4PM Help: Stuck in low pass

I have a 4PM board I am working on and it does not want to switch modes. Flavour works but its stuck in a low pass mode regardless of what i set it too. I noticed that the BOM calls out 74 series logic chips and i have the old 4000 series ones in there for the switch and mux, could that be it?

I really doubt it - especially because the “flavour” function appears to work.

Do you see the voltages at the pins 9, 10, 11 of IC9 change (from 0 to +5V or back) when you change the filter mode?

how fast should it change? Should I be able to see it with a meter or would I need a scope? On the meter they do nothing… Also worth noting, blinky lights dont do anything either

They should only change when you change the Filter Setting. The Blinkenlights indicate high Resonance Volumes when playing…

i know, problem is they are not changing. i built two of these last week without issue

So pins 9, 10, 11 of IC9 are stuck at 0V no matter what is the filter setting?


You do have both of the connectors hooked up?

Try removing the 4053 and 4051 (we’ll assume they are damaged).

What are the voltages at pins 15, 1, 2 of the 74hc595 when you change filter mode?

What are the voltages at pins 3, 4, 5 of the 74hc595 when you change the resonance flavor?

hmm. not getting anything out of the 595 on any of those pins

Strange, because you mentioned that the resonance flavor had an effect… If it had an effect it must have changed the routing on the 4053…

It might be a communication problem between the two boards… You’ll need to trace with a scope or logic analyzer the digital signals coming out of the EN / Q7’ / CLK pins. EN is the chip select / enable (goes low during a transmision) ; CLK is the clock / you should see 8 steps ; Q7’ is the data - flat when the filter mode is set to 0 and you should see this “counting” when you scroll through the settings.

on the en there is constant traffic (square pulses), clk I get FAST but erratic pulses, nothing on q7 though.

found it. One of the 595 pins had a microscopic bridge to a scratch in the solder mask shorting it. Had to bust out the microscope to find that one…

Thanks for the lesson though, always nice to learn something :slight_smile:

This shouldn’t happen - you should even see the bits from the LEDs getting shifted away. Can you swap boards and check it if this comes from the filter board or the control board?

If this comes from the filter board, it looks like there’s something wrong with the 74hc595. Maybe a short between pin 14 and an adjacent grounded pin?

Was it pin14?

ooh youre good :slight_smile:

Is there actually anything the man doesn’t know ?

if he doesn’t know, who does?


Who’s she?