Fixing this Filter (Octave The Cat)

Octave The Cat. Fc filter cutoff has stopped working. Trying to find where the signal is lost as I follow the circuit. Q slider shows activity and raises/lowers volume. Modulation knobs show activity on the scope but no audible change when moved. Same with modulations switches. Rest of the synth works.

Can someone tell me what the signal should look like on the scope at the inputs and outputs of ICs 1458(a and b) 307 and the 3080s? Also the transistors. +- Voltages seem to be at correct pins of the ICs.

Things ive done:
Rebuilt the slider- tests good at 0-100kohms

Sprayed other pots and sliders and switches

Diode Tested transistors/fets with the meter - seems ok.

Tested the 1458 and 307 in an ic tester. Popped ina new 1458 I had.

I see audio on the outputs of the 3080s. No signal on the inputs. No audio signal or very very faint at the inputs of 1458. I’m not well versed with OTAs so I’m not sure how to test them?

Schematics here:


I just dealt with an OB-SX with a “dead” voice card. It turned out that an interstage coupling cap was no longer passing audio. Check signals before and after.

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Thx for responding. If you have a minute here’s the filter schematic. Can you tell me the capacitor number(s) you suspect? I’m guessing it’s these 220pf polystyrene units you mean? c55-58? Also the c52 ceramic disk off of q14? Using the scope, Should I see +-15v or should I see a audio waveform on either side of these caps?

Here’s the picture of the schematic for those who don’t want to chase for it.

There are two paths I would try to follow.

  1. The control path.
    Do the following checks with LFO modulation and keyboard tracking both set to minimum.
  • Check that the voltage across C63 varies as you move the “Fc” potentiometer.
  • Check that the voltages across R211, R162, R167, and R173 are about the same and vary when you move the “Fc” potentiometer.
  1. The signal path.
    Do the following checks with an oscilloscope, with VCO1 and the noise source both muted to have only a simple waveform, e.g. saw or square, from VCO2 as input.
  • Check that you see the signal at the output (pin 1) of IC33A.
  • Check for the signal at the output of every successive stages (junction of the transistor, the 10k and 100k resistors) Q8,R212 and R213 for the first stage and the three other similar points.
    With the filter fully open and no resonance the signal shouldn’t be modified much after each stage.

Hopefully the signal will disappear somewhere along the way and you’ll be able to focus on the components of a specific stage. If the signal comes out clean of the fourth stage, the problem is further downstream, around the VCA.


Excellent detailed response and greatly appreciated!

Right off the bat c63 shows no variable voltage when sliding Fc and measuring DC voltage across c63.

Using the scope, audio waveform is visible at ic33A output pin 1 up to one side of the r207/100k resistor. Other side of r207 signal is gone. I see the signal again at each intersection of the four 3080/5457/resistors but Fc has no effect.

Ic33b output pin 7 shows no audio signal. There is that calibration trimmer involved here I haven’t touched it still lock tight sealed.

Sounds like the problem could be somewhere between the Fc pot and C63.

The schematic clearly shows that the connection between C63 and ground is broken. (Joke)

R207 and R208 form a voltage divider of 202/(10000+202) = 0.0002 so the signal is greatly attenuated on that side of the R207, that’s normal (and why I said to check the other side of the R207).

IC33b is not in the signal path, it is in the control path, so no audio signal there is normal.
The output of IC33b should be a voltage that changes with the Fc pot.
You can also turn up the LFO filter control and you should see the output IC33b follow the LFO.

You checked IC33 and the transistors already, maybe, with the power off, you can check continuity between the various components between the FC pot and C63. Try to check continuity by touching the component legs directly with your meter’s probes, that way you’ll detect any bad solder joint in addition to possible crack in the PCB traces.

After removing IC33 you can also measure the value of the resistors and potentiometers in that portion of the circuit, one could be open.

Like for example R193 on the schematic. (Again joke.)