Recently I spoke here in Skooter Blog on a Atari 2600 the Polivoks with American Board Rev. 17, I acquired, with the intention of installing a against 2600Tim RGB Worhington.
But before installing the against, removed the transcoding the Polivoks had made him, in other words, undo the change that the Polivoks has made for the console to work with the standard PAL-M, used in Brazilian TVs the years 80. In this way, He started working in the NTSC standard, as he was originally.
I strongly recommend reading the the first part of this article before proceeding, because I made an introduction about the color systems PAL-M and NTSC, talked about transcoding the Polivoks was on these systems and, Finally, talked about the pros and cons of removing this transcoding.
Well, decided to remove the Polivoks is the transcoding, I tried to study the circuit board and try to identify what had been modified. Photographed everything I could find in case you need to revert the process.
Of course the first modification that jumps the eye is the label of transcoding the Polivoks was wearing, as can be seen in the photo below. She is glued using a foam with adhesive on both sides. Underneath it is the AUNT, main IC of Atari 2600. The plate is connected to the main Board for four-wire.
Looking at the bottom of the main Board it is also possible to identify some foreign elements: at “jumper” black, that I be a resistor capped, and a ceramic capacitor. It is highly unlikely that Atari has placed these side components “wrong” the Board. Everything indicates that were added by the Polivoks.
Taking off the foam where the label of transcoding is possible to identify the points where it is soldered on the motherboard. You can also identify a missing, identified by R234. It is unlikely that Atari has put a resistor in the project and then decided to let it out. Probably he was removed by the Polivoks.
Before you go making any silly, I decided to search on the Internet if someone had posted a recipe for cake ready to do this-transcoding. But unfortunately not found. But, some clues that I found helped me create my own recipe.
First I found an article on the website Luccas Electronics, the Edward Fatu, where he explained how to install a/V output on the Atari 2600, process in which it is necessary to remove the transcoding. But, It details the process using an Atari 2600 with National Board of Polivoks. He even mentions the American Board and says that all you have to do is swap the Crystal, remove the label of transcoding and splice the wires yellow and white that were connected to the label.
But there is a problem, in the article Edward doesn't talk if you're referring to the label of small or large transcoding. Little has 4 fios, While the big has 5 fios. Furthermore, as you can see in my pictures: There is no white wire!
Nay, in my two Atari Rev. 17, the colors of the wires are different, see this picture of my other Atari 2600 with American Board Rev. 17. In other words, There is no default colors.
On both islands the paddle is identified as PCI335X. It's the same Board, only the wires have different colors. The points of the wires on the plate are numbered, and these are the colors of the wires in each number, on both consoles:
Atari 2600, American Board, Rev. 17, with chips in sockets:
1 – Brown
2 – Orange
3 – Blue
4 – Yellow
Atari 2600, American Board, Rev. 17, with chips without sockets:
1 – Yellow
2 – Blue
3 – White
4 – Orange
With this blurred colors in and on the model of the plate it was clear we couldn't go that route. Furthermore, It seems to me unlikely that just join the wires, keeping the resistor missing, the resistor and the capacitor to more, was to generate a unique equivalent circuit. It may be generated a signal NTSC TV to understand, but not necessarily be a NTSC signal entirely within the specification. Different TVs have different tolerances to out of specification, so I think it preferable to leave the circuit 100% equal to the original to ensure the operation on any TV.
Another site that I found was the information PICOLO's Online, Claudio H. PICOLO. He reports have done this-transcoding on the Atari 2600 from a friend, whose plate was Rev. 17. He reports having found the same components that found the most: a flux capacitor 1, 5nF and a resistor of 560 ohms. But he doesn't say how proceeded with this modification.
Por fim, I found a video on Youtube of Electronic MRM, where he talks about removing the transcoding in an Atari 2600 with Rev. 16. It does not show the procedure, but cites the presence of two strangers on the underside of the Board, that need to be removed. He also cites the cutting a trail, where needs to be put a jumper to fix it. Look at the pictures of my license plate under the intruder capacitor there is a trail cut. So the idea is to replace the capacitor for a jumper, restoring the trail. With this I discovered that the modifications that the Polivoks made on Rev. 16, with big transcoding card, and in Rev. 17, with small transcoding card, turned out to be quite similar in the end.
At that time it was already clear to me that it was necessary to remove the rogue elements and rework the track cut. But still lacked something that puzzled me: the absence of the resistor R234. I needed to find out what the value of it in order to fill the gap with a new resistor. Looking for this information, I ended up finding a Atari service manual 2600. Through it, I discovered that the R234 is a resistor of 820 ohms.
And so, gathering evidence, I built my own cake recipe, with the following steps:
- Remove the label of transcoding.
- Change the crystal oscillator 3,575611 MHz (PAL-M) for a crystal oscillator 3,579545 MHz (NTSC).
- Remove the flux capacitor 1, 5nF the bottom plate.
- Remove the resistor from 560 ohms the bottom plate.
- Add a resistor of 820 ohms na posição R234.
Agora faltava ver se o bolo ia ficar bom. 🙂 Estava confiante que essa mudança iria trazer o sistema NTSC de volta ao meu Atari 2600 the Polivoks with American Board Rev. 17. Mas para ter certeza não tinha outro jeito, era preciso testar.
Então mãos à obra! Na falta de uma bancada para trabalhar, improvisei com uma pia de granito. Ainda pretendo ter uma bancada para este tipo de trabalho, mas por enquanto ainda não tenho. 🙁
O primeiro passo foi trocar o cristal oscilador de 3,575611 MHz (PAL-M) for a crystal oscillator 3,579545 MHz (NTSC) – yes, acabei de inverter os dois primeiros passos da minha própria receita, but no problem. A plaquinha do 2600RGB exige dessoldar o cristal e monta-lo na horizontal para não esbarrar nele. Para evitar esse problema optei por usar um cristal meia caneca, It's small and not bump into other components.
Also I used to remove the flux capacitor 1, 5nF, He was a soldier between an original oscillator's leg and a leg of a transistor beside. I used one of the legs of the oscillator itself again to do the jumper with the leg of the transistor, restoring the trail cut.
Time to remove the label of transcoding. I took a few more pictures to mark the points where it was welded shut and then proceeded with the removal.
By Curiosidade, I decided to test the Board as was, even with that resistor on the bottom and with the resistor R234. With this TV have exhibited colors and identified the system as NTSC. Note, however, This NTSC signal probably should still be slightly outside specification. Works on my TV, but another TV might not be so tolerant. Anyway I can see NTSC color palette, much prettier than the palette PAL-M Polivoks modification.
The next day I decided to continue with the contract. Not enough work. The service has to be complete. So let's go to the removal of the resistor of 560 ohms the bottom plate.
Por fim, We will solder the resistor of 820 ohms na posição R234, leaving Atari Board 2600 as it was originally.
All ready! It's time to retest the console, now I totally-transcoded, viewing an NTSC signal pure.
And so the transcoding completed successfully, to Atari 2600 the Polivoks with American Board Rev. 17 viewing an NTSC signal pure, free of bad color palette of transcoding PAL-M. What did you think of the picture NTSC? Leave your comment.
In the coming articles I will show how I did the cleaning and some repairs on the carcass of this Atari 2600 the Polivoks with American Board Rev. 17. I'm also going to talk about how I did the removal of rust and as I left the switches Shining again. Por fim, I'll start another series of articles to show the installation of against 2600Tim RGB Worhington, that left this Atari 2600 with the best image of Atari 2600 I've ever seen. Stand by…