Purple GTIA question

[seastalker]

On both 600xls and 800xls, in doing the chip swaps, often a GTIA swap results not just in a screen color change, but SPECIFICALLY a purple one instead of the BLUE ready prompt. I've noticed a link to purple chips and troublesome boards. I've adjusted the potentiometer to make it blue again just fine, but then I'm curious. From a tech nerd level, I'd like to know the REASON for this. Is it that not all GTIA chips are factory adjusted the same, or is that a purple screen from a GTIA means it has degraded in some way or slowly dying? Anyone encounter this?

[+tf_hh]

On both 600xls and 800xls, in doing the chip swaps, often a GTIA swap results not just in a screen color change, but SPECIFICALLY a purple one instead of the BLUE ready prompt. I've noticed a link to purple chips and troublesome boards. I've adjusted the potentiometer to make it blue again just fine, but then I'm curious. From a tech nerd level, I'd like to know the REASON for this. Is it that not all GTIA chips are factory adjusted the same, or is that a purple screen from a GTIA means it has degraded in some way or slowly dying? Anyone encounter this?

 

The answer is simple: Analogue - The affected part at the internal circuits of GTIA are dependent on the input voltage of pin 17 (CADJ). As nearly all chips, there are tolerances. In the 70s, where GTIA & Co. was initially developed, these tolerances where huge in relation to modern times. Even the same manufacturer of GTIA and same datecode may result in slightly differences in color. Using NTSC this effect is more visible than using PAL.

[+DrVenkman]

That's exactly correct. I've had two GTIA failures in my experience, and in each case, swapping the new GTIA for the old resulted in a color hue shift as to compared to the prior chip. This is fairly well known among folks that have fixed multiple machines, but perhaps less well-known in the general hobbyist base.

[seastalker]

Thanks- I've swapped some chips these past years and I'm curious at the finer details of such nuances. It makes for an easy adjustment, but I suppose it also makes for many an Atari user to have slight nostalgic variances as to what their true shade of blue was.

[+DrVenkman]

Thanks- I've swapped some chips these past years and I'm curious at the finer details of such nuances. It makes for an easy adjustment, but I suppose it also makes for many an Atari user to have slight nostalgic variances as to what their true shade of blue was.

 

And that's without even getting into the variations in those old analog color TVs most of us were using for displays back then, too. Lord knows my old hand-me-down Zenith tuned to channel 3 wasn't the ideal computer monitor but something like that was what most of us had circa 1983.

[seastalker]

Same- don't think it was Zenith, but likely Sony through RF switch - moderately lighter and whiter shade of blue. It was still glorious or I suppose we won't be talking about it today.