Greenish 600xl: it might be missing O1

[manterola]

Hi all,

My new old beaten 600xl has color problems. The color potentiometer does not fix it. So today I checked the schematics and looked for some suspicious capacitor to change, then I realized that the O1 signal is the input signal to create the signal that goes to gtia pin 17. So I checked continuity between the potentiometer and gtia. No problem, then I discovered that O1 (phi1?) coming from Sally was disconnected!

I still need to replace the Sally socket, but I want to document this here, so it can be helpful in the future for someone with same problem.

But also I would like to know more about this circuit , its objective and why it uses O1.

And how the lack of O1 might be the reason of the failure to get an stable image using UAV (currently I am just using the modulator output, UAV is shelved until I solve all these other problems)

[Rybags]

GTIA doesn't use o1

 

The clock and colour related pins on GTIA:

16 PAL (input) - only used on PAL systems, is the 4.43 MHz signal from the second crystal which is used to generate the colourburst and colour waveforms.

17 CAD/DEL (input) - input from the colour adjustment trimcap

21 COLOR (output) - colour output. Is a square waveform which is approximated to a sinewave by external circuitry

28 OSC (input) - main oscillator input (3.59 MHz) - this is used for internal operations and divided by 2 which is then used throughout the system as o0

29 Fo0 (output) - fast phase 0

30 o2 (input) - Phi2 input

 

Without the clock signals, the Atari won't work. In theory you could get away without the PAL input on a PAL machine but you'd end up with a B&W screen.

It's my understanding that the clock signals are generated as such:

Fo0 originates from GTIA and then is passed to Antic which generates o0 from it. The 6502 generates o1 and o2 from that. These are the same frequencies with phase delays between o0, o1 and o2.

[ClausB]

GTIA's color circuit was designed to operate with the 12V power supply in the 800 or 400. But the XLs only have 5V supplies so they used Phi1 to drive a charge pump which generates enough voltage for the color circuit.

[manterola]

Great! It makes all sense now. I measured the voltage at the potentiometer and it was 5v full right and it was too green.

Now I can get the familiar blue screen with the potentiometer in the middle.

I connected pin 3 of Sally to the front end capacitor using a cable, while I am waiting for new sockets, and color problems are gone. This 600xl is slowly coming back to life.

[gozar]

Great! It makes all sense now. I measured the voltage at the potentiometer and it was 5v full right and it was too green.

Now I can get the familiar blue screen with the potentiometer in the middle.

I connected pin 3 of Sally to the front end capacitor using a cable, while I am waiting for new sockets, and color problems are gone. This 600xl is slowly coming back to life.

 

Nothing to add to the conversation, but the set up looks way to familiar. I currently have that Magnavox LCD TV on the left, and used a white cabinet almost identical to this one for my set up in the dorm room at college. :-)

[manterola]

 

Nothing to add to the conversation, but the set up looks way to familiar. I currently have that Magnavox LCD TV on the left, and used a white cabinet almost identical to this one for my set up in the dorm room at college. :-)

Did you get problems trying to get the Magnavox TV "synchronized" with the Atari ntsc signal?

I switched to a computer monitor and svideo to svga converter, just because that problem drove me crazy.

[gozar]

Did you get problems trying to get the Magnavox TV "synchronized" with the Atari ntsc signal?

I switched to a computer monitor and svideo to svga converter, just because that problem drove me crazy.

 

Almost every time. I unplug the video cable and plug it back in until the screen is correct. Now I use a VBXE through a GBBS-8220 into the VGA port, so no problems now. :-)