[help needed!] KORG V-C-F FK-1 Traveler filter (aka Univox Korg Synthepedal)

[Pepe]

In early July I purchased a very rare KORG V-C-F filter unit (without the accompanying pedal for the Remote jack). Somehow it got lost in the mail and after a very crazy and nerve-stretching odyssey with the German post service the quite beat-up parcel finally arrived in early August.

At first the V-C-F didn’t work and I suspected the blown fuse as the main culprit. I was right! When I replaced it the unit was alive again. Only the pilot lamp is still faulty.



The sound of the filter, however, is not how it is supposed to be. I have an original KORG 800DV duophonic synthesizer that contains a pair of this same highpass/lowpass filter combination, so I know how the sound should really be. The lowpass filter is set way too high, so everything sounds very thin.

In the meantime I did a full recap with high quality electrolytic capacitors, but that didn’t change the overall sound. (The pictures below were done before the recapping.)

I suppose that the previous owner fiddled around with the trimpots for whatever reason after the fuse was blown. Now I would like to know what the five trimpots are exactly doing. Thanks a ton in advance for any help!

There is no service manual around, in the internet only this same lo-fi photograph of the schematics can be found, so I share it here:
KORG V-C-F schematics

We have a 10 kOhm trimpot that seems to adjust the main voltage for the circuit. This one is quite big and can be turned with bare fingers. It might look very inviting touching this one if you have absolutely no idea what you are doing, so I guess this could very well be the one that the previous owner has fiddled around with.

And then there are each two trimpots in the circuit path after the 10 kOhm slider potentiometers of the highpass and lowpass ("Traveler") filters. I can only guess that these 100 kOhm trimpots define the right frequency for each filter, but what do the two 1 kOhm trimpots do? Do these define the intensity of the "Expand" function?

[Pepe]

I marked the trimpots in the photo of the schematics (open in new window to see it in full size):

[Pepe]

I received a few parts for various projects today. I replaced the blown light bulb with a new one and that red light looks very sexy!

I checked the voltage that seems to be adjustable with the big 10 kOhm trimpot. In the schematics it says that it should be 15 V. The result is 15.16 V. So I think that's okay. So it looks like nobody has fiddled around with this one.

I don't have the ability to read in the schematics where the lowpass filter is, so while the box was already open I checked where the wires from the LPF slider potentiometer are leading to. I'm very sure that it is the part with the third and fourth trimpot (from the left) in the schematics photo and above this the part with the photo coupler on the right.

As it seems to me that only the lowpass filter is misbehaving in the overall performance (turned up way too high, sounding very thin), I am certain that I have to tweak the fourth trimpot (100 kOhm). Dear tech experts, does that sound right to you as well?

[fuzzbuzzfuzz]

How did you get on with this Pepe?

Unfortunately I'm no major tech head alas...

I notice another of these currently on the auction site. I expect it will sell for a pretty sum...but I'm watching it.

Do you have the FK-5 footpedal btw?

[Pepe]

Unfortunately the unit is still collecting dust on the chest of drawers in the bedroom.

For months I had the plan to fiddle around with the trimpots until it sounds like the two calibrated HP/LP Traveler filters of my KORG 800DV duophonic synthesizer. But I need enough free time for this. For that I need our 3-year-old daughter to be out of the house for a few hours. That doesn't happen too often, as it is my wife who is working full time.

I don't particularly need the dedicated pedal, as it is a very easy thing to modify an expression pedal to work with the 5-pin DIN jack of the filter unit.

[snufkin]

I regret selling my one of these, they are not as extreme as I think people expect them to be but a great sound none the less.

I hope you manage to track down the source of the fault, I know how it is with kids etc