The 4023 filter module

Bad filter behaviour

The 4023 filter module is a 2 stages 12 dB/oct, potted in a module (filter schematics). The cutoff frequency is driven by a LM301 as a unity gain inverter. The problem is the filter opens when cutoff slider reaches ~70%, with a noisy sound.
I replaced the LM301 (which was wrong), but the problem is still present.
As filter's pin 9 (output of stage 1) already shows the same behaviour, I suspect someting is wrong on the exponential converters.
Before opening the module, I measured the voltage on filter's pin 3 & 4 with different keys:

CutoffCVVoltage on pin 4 (mV)Voltage on pin 3 (mV)
C1C2C3C4 C1C2C3C4
MinMin 10 10 10 10 74 74 74 74
MinMax 76 62 48 35 -1560 -2460 -3140 -3920
~50%Max -10 -23 -37 -50 -1560 -2350 -3120 -3900
MaxMax-120-136-150-163 -1550 -2320 -3110 -3890
MaxMin -90-90-90-90 8.98.88.78.6
~70%Min -8-8-8-8 8888
As we see, filter opens when cutoff goes below 0.

Opening the module

Two kinds of potted modules exist: those fully potted with hard rosin and those potted with rubber. Unfortunatelly, mine is the first kind (referenced 72-44, hence made in October 1972); unpotting the module is a tedious process and requires dangerous solvents. The best option is rebuilding one from scratch...

Building a new module

The 4023 uses 2 OTA (CA3080) in T8-C "metal can" package. These are becoming quite rare and expensive now; hopefully, CA3080AE in DIP-8 packaging are still available. However, this means designing a new PCB.

Because several Odyssey has already encountered the issue, some PCB are flying around. Here is one . This clone has been designed for a modular implementation, but could it could easily be used for a replacement (example here , in French).

I tried, quite unsuccessfully, to etch a PCB using the laser printing + iron technique. That was a big mess. So I decided to give this work to ITeadStudio in China (http://imall.iteadstudio.com/open-pcb/pcb-prototyping/im120418001.html, but I had to draw a new PCB from scratch using Kicad. Here are the files:

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