[sdiy] Unknown VCF PCB

[Terje Winther]

Hi all,
I recently got some modules/PCBs. Most of them was digisound modules,  
but also a Tau VCO, some old Thomas Henry stuff and some PAiA stuff.
All of them without any ICs, so it will be some work to get this  
playing.

However, one of the PCBs was an unknown filter, see this very bad  
mobile photo:
http://www.wintherstormer.no/VCF.jpg

It is marked "VC - State variable filter", and the next line reads  
"ARTS  85"
There are spaces for "coarse", "fine" and "Q", and the power  
connections are spread out over the PCB, so no typical power connector  
to identify this, but it is labelled av +/- 15 volt.
Also of interest is the fact that there are 3 trimmers (missing, so I  
don´t know their values), no transistors on the board, and some 100pF  
polystyrene cap used.
I have followed the traces, and the bottom 14-pin IC is clearly a quad  
op-amp of some kind, probably to buffer input/outputs and/or make the  
various filter types.

So my guess is that the top IC is the filter core. However, I have  
tried all the various suspects: CEM-chips, LM13600/13700, various op- 
amps, even CMOS-chips, but none of them fits into the required 18-pin  
required socket configuration. Also of note is that leg numbers 5, 7,  
10 and 14 on this IC are not soldered in, and consequently not  
connected to anything.
It also seems that none of the legs are in direct contact with +/- 15  
volt, so could this IC be some matched transistor device?

Any response appreciated.



Terje Winther
terje.winther at wintherstormer.no
http://wintherstormer.no/