MFOS Multi-Mode Filter (Dead)

Having received all of my parts, I’ve attempted to prototype something a little bit more complex, with a total of 19 parts. One of the big problems I have had is that the parts I ordered haven’t been of the best values. I seem to be missing some very common part values, like 1uF capacitors and 20k resistors, even though these are very common, and are used in many of the circuit diagrams I have access to.

So, with that out of the way, and knowing that I can easily put resistors and capacitors in series or parallel to get other values, I started to work on this filter.

It turns out that most of the important capacitor values in this circuit (the drain capacitor for the biased output of the LM13700, the capacitor at the input stage of the filter) had to be made using other capacitors, as I was not shipped 560pF capacitors, nor 1uF capacitors. I was also not shipped 20k or 200k resistors.

This made prototyping the circuit unnecessarily complicated and large. It also made diagnosing errors a more complicated task.

I started from the pins of the LM13700 and worked my way out from there, from pin 1 to pin 16. Just as an idea, there are 4 pairs of resistors meant to make up for the lack of 20k resistors, and 3 pairs of capacitors making up for the lack of 560pF and 1uF capacitors.

Once the circuit was built and I went over the diagram one more time, I powered the circuit.

The oscilloscope showed a DC bias, which was not expected, as I have set up the batteries to allow +9V, GND, -9V. It would also show some kind of oscillation, even though no input was connected.

The LM began to smell, and I touched it and it was hot. So I fried the OP-Amp somehow, and it is impossible for me to tell where I went wrong because, as you can see, there are no voltage reference points on the diagram so I can measure with the multimeter, and the breadboard looks like a nest of wires.

I would like to revisit this circuit once my second power supply comes in the mail, but until then, I have to disassemble this and throw away the LM.


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