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

[laurie]

I feels like something heat related. How does it respond to being heated up with a hair-dryer?

[Pepe]

I can check that in the next days. Do you suspect something in particular that is prone to failure? Transistors, JFETs, the C1226 with the corroded metal part, (zener) diodes? I wonder if I could narrow it down by touching the components and check if something feels too warm to be normal.

Here is a picture in high resolution of the current state (open in new tab for full size):

[Pepe]

Well, I tested it a few minutes ago. It is definitely a heat related problem! As soon as I held the hair dryer for a few seconds over the circuit board that has not been used for longer than 20 seconds, the effect sound vanished. And then it took more than half a minute until the sound came back. Without the hair dryer the sound vanishes for the first time after a couple of minutes, then it reappears after around 7 seconds or so.

C1226 feels warm, but not really hot. I wasn't able to detect other components that became warmer than usual. But the board is quite packed and I cannot put my finger on some of the smaller components.

[Pepe]

We might have to keep in mind that the unit arrived with a broken fuse. If we assume that the one component that is getting too warm was also originally causing the fuse to blow, then which one would be the likely culprit? The C1226 maybe?

[laurie]

The 2SC1226 could be the problem.

The best way to confirm is to buy a can of freezing spray. Heat up the board with the hair dryer until it stops working, then use the freezing spray on one component at a time until you find the one that miraculously makes it start working when the spray hits it.

Something like this - whatever the equivalent is over there:
https://mgchemicals.com/products/electr ... eze-spray/

[laurie]

If it IS the 2SC1226, then you could check the voltage of the +15V supply rail when it is in a failed state - it likely won't be +15V if it is the 2SC1226

[Pepe]

Maybe I can do it the other way round, by building a funnel out of cardboard to lead the heat to the 2SC1226 only.

There is a NOS 2SC1226A for sale at ebay/Germany. To me it looks as if there is no difference at all, looking at the datasheet. At least I cannot find anything that explains the added "A" in the name.

Or is there a modern substitute? Somehow I cannot find one, but maybe I simply don't know the correct webpages for it. Or the magic key words.

[laurie]

Let's find out what the actual problem first, eh?

Yes spot application of heat might work.

[Pepe]

Okay, the 2SC1226 is working properly and added heat doesn't affect it in any way.

I narrowed it down to the part of the circuit where the 2SK-30 (labelled K30A) is. The hair dryer became too hot to test it right now. While it is cooling off I also leave the KORG V-C-F without power. Then I can test if the JFET doesn't like heat. I found a small plastic funnel that fits right over the 2SK-30, so I have it pretty much isolated from the rest.

Let's see if my first assumption was right ...

[Pepe]

Nope, it's not the 2SK-30, either!

But my experiments led me to believe that it is this red diode:



I tested it a few times. A couple of seconds of heat, only applied to this diode makes the fault appear any moment. If it is not been treated with the hair dryer and the fault pops up, I can make it disappear more quickly when holding the head of a screwdriver against it to make it cool down a bit. To me that doesn't sound like a coincidence.