[sdiy] Does this look like phase reversal?

Guy McCusker guy.mccusker at gmail.com
Thu Sep 7 10:23:52 CEST 2017


I'm looking for some help troubleshooting the simplest circuit I've
built in ages, a CGS28 sequential switch.

There seems to be trouble in the input stage, which is a TL074
configured as four comparators. I've been poking around the clock
input (pins 3, 2, 1 on the schematic.)

The TL074 is powered from +12V and 0V. Pin 2 (-ve input) is biased at
1.55V and measures fine. With no input CV, pin 3 (+ve input) is at
0.88V; I can't see where that is coming from and seems way too high to
be input bias current with the 100k resistor to ground. (It goes away
when I remove the IC from the circuit so it's not coming from an
obvious construction error elsewhere.)

With those inputs, the output on pin 1 sits at about +11V. But it
should be negative i.e. near 0V, so something is wrong.

When I apply a 0-5V square wave to the input, I can see it clearly at
pin 3, and the output goes between 11.4V and 11V, with very narrow
spikes to about 0V at the transition points.

Does this sound like phase reversal in action? Or something else? And
would that explain the apparently excessive input bias current as well
as the weird outputs?

I've tried a few different TL074s from different batches, with similar
results, though the actual output voltages differed a bit. i noticed
that the TL074 data sheet specifies common mode input down to the
negative supply + 4 V, so the inputs here are certainly out of range.

Ken Stone originally specified an LM324 here which should be ok with
input down to 0V. Should I try one of those?

Alternatively I might have messed something else up --- if you have
any ideas I'd love to hear them.

Annoyingly I won't be able to tinker with this for a couple of days
but the benefit of your expertise will be much appreciated.


More information about the Synth-diy mailing list