R38

[800xl_1984]

Any tips on replacing the R38 NTSC color adjustment potentiometer in an 800XL? I have two spare motherboards, so I'm good on parts. Just wondering if there is anything I should look out for in the process.

 

Should have mentioned: Video color cast is changing wildly. Replaced GTIA with no change. Barely touching R38 results in crazy color shifts, and it's impossible to find a place where you can set R38 and the color stays fast.

Edited December 8, 2019 by 800xl_1984

[JR>]

What kind of power supply are you using? Have you checked its voltage stability?  Fluctuations can cause color shift.

[Sikor]

It is standard potenciometr resistor, so, what ii is your problem? Use any new with proper resistane.

And yes, bad potenciometr made problems as you wrote.

Edited December 8, 2019 by Sikor
more info

[1050]

Changing it is likely to result in no joy oddly enough. The problem is that it's dirty and needs a proper cleaning with deoxit or other contact cleaner. Part of that process is to wet the wiper and carbon arc, then move the wiper via screwdriver slot full range back and forth a dozen times. Let it dry, move it 12 more times dry and clean it again wet. It should become very stable in setting the color at that point and the new one you replace this one with will need the same treatment anyway.

 

It's the nature of some potentiometers to behave like this and it's not really bad in the first place. You can try it with some WD-40, but eventually most will want a genuine name brand contact cleaner for that kind of work.

[800xl_1984]

1 minute ago, 1050 said:

Changing it is likely to result in no joy oddly enough. The problem is that it's dirty and needs a proper cleaning with deoxit or other contact cleaner.

 

Thanks for the tip. I will give it a try.

 

J.

 

[800xl_1984]

Cleaned the potentiometer with Deoxit as suggested and the color seems to have stabilized. Thanks for the help.

[KLund1]

I have a similar problem with this 800xl color potentiometer. The unit I'm trying to bring back to life has lost the top plastic part. It does not make good contact between the halves. It needs replacement. Easy fix if I have the right part.

Could one of you parts guru's point me to the correct part on digi-key, please. I think it is a 500K, at least that is printed on another 800xl I have. (an aside, I looked at the Best Catalog, and did not see any potentiometers listed. Guess he does not have everything in that book.)

[manterola]

I had a similar problem once and it was not the potentiometer, it was a bad connection in the phi1 signal of the 6502, that creates the voltage level need for the R38 color adj.

Maybe you can check that, too.

I can also send you one potentiometer if you want, for the price of shipping... I am not 100% sure it is compatible with 800XL, though.  .... It is 500K but the footprint, not sure...   You decide.

 

[KLund1]

Thanks thanks. The 800XL was a 3 point connection not 4.  Very much appropriate the kind offer. This community is always helpful.

Any know the right part to buy.

Again thanks

 

[KLund1]

Still looking for a little help finding this correct part.

I am unsure how to select one of these on Digi-Key?

Anyone?