Centipede Screen Stretch (Left Side) (K4625)

[jasonsmith]

Thought I would start a new thread and leave my blowing resistor mishap behind me LOL.

Here is the actual issue.


https://dl.dropboxusercontent.com/u/85675826/2014-04-05%2020.23.43s.jpg

- I put in my spare K7901 monitor and the problem went away so it is my 4600.

- A full cap kit has been done.
- I've reflowed all the headers.
- I've adjusted all the H and V pots with no change.
- Have not adjusted the H or V raster positions.

Issues that I know of:

- HOT isn't doing much if anything. I've backed the screw so far it fell out with very little change. Maybe it's transistor is bad, have not checked it, don't know if it would create this issue.
- My B+ maxes at 124.8 or so volts (should be 127v), and turning the pot doesn't do much if anything either.

 

[jasonsmith]

Now something very interesting that I just noticed. If I widen the screen image with the vertical size pot and force the bad area off screen the resulting image in that area is good? Meaning that the image signal in the left two inches is corrupted for some reason.

Could this be an interface board thing??

[jasonsmith]

Okay so as John mentioned in the other thread this could be an adjustment issue. I tried to adjust it out but it didn't work. I found this website and apparently Centipedes all over suffer from this very issue. http://arcadevidiocy.com/wordpress/2013/05/16/centipede-monitor-sync/

I tried adjusting the horizintal osc which made it a tiny bit better but not fixed. Now both my Hor Hold and that OSC pot are maxed, I need more adjustment. 

[jasonsmith]

I want to increase the resistance value of this potentiometer. Or actually change it's range.



Right now it is zero to 50k ohm, I want it to mod it to be around 25 to 75ohms.

I want to add around 25k to the start of it's variability.

There's 3 legs on the thing, where should I put the 25k resistor? I'm thinking onto the leg after R359??

[jasonsmith]

Took it out and played with it a bit and I was correct. If I add a 25k resistor to the non ground leg it starts the pot at 25k and goes to 75k vs 0 to 50k.



I heard of a mod similar to this for a Centipede. I'm thinking the HOR Hold circuit just wasn't built to take the age of these components. As you know I'm suffering from some stretch or curl on the left side of the screen. Adjusting the hold starts to take it away but the hold maxes out. If I dial it back from max it's pretty quick to lose it's hold so that telling me the first 50% of the range is doing nothing. Going to get a resistor tomorrow and give it a go.

[jasonsmith]

Now what if I put a 100k in the spot of the 68k at R359...? Wouldn't that do about the same thing?

[iankellogg]

yes you could just replace the top resistor to a 100k. just make sure you replace it to the same watt value.

[jasonsmith]

Thanks for confirming that Ian, I'm going to try it today and see what happens.

[jasonsmith]

Well an interesting outcome. Upping the resistor value does make it possible to adjust out the bad curl. My guess is I'd need to add an extra 50k to get it out completely. BUT...

It causes the game to go bonkers and half reset or stumble with beeps power on/off cycles after about 2 min. So needless to say that mod is out and well enough I guess. I should find the real issue instead of trying to compound matters.

Here is the million centipede question. What is causing my Horizontal Hold circuit (Hold + OSC adjustment) to be so week? about 50k ohm week??

[iankellogg]

I'll have to look St the schematic. But as far as I know this mod should not affect your game play. There might be something else at play

[jasonsmith]

Thanks Ian.

I was thinking that as well but when I stuck the original 68k resistor back in the reset issue was gone. I metered the monitor voltage and it's the same with any of the mods and also while it's resetting it's self.

[jasonsmith]

Now here is something that should be noted. The manual calls for a 1/8w resistor in this spot. I wanted to use what it called for just in case, rather have it blow then something else blow. Take a look at R359 in the pic though, there's a huge difference in size. Is this just due to new manufacturing methods or is the board filled with 1/4w resistors. The 68k that I took out was the same size as the one below R359.

[iankellogg]

dont worry about putting in a larger value wattage as long as it fits.

[jasonsmith]

You think the 1/8w might be heating up and causing the reset issue?

[iankellogg]

no, you would have toasted the resistor if it was the wrong value. they ether burn up or not. really no inbetween.