KORG V-C-F FK-1 Traveler filter (aka Univox Korg Synthepedal)

[Pepe]

It is one of those two diodes, in any case leading to the photo coupler of the LPF:



That could explain why the fault did appear even more quickly whenever I adjusted the fader for the LPF. I forgot to mention that. So all in all the vanished sound might simply be a result of the LPF being completely turned down, so that no signal can be heard anymore. Does that sound logical to you?

Could it just be that I have to swap this diode for an equivalent of the 1S1555? I have plenty of 1N4148 in my parts box. And would it make sense replacing them all while I'm at it?

[laurie]

Nice. I don't think there is any need to replace all the diodes - they rarely fail. Replace the likely one(s) with 1N4148 and that should do the trick.

However... because those diodes rarely fail, could it be the solder joint? Did you re-flow those component legs already? Or maybe a hairline crack in the trace?

[Pepe]

I resoldered all the connections and the tracks. Twice. That's why I was suprised that there was still a fault.

[Pepe]

Groan! I replaced the diode and in the first minutes it sounded like it was a full success. I was almost doing a little jig, but then the stupid fault appeared again! With Balance fully turned up the sound becomes completely dull. It doesn't vanish as I wrote earlier, so it is indeed the LPF shutting completely down.

Turning "Threshold" or "Expand" has no effect on the sound when it becomes dull. This part of the circuit seems to work properly.

Is it something before or after the photo coupler? Or can a photo coupler work intermittently over time?! The photo coupler is next to the diode that I replaced. Did the heat of the hair dryer, although I used a funnel, affect the photo coupler and NOT the diode as I could have sworn?

Is there a way to check or exclude if the photo coupler is faulty? Can we somehow check if the photo coupler is receiving and sending the correct signals/voltages while the fault comes up? I hope it's something else, because it is a VERY expensive replacement part ...

Or could it simply be the 100Ω resistor that is also next to the diode? Oh, it's so annoying at the moment!

[laurie]

My money would be on the photocoupler. The LDRs in them sometimes fail, and so do the light-sources.

[Pepe]

I bet that it is very hard finding a substitute for the HTV P873. Just replacing it with another 5-pin photo coupler like this one wouldn't make too much sense, I fear?

Could I swap the two photo couplers to see if the problem shifts to the high pass filter? Or is it not recommended (because of the heat of the soldering iron maybe)?

[laurie]

swap the two photo couplers to see if the problem shifts to the high pass filter? Or is it not recommended (because of the heat of the soldering iron maybe)?

That would be something to try.

[Pepe]

I am massively surprised. Apparently it is NOT necessary ordering a replacement part!

I exchanged the photo couplers for the LPF and the HPF and reflowed the two solder spots of the 100Ω resistor next to the replaced diode (I lifted the resistor a bit, because the legs are insulated for the most part, to ensure that it now has definitely full contact).

And now the V-C-F is magically working properly! For over 40 minutes in a row, whereas prior to that the intermittent fault had appeared after four to six minutes at the latest. No matter how much and how quickly I maltreat the LPF and HPF faders, the effect is working like a charm.

Maybe it's still too early to do a jig and I have to observe it for a few hours in a row tonight. But so far I'm about to say that the patient is healed!

[laurie]

Excellent. So ultimately likely a dodgy joint.

[laurie]

Your comment "lifted the resistor a bit, because the legs are insulated for the most part" is familiar. I've seen that before. Resistors from the ~1970s with painted legs looking like they have a good solder joint, but the paint is causing issues. I've had to scrape the paint off the resistor legs a couple of times in the past.

I *think* you might have a smoking gun here...