K4600 focus pot replacement

modessitt

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Easy to do?

Did a TV tube swap tonight, installed new caps and screen pot. After a problem with no blue that was traced to the interface board, I have a pretty nice picture.

Except for one thing - I can't get it completely focused. It starts dialing in, but just as it seems to be about to focus perfectly, I reach the end of the pot's travel motion.

So, I need to replace it, unless there is a way to add a resistor in series to make the pot range different (assuming the focus is on the upper end of the value and not the lower).

Unfortunately, my spare 4600 flyback is at work.... :(
 
Check the solder connection at the tube socket for cold or crappy connection.

You know, now that I think about it, there is something weird about this neckboard (or I haven't worked on a 4600 in a while and don't remember).

There is something wrapped in black plastic/tape that goes from the focus wire input to the neckboard. I'm guessing there is a cap inside? If so, might it be the cause of my lack of complete focus ability?

SDC11430.jpg


Checked the entire neckboard for cold solder, but don't see a problem:

SDC11432.jpg
 
no clue what that is, but I just checked my 4600 and it has the exact same thing as yours.

I was talking about the red wire going into the neck socket for cold or bad solder joint. open up the neck socket and check it.
 
no clue what that is, but I just checked my 4600 and it has the exact same thing as yours.

I was talking about the red wire going into the neck socket for cold or bad solder joint. open up the neck socket and check it.

Look like it is labeled as C405 (seen in my 2nd pic). It is a 100pf 12kv cap between the focus line and ground. I guess if it is devalued, it could cause a problem with the focus....
 
Hey Mod can you post your results? I have a 4600 doing the same thing and I just assumed the flyback was the cause.
 
I checked the solder at the socket. Looked good but I redid it anyway. No change.

Replaced the focus pot. Still does the same thing. Perhaps it has to do with the position of the yoke on the neck? I had to back it off slightly to get a perfect picture color-wise.

It's pretty damn close, so I may just leave it alone. If I drop the brightness down slightly, it looks better. I need to look up the vertical and horizontal shift jumpers for the 4600 now...
 
Okay, my board doesn't have the position jumpers seen in this photo. Can I just add them? Where does the other end of the jumper wire attach for each?

4600controls.jpg
 
Been working on my 4600 all day and I'm having the same issue with the focus. I reflowed all the solder points on the neck socket and the red one Dokert said also no change any more suggestions??

Andrew
 
Not yet. I don't have one handy at the moment. The 12kv is the tough part...
 
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