Need to Replace Convergence Pot
 

So a pot on my convergence board gave up its shaft. Not all the way into the pot, but close enough. Since the shaft is completely discolored at the break, I'm sure it's due to heat and could be the reason I'm having so much trouble getting the blue horizonal on the left side of the screen.

So here's where I need assistance. I have the Zenith OEM replacement kit. It comes with the pot, WW resistor and jumper lead. If I'm understanding the kit correctly, this pot is a replacement for other convergence board version and the entire kit needs to be installed. In my case the pot on mine is the same as in the kit, I have the jumper installed and there is a resistor located as in the instructions. However, what I have is a film resistor and I cannot figure out its value as the colors seem well faded and there are only 4 visible lines on it. I'm assuming the silver is the 5th line, so line one might be gone. I am attaching pics of the resistor and the pot as well as the instructions.

I'd also like to know if it's okay to cut the legs off the old and new pot and solder the new onto the old legs? Otherwise, I'm going to have a difficult time trying to remove and install 3 legs in the solder buckets.

Your original control has the same part number as the replacement. I would just replace it since it appears the modifications have already been done. It's okay to cut and solder the terminals.

I figured the kit instructions were meant for those without this exact pot, but didn't know if the WW resistor was better than the film in this circuit, or if the values were different and the WW was the updated value.

Whoever thought it was a good idea to use solder buckets on anything other than a single lead didn't consider the pain at removal.

You need a 75W+ IRON (not a gun) with a big tip that you can touch to the above chassis 'pins' of the solder pots and touch to 2 solder pots at once. Feed some solder to the tip to get good thermal conduction to both pots. I do this with a gigantic 'stain glass' weller iron and it does fairly well.

Well it went better than expected. I looked into an empty socket and saw it was tapered and looked into one with a leg and I saw no visible solder like the buckets on the chassis. So I decided to give it a shot.

I am certainly not the first person to mess with this pot. I think it had a different pot to begin with and someone replaced it. Why they soldered the retaining ears is beyond me as I didn't have to on the replacement.

Here's how I did it. I bent the retaining ears so it wouldn't interfere with removal. I then grabbed one leg with the smallest needle nose I could find so as not to act as a heatsink. I pulled on the leg as I held my iron with the biggest tip I have to the other side of the socket.and within 10 second, the leg came out. I did the same to the other leg and the pot was out. I realized after moving the wire that there was no center leg. I then placed the new pot into position, lined up the retaining ears the the opening in the board, heated the tip of one well and the leg slipped in. I did the same for the other leg. I measured everything and the pot goes from just over 1ohm to over 85ohms.

Whoever replaced it last time made it much harder this time. I wanted to test the removed pot, but it was disintegrating in my hand. So much plastic had dried out and crumbled I just tossed it.

The ears were soldered because the case of the control is the wiper connection.

Oh, so I should solder the ears of the new one?

I decided I'm going to install all the new pots I have for the board. I found another pot that causes the screen to shimmy as I'm adjusting it, and it also continues to shimmy intermittently afterwards until I wiggle the shaft. Figure it can't hurt to use the NOS pots I have.

I've also decided to remove the chassis, put it on the bench and replace all the electrolytics I put in 13 years ago. While I'm in there I'm also going to check all resistors. I know I won't see the true value being in circuit, but I can see if it's way out of spec or worse. I'm also going to remove the tuner again as it gets stuck at times switching channels and I have to go up and down the channels again to get it where I want. Figure something just needs cleaning/lubing.

It would be a good idea. An intermittent connection may not show up for years, but why take a chance.

I'd only replace the pots that are actually causing troubles, and honestly first I would try to clean those pots before considering replacement.... Don't want to open a can of worms.

They have been cleaned twice, but I get what you're saying. As it is now, I can't get the same results twice when adjusting. The first time I tried, I had a few bad converged areas and lived with it. Years later I did the same procedure and it was almost perfect. I then tried to touch up those areas, but couldn't and then couldn't get back to where it was before and now have even more areas that are off. I only made small changes based on the area and the pot that controlled it and still they didn't allow adjustment back to where it was.

The only thing I can think of is pots that are not up to the task anymore.

The oddest thing I ran into was during the static convergence. I moved the blue away so as to align the red and green. I got it dead on, but when I moved the blue in, the red and green would separate as soon as the blue got near the center of them. I could see the red go up and the green go down. I had to compromise on the blue a bit to where the red and green came back to their original positions. The dot is pretty much white in the center with a bit of color around the edges as I'm sure it is supposed to be.

The convergence controls always have some interaction with each other. When going for red green convergence instead of moving blue off to la-la land, it's better to keep it close to the other two and bias the blue gun of the CRT to emissions cutoff so the blue phosphor doesn't light.

I think I will connect the gun killers of my pattern generator on the blue next time. That way I don't have to redo grayscale. The SM has the grayscale being done before convergence, but I'm not sure if that's truly necessary.