Another 600XL Video Experiment -- A800 color circuit

[reifsnyderb]

While I had my 600XL upgraded to have S-Video by adding the 600XL S-Video circuitry in the empty component spaces on the board, the video always looked a little "dirty".  Every now and then, I'd experiment with the video by trying a couple things I found in some of the old magazine articles I found online.  But, nothing made much of a difference.  As a test, I was going to try using the XE color circuitry but didn't yet order the inductor.  Anyhow, I got to wondering what the difference was between the 800 circuitry and the 600XL circuitry.  The 800 always seems to have really good color anyhow.  So, I took a look at it.

 

Here's the 800 circuitry:

 

 

Here's the 600XL circuitry:

 

 

My 600XL had a few modifications from this, such as the RF and composite circuitry is omitted but was very similar.  Due to some other experiments, based on old magazine articles, some components were different, too.  For example, I have R123 as a 75 ohm resistor instead of the 390 ohm resistor.

 

So, after studying the circuitry, I decided that I could remove the following:

 

R130, $128, R129, R131, Q7, R133, R135, and C110

 

Replace C11 with a 3.3k ohm resistor, jumper the location of R135 to ground, and jumper Q7, I'd have the same circuit.

 

I didn't change out C110 because I don't have a 10pf capacitor so I left it at 3.9pf.  I am not an electrical engineer, but given it's location and use, I am wondering if it would make much of a difference anyhow.

 

So, I came up with this:

(Some other components are different, due to previous experiments.)

 

(Note:  I just noticed an error on posting.  C115 was upgraded to 100uF.)

 

 

The video now looks almost like that of an 800!  The colors are much better.   🙂

 

 

 

 

 

[Beeblebrox]

Very interesting. I've always found the 600XL, (at least the PAL versions), to have the best stock video quality of all of the A8's imho. Once chroma is hooked up and c109 is removed it's fantastic, on both av composite and of course on the superior S-video. 

 

However I am very intrigued about your findings using the 800 video circuit and may well have a play around at some stage. 

 

By far the worst stock video, (again my experience being PAL a8s), is on the XE lines. I've have many many 65XE, 130XE and a good few 800XE's and all of them have been horrible until either I've upgraded with UGV or SpectreAV or sorted with an internal passive components tweak, (eg super video XE tweak). This is on both av composite and svideo. 

 

One thing I've noticed is that with all the PAL XEGS I've had, the stock av output has been pretty good. Way superior to the XE lines. So it seems the XEGS had a pretty good stock video out the factory. As some stage I'll do an S-video mod for my own XEGS I expect.

Edited October 21, 2023 by Beeblebrox

[reifsnyderb]

 

What I find odd is that Atari used a different video circuit for almost every line of computers.  I would have thought they would find something that works great and stick with it.  Here's the schematics for the 130XE:

 

 

Here's the XEGS:

(The XEGS only has composite video.  😞  )

 

I modified my 130XE as follows:

(The 10uF capacitor, by R116, is Tantalum and the RF modulator is disconnected.)

 

My thought is that the simpler circuitry should have fewer losses.

 

 

 

 

 

 

 

Edited October 21, 2023 by reifsnyderb