ATARI 800 COLOR BURST CRYSTAL

[Sean39]

Hey

 

My Atari 400 and 800 have different positions on the tint of the TV. My Atari 400 when it did work would be right

on the colors with the tint center right in the middle. My Atari 800 is not like that at all I have to move the tint more

towards the red side or all my color are off. I am figureing that the Color burst crystals were not always on frequency

back in the early 1980's. Now will this only affect my channel 2 and 3 output on the computer, or will this affect

the colors on the monitor output too. DO I need to change my color burst crystal out to one that is more on frequency

so the colors are more on. If this does not affect the monitor output I rather leave it alone since I will be changing

to a monitor instead of a TV set. Does anyone know if the color burst crystal affect the monitor output?????????????

 

email: seanradios@yahoo.com

 

Sean39

[Rybags]

AFAIK, NTSC Ataris base all their timing on the single 14. <whatever> MHz crystal.

 

Colour generation is based upon an internal delay generated within GTIA - it can be adjusted by a colour trimmer pot.

 

Not sure about the 400/800 (someone else can confirm), but on other machines it's accessible by a small hole in the bottom of the case.

 

If you do a bit of a search of the forums, there are programs to display all colours at once which can be helpful when you're doing such adjustments (subject was something along the lines of "Palette")

Edited February 5, 2008 by Rybags

[1050]

The Color burst crystals (3,579,545 Hz) of the 80s were selected to the same exact tolerances (+or- 20 Hz or .0006%) as those of today and much earlier - crystals have been around a long time and that part of the technology hit a dead end pretty much by the 40s. So pretty sure that's not it.

 

There is a color adjustment slug on the bottom near the label of your 800 that should allow you set it for the best overall picture. The method of adjustment is to use a small hex shaped, plastic or fiberglass tool for such purposes possibly sold by Radio Shack and the like. Lower the tool into the hole and engage the ferrous slug and turn it. As it turns, the external threads on the slug force it in or out of the coil and change it's inductance which alters the phase relationships (and thus color) of the color signal sent out of the computer.

 

You could use a metal allen wrench key to move the slug but remember that the metal key will affect the picture shown drastically so you have to remove it with each quarter turn or so remembering to only go in the direction of the best picture. It's a bit more problematic but sometimes the best plastic hex driver isn't strong enough to turn those slugs and if you happen to already have an allen wrench set, you don't have to buy anything.

 

Your monitor has it's own crystal so adjusting your 800 will not affect your monitor. Adjusting your 800 would affect the color shown on your monitor only when it's hooked up to the 800, however.

 

You certainly can try a replacement crystal - it's not rocket science after all. Do wait for the monitor though, as the problem may be with your TV modulator on the computer. And that simply gets bypassed when using a monitor.

 

 

Rybags - you stand partially corrected. USA 6502 clock is 1,789,772.5 Hz which is exactly half of the NTSC color burst freq. and not by any accident. Coincidently, 14.31818 MHz is 3,579,545 Hz times 4. Just handy dandy fun facts to throw around. Only XE and XEGS (Freddie) were driven by the 14.31818 MHz crystal "cans", all the rest were driven by 3,579,545 MHz crystals proper. But, the 400 schematic clearly shows 3,579,575 MHz - I can't imagine why? PAL units of course use different frequencies of which I am not up to speed on.

[+warerat]

There is a color adjustment slug on the bottom near the label of your 800 that should allow you set it for the best overall picture. The method of adjustment is to use a small hex shaped, plastic or fiberglass tool for such purposes possibly sold by Radio Shack and the like. Lower the tool into the hole and engage the ferrous slug and turn it. As it turns, the external threads on the slug force it in or out of the coil and change it's inductance which alters the phase relationships (and thus color) of the color signal sent out of the computer.

 

The GTIA color delay pot on the 800 is located on the CPU board, not the bottom of the machine like the XL/XE series. You must disassemble the top of the machine to access it from a small hole that's in the metal RF shield on the rear. The pot is resistive, not inductive, and the voltage controls the internal analog delay line that provides the color shift.