Just Got My First 2-Port - Weird Factory (?) Mods

[+DrVenkman]

So a friend recently sold me a spare 2-port unit that he had sitting unused. I intend to put a UAV into it in a couple weeks and leave my trusty 4-port untouched and then keep that one as a backup. I have been hesitant to AV mod my 5200 until I have a spare system - so long as @MakerMatrix and I are building and selling gamepad controllers, I need to ensure I have a working system at all times in order to test each new controller I build.

 

At any rate, I had some time this morning so I decided to disassemble the system and take a look inside. I was also seeing some weird keypad behavior so I decided  to replace the 4052 keypad mux chips and adjust the color pot while I had it apart. When I took the system out of the case and got the gods-be-damned RF shield off, I discovered a series of board mods that appear to be factory original. By and large they are very neatly done, and the bottom shield tabs through the top shield slots were all twisted exactly like they had been leaving the assembly line. Therefore I'm inclined to believe these are original.

 

The first and most obvious mod are these bodge wires from the 74LS00 chip to another 74-series chip near the DRAMs and to a via under those green chiclet caps. Next, under the board I see a number of pins from POKEY are bridged and then run to what appears to be the leg of a diode or small glass cap. And then also under the board, I see a pair of pins from one of the 74-series chips are solder-bridged. Finally, I see that pin 11 of GTIA is bent up and out of the cocket. From the white surface oxidation on that leg, it's been bent up and out like that for many years or decades, probably since new. Per the pinout, this is the Trig3 signal, which I guess isn't used in a 2-port system. But then again, is Trig2? I need to look at the 2-port schematics I guess to see if I can make any sense of these mods.

 

One unpleasant surprise from this morning was that the 4052's were soldered on this machine; on my 4-port they were all socketed from the factory. So I desoldered them, installed sockets and replaced all 3 with modern replacements. That cleaned up the keypad issues, fortunately. BTW, it was a serious pain in the ass to get the original chips off the board. The solder was very oxidized and hard to remove, even adding fresh 63/37 leaded solder and my Hakko FR-301. I ended up getting most of the solder out with the Hakko, then using hot air to melt the sticky remaining bits and lift the chip out of the board. Of course I installed new machine-pin sockets for all three in case anyone ever needs to replace the mux chips again. 

 

Anyone have similar weird stuff like on their 2-ports? 

 

 

 

 

 

 

Edited January 5, 2020 by DrVenkman
typos

[+DrVenkman]

Okay, an afternoon of research and I've found the answers, I think.  

 

From the Atari 5200 Field Service Manual updates document on Atarimania, I found the following Tech Tip:

 

This accounts for the visible bodge wires between the LS00 and LS51 chip, plus the bodge from the LS00 to the via. It also accounts for the solder bridge under the board between two pins of the LS00. It's weird on this system though, because it says this factory mod was done due to a shortage of LS258 chips, so systems were built with LS158's instead. However, my system has the LS258 chips. Weird. 

 

The 2-port schematic indicates that those 4 bridged pins on POKEY are not connected, so that bodge ties them all together and grounds them to prevent the lines from floating. The schematic also indicates that pin 11 of GTIA connects to pin 4 of ANTIC. On a 4-port, it would be used for the port 4 Fire button.  However, pin 4 (Trig3 on GTIA) isn't needed for a controller port on the 2-port, and per the chip pinouts, pin 4 of ANTIC is LP - the Light Pen signal. Since there is no light pen or light gun for the 5200, there is no need to connect this line to the GTIA. So I guess that explains why the GTIA pin 11 is left bent up and out of the socket.

 

Anyway, mysteries solved.  

[Shawn]

Yes those are all factory mods. I have had a few new old stock 2 port motherboards over the years and they all looked exactly like yours. The ones Best Electronics sells are mostly, if not all, like this as well.

[EpikalPuppy]

I was wondering what was going on inside my 5200, I guess I know now.