but it is impossible to filter out the high frequencies with the LPF. I’ve attached a file, this is on the “Teebee” preset. This is playing several C notes each an octave apart going up. It sounds like FM modulation. Any clues?

Is the 1x6 connector connecting the two boards?

Hi, yes it is.

I’ve got an osciloscope if that helps. I’m guessing it is either the CV to the filter or the filter itself.

One more thing that I noticed. If I go to System > CV Mode if I change from “4cv in” to the next setting it makes the screen go crazy it says “programmer” at the top but the text underneath jumps about so fast i cant read it. If I keep turning knob 4 I get to System CV Mode > Pedals.

Not sure if this is relevent.

Just dont set it to progammer if you have none attached.

This is what a filter sweep sounds like on full resonance with LP4 filter mode.

Ok got it about the programmer, that’s for the optional programmer with all the extra knobs. Good so thats not a problem.
Does sound like the CV control has lots of digital signal on it perhaps it’s not being filtered correctly, I’m going to check the capacitor values.

Cap values appear to be ok.

I doubt this is a problem with the CV or the filter - more with the digital pole selection/switching.

Probe signal (with the scope or a cable connected to your mixer/audio card) at IC4 1 ; 7 ; IC6 7 ; IC6 1. You should see the 4 stages of the filter. The signals there must be clean.

Digital switching is done by IC8 / IC9 / IC10. Try resoldering all of them.

Check that IC8 pins 15, 1, 2, 3, 4, 5 have stable voltages, either 0V or 5V, which changes only when you modify the filter mode or resonance mode setting.

Ok, thanks good advice.
I just recorded this example by the way which is without the 6 pin connector. This is preset one I think, with resonance turned up. It sounds ok. So I think that agrees with what you say, the filter is ok, but pole switching is bad.
Thanks very much i will test as you suggested, could take me a day or two.

Does your PSU output a clean DC ? Have a look here

Hi, it didn’t take as long as i thought!

IC4 and IC6 at poles 1-4 sounds clean.

On the default setting, IC8 Pin 1 is at 3.5v 2,3,4 at 0.1v and pin 5 is at 1.3v That does not sound right.

Thanks Georges, I think the PSU is ok, I have another Shruthi and it works ok, and I have tried 2 PSU’s.

Check for a bad connection between the two boards on the 1x6 connector. Check that you have not swapped the 595 and/or 4051/4053 with another chip.

Sometimes It’s just a bent leg… I had a lot of headaches debbuging stuff only to find out, after removing a chip, that a leg would be bent inwards.

I have checked every leg of the chips for continuity to the circuit board, and have checked the 6 pin continuity from one board to the next. All ok.

I have swapped the 595 for a different brand, no change.

The other 2 chips are CD4051BE and CD4053BE.

I’m a little stuck now.

This is what the signal looks like on pin 1 of IC8. On my scope the part on the far right, where there are 2 long low pulses, appears to be jumping up and down.

Huh? Is your probe set to 1x or 10x? And are you using AC or DC coupling?

Cool. Blue!