Reverse engineer vintage inductor EQ (telefunken style) with me

[ion]

Hi, while my diy tape recorder is resting for a while i wanted to present you a funny interesting little vintage filter board, inductor based from former eastern germany.
got it for cheap. Cant say now to inductor filter.

i ve to completely reverse engineer it and have no shematics.
it looks most close to a Fixed Filter Bank like the moog 907 or a clone like this: http://www.analog-monster.de/mmt914.html

of course its obviously not wired for that applicatiom but given its 6 pos switch seems to be a preset filter.
very interesting is the the upper row is a classic fixed inductor setup while the lower row has 2 pins standing out wich are interconnected and trim pot.
i am pretty sure the trim pots would be replaceable as filter style pots and the 2 pins should work as signalflow points wiring one to the next inductor and so on.

so the aim is probably some kind of FFB. Does anyone know or see clear signal path or pin meaning Like for power/ground? Pretty sure its 24v standard

 

[ruffrecords]

Looks 100% passive to me, no power required.

Cheers

Ian

 

[Bo Deadly]

Since there are no visible active components, clearly no power is required. However, it is likely driven through probably multiple amplifiers possibly with this circuitry in the feedback paths of those amps. You need to trace it out completely and then figure out how to make another PCB with the right connector and op amps to drive the fitlers and handle I/O. To trace the PCB take a good picture above and below, mirror the top image, make it high contrast but light gray scale. Then print out both sides, hold it up to a window and use a fine point marker to draw traces. Then use some schematic capture software to make a nice neat schematic. But maybe first trace everything connected to one inductor and just scribble something out on paper. It could be that you have 12 circuits that are mostly the same in which case you might skip the full procedure outlined above.

 

[ruffrecords]

There are six pairs of inductors and a 6 position switch. it is likely a HPF of a LPF.

Cheers

Ian

 

[ion]

Yes i was assuming they are pairs, but would that hint to stereo?
you mean the whole thing being a hpf/lpf? Never knew they were build as inductor units but it would explain the switch as not a musical but technical filter.

 

[ion]

Since there are no visible active components, clearly no power is required. However, it is likely driven through probably multiple amplifiers possibly with this circuitry in the feedback paths of those amps. You need to trace it out completely and then figure out how to make another PCB with the right connector and op amps to drive the fitlers and handle I/O. To trace the PCB take a good picture above and below, mirror the top image, make it high contrast but light gray scale. Then print out both sides, hold it up to a window and use a fine point marker to draw traces. Then use some schematic capture software to make a nice neat schematic. But maybe first trace everything connected to one inductor and just scribble something out on paper. It could be that you have 12 circuits that are mostly the same in which case you might skip the full procedure outlined above.

Thats a bit too much for me, tracing and creating a new pcb. A passive hpf/lpf needing gain makes sense. I still think i will test to isolate the 6 bands and give them each a potentiometer to play and chain them serial.

 

[ruffrecords]

Yes i was assuming they are pairs, but would that hint to stereo?
you mean the whole thing being a hpf/lpf? Never knew they were build as inductor units but it would explain the switch as not a musical but technical filter.

The early Neve HPF uses inductors and there are some famous vintage EQ boxes that are the same so it is definitely not unheard of. Stereo? - could be - one inductor and a capacitor will make a HPF.

Cheers

Ian

 

[ion]

Managed to decrypt the paper markings on those inductors wich are saying 43096

 

[ruffrecords]

You really need to trace out one circuit so we can say for certain what the EQ should do.

Cheers

Ian

 

[ion]

So what both of you mean is following one signal from in through 1 filter to out, will do

 

[ruffrecords]

So what both of you mean is following one signal from in through 1 filter to out, will do

Yes, just tracing one should do because we expect the others to be identical in topology but with differing component values.

Cheers

IAn

 

[ion]

So after i got some time i think it will be pretty easy to use as octave filter as intended.
i started wiring up the pins for some tests with audio but was wondering:

-if i mod this to route the signal through all filter stages like a filterbank, could i simply jump from first input to 2nd to 3rd etc until the last one or would this add problems with the signal like phasing/gain issues?

-or would i have to take each signal directly after its filter stage and route to the next input pin?

-i suppose in this application the octave 6 pos switch doesnt do anything anymore.

-what would be a common resistance value of the 6 trim pots inside to add hands on pots?