128 games builtin 2600 jr video mod help needed.

[Bignumbas]

I recently aquired an Australian 2600 jr with 128 games on board. I want to do composite out mod but the circuitry does not match anything  I can find.

If anybody with better diagnostics skills can help or has an actual schematic it would be much appreciated.

I have included some system photos. 

There is a 4 position bank switch for choosing each set of 32 games on the upper left corner of the mainboard.

On the case it is labeled as Game Group with 1,2,3,4 above, it did not come out clearly in the case photo so I took another photo of just that section.

 

 

128-in-1 Junior Console Games List.txt

Edited December 20, 2019 by Bignumbas

[Bignumbas]

Ok I got motivated yesterday afternoon and did some circuit snooping comparing this board to a C12283 version available on this site.

It cannot be that much different except for the 4 game roms circuitry added on I thought.

So I traced audio from the TIA pin 13 and pins 2,10,12 & 15 from the 4050. 

I tried a transistor mod but that did not work any better than not having it .

I am posting the photos of what I ended up with. All I had to do to tweak it was adjust the inductor L10 to get rid of those sloping lines you get and the pot next to the voltage regulator.

Plus the RF ciruitry is still functional, it does not appear to interfere with the composite signal that I have noticed.

I am going to drill a holes either side of the RF socket and mount the rca sockets. It should look neater than putting them on the side.

I will post more photos later over xmas.

I have a 32 in one cart (64k rom with 2k games) coming that I have made a 16 in one rom with 4k games but I am thinking now that I could just replace the roms onboard with 4k game roms.

Luckily the onboard roms are already socketed from the factory.

I also found that the TV/BW switch auto returns to Colour when you select B/W,  it is actually used as the game change switch for the onboard games.

I have no instruction manual for this console.

[Bignumbas]

Ok I changed the video out connection point to TP3 that is where all the lum,sync etc combine after leaving the 4050 chip. It will make it easier if I have to change stuff later.

The audio is on a capacitor which is directly connected to TIA pin 13 which is what was usually done in other mods

The output connections look neat where I have placed them on the back.

I prefer them on the back than on the side where they seem to be put normaly.

My only problems now are that games don't seem to immediately start but hang on powerup. Electrolytic cap replacement?

Also the video seems unclear a bit slightly out of focus but it was like that using the RF so may be normal? 

I found a post here to use an op amp for output so I will look at that.

I will also do a labeled photo to show where all the needed connection points are on this board with a description of what I found out. 

I tried the 16 in one rom I made in socket 4 and it works. I think there may be a problem with ntsc games working properly though.

Further investigation is needed on that one though.

 

 

Edited December 21, 2019 by Bignumbas

[0078265317]

Nice.

[Jason_Atari]

Nice job, the Jnr version with 128 built in games are not that common.

[Bignumbas]

I have done a basic diagram of what I found tracing the circuit.

I also discovered that the TV/Monitor I was using was not working properly, it has been decomissioned.

My video problems were the screen misbehaveing not the 2600jr. I would still like to try out the op amp but the chips won't be here till well after xmas

Looks really good now I will post some more pics when the sun goes down and I don't have any back light.

One of the games on my homebrew rom was a math game and needed a number pad, don't know where that came from, maybe one of the game dumps was wrongly named.

The  home brew rom worked. and you only had to flick the select switch once to go to the next game, which I found odd because the other roms had 32 games a piece.

still a work in progress.

 

[Bignumbas]

I have a problem now of the sound randomly starting or no sound at all and when it is going it makes the screen flicker.

You can see a bit of it in the first games with the green. There is banding across the screen under the spaceship, it seems to be made by the sound.

So I guess I need to isolate it from the video more somehow.

It might be the power pack not having enough output due to bad caps I will try another PS tomorrow

Stay tuned, maybe its a mainboard problem.

Edited December 21, 2019 by Bignumbas

[ChildOfCv]

On the electro caps, look for residue on the board and signs of corrosion around them, as well as bulging.  Any of those are dead giveaways that the cap needs to be replaced.  And if there's residue, clean it off--it will eat through traces and even the board itself.  Otherwise, check the voltage output from the regulator.  If it's low (or even high), replace the regulator too.

[Bignumbas]

Solved the video interference.

Thanks, I checked the reg and it's 5.02v.

I moved the audio to a direct link to TIA13 and connected the sound RCA socket to it with a 1uf electrolytic cap in line

I replaced all the electrolytic caps which did improve the quality of the video a lot, I do a lot of vintage stereo amps and even if they look good they can still be bad. They did all measure higher values.

The clincher was that the video was still being interefered with by the sound, especially deep bass so having no schematic I just removed the Q4 transistor which is the same function as the other jr's.

Perfect video now, I would say it's almost HD now, no interference at all and solid colours. ?

Obviously no sound to RF but the RF is crap anyway. 

Final photo of the finished mod:

 

[ChildOfCv]

It sounded like you were also seeing corruption of the graphics, hence the suggestion.

 

Yeah, audio can interfere with video since it's FM modulated at 6MHz above the video.  So after suggesting that op-amp circuit you saw, I decided to improve it with a low-pass filter to keep the audio away.  It ought to work well in other types of system too that use this scheme:  Intellivision, TI, CoCo, etc.  But the first question is how well it works at all...