Atari 7800 Audio Video Issues

[ChmA2678]

I appreciate all the info and I'll check into those other mods.  I've ordered from Bradley before and haven't had any issues whatsoever and he's always very helpful, quick to respond and deliver.  In this instance he seems to be pretty set on me getting the video fixed first.  I don't believe they do repairs or mods anymore since he recommended I look for someone to repair it from the atariage site.  I sent an order in this morning and haven't heard anything from him even after I said that I would be getting a working board and mod that one.  I'm sure he's just busy but this is a new experience from this company apart from what I've originally experienced.  

[ChildOfCv]

For that matter, do you happen to have a breadboard?  If so, you can set up a simple voltage follower op amp circuit and wire that into the TV composite to see if you get a good image, at least.  This would require no modifications at all to the Atari, so it would be a good initial check.

[ChmA2678]

8 hours ago, ChildOfCv said:

For that matter, do you happen to have a breadboard?  If so, you can set up a simple voltage follower op amp circuit and wire that into the TV composite to see if you get a good image, at least.  This would require no modifications at all to the Atari, so it would be a good initial check.

While I can solder, follow directions and test simple components with a multimeter; my knowledge of electronic components are extremely limited.  With this foray into trying to diagnose my 7800 video issues I have to admit becoming more interested in the technical aspects on how everything works and why with all the circuits.  I can swap out parts as needed but it's interesting to think beyond that.  Back to the point, I do not have or ever used a breadboard but I know what it is.  I'm sorry to say that even if I did have a breadboard that I don't know what a voltage follower is or how to make one to connect to the tv.  I really appreciate your ideas and you've been of great help troubleshooting the issues.  I just which I were more technically capable to follow through with some of the suggestions.

[ChildOfCv]

Well if you're willing to buy a few parts, I can tell you how to put together a circuit.  First you'd need an op-amp.  Looks like a decent one that can handle the frequency and plugs into a breadboard would be AD8055, which costs $2.80 on Mouser.  You can get a small breadboard with assorted jumper wires for about $10.27.  75-ohm resistor 10 cents, and a 10uF capacitor for 31 cents.  A package of 5 J-clips to breadboard plugs would cost around $5.  So, about $18 plus shipping.  If that sounds like something you'd like to embark on, I can give you a specific parts list and a breadboard wiring diagram.

 

Of course it would probably be easier to let one of the Atari repair guys fix your system

[ChmA2678]

53 minutes ago, ChildOfCv said:

Well if you're willing to buy a few parts, I can tell you how to put together a circuit.  First you'd need an op-amp.  Looks like a decent one that can handle the frequency and plugs into a breadboard would be AD8055, which costs $2.80 on Mouser.  You can get a small breadboard with assorted jumper wires for about $10.27.  75-ohm resistor 10 cents, and a 10uF capacitor for 31 cents.  A package of 5 J-clips to breadboard plugs would cost around $5.  So, about $18 plus shipping.  If that sounds like something you'd like to embark on, I can give you a specific parts list and a breadboard wiring diagram.

 

Of course it would probably be easier to let one of the Atari repair guys fix your system

Yes it would be easier to let the Atari guys fix my system but where would the fun be in that.  Learning new things is always a blast especially when you get everything figured out and it works!  So yes I'll take you up on your offer and pickup the parts needed.  Thanks!

[ChildOfCv]

Okay, so here's a list of things to buy and also a board to wire up.  The part numbers in the PDF can be clicked to go to the part itself.

 

I drew the breadboard the way I did because each vertical row is connected on all of its pins.  So the red lines are just where you have to stick a jumper wire.  The circles are where the grabbing jumpers plug in from the Atari.  If you don't have spare wire, you can sacrifice a couple of jumpers by soldering them to the RCA end.  The inputs come from the Atari and list the pin numbers on the RF module that you'd need to connect to.  1 is ground, which should be easily locatable, and the rest will be in order.

 

So, with 3 of the J-hook connectors, one jumper wire, and a connection to the RF connector end, you have a test composite mod.

 

Cart_Aug21_0310AM.pdf

[ChmA2678]

11 hours ago, ChildOfCv said:

Okay, so here's a list of things to buy and also a board to wire up.  The part numbers in the PDF can be clicked to go to the part itself.

 

I drew the breadboard the way I did because each vertical row is connected on all of its pins.  So the red lines are just where you have to stick a jumper wire.  The circles are where the grabbing jumpers plug in from the Atari.  If you don't have spare wire, you can sacrifice a couple of jumpers by soldering them to the RCA end.  The inputs come from the Atari and list the pin numbers on the RF module that you'd need to connect to.  1 is ground, which should be easily locatable, and the rest will be in order.

 

So, with 3 of the J-hook connectors, one jumper wire, and a connection to the RF connector end, you have a test composite mod.

 

Cart_Aug21_0310AM.pdf 297.83 kB · 1 download

Fantastic, thank you.  I've got the parts on order and am looking forward to bypassing the rf module to see if that's the culprit.  Nice job with the diagram, very easy to follow.  There was something I had a question about with the bypass.  Don't you have to remove a couple resistors next to the rf modulator when doing a mod?  What do those two resistors do and will it impact this test in any way?  

[ChildOfCv]

The cheap mod requires you to remove some parts because the simple transistor amplifier affects the circuit that it's tying into.  The op-amp has such high input impedance that the rest of the circuit shouldn't even notice its presence.  So for this, you don't have to change a thing.

 

The only potential complication I can think of is the sound carrier that's already mixed in.  But the TV will probably filter that out.

[ChmA2678]

I got the circuit completed and bypassed the RF Modulator.  The picture isn't perfect but it's 100 times better than it was.  Is it suppose to be a cleaner picture or is this just a quick test?  There obviously isn't any sound since it wasn't connected.  So is it safe to assume that the RF Modulator is to blame?  I also got it to load 7800 games.

[+-^CrossBow^-]

Yeap...I would say the RF modulator is to blame but my thinking was already that when I was able to identify Battlezone from what you were showing before. That told me the logic was working and it was all video output at that point. And even the most basic of composite mods still look better than that is most cases so I say you pop a UAV in there next and forget about the RF. If the RF is that important to you... I likely have one or two from 7800s I've modded where the client requested I remove it.

 

[ChmA2678]

Putting a mod in is the direction I was heading.  I wanted to keep dealing with Best Electronics but they were basically insisting that I find the problem and get the video working before doing the mod.  I've tried to order it twice now and get a reply back at how nobody would do the mod without the system working and the order wasn't filled.  So I decided to figure out where the problem lies and learn a bit along the way.  I've had a really good experience with them in the past so I'm not sure where this is all coming from.  I'll try once more before checking other sources.  I do thank you though for offering to send me a replacement RF.  

[ChildOfCv]

You may still want to check if the MARIA chip is working though.  Try one of your 7800 games to see if you get a screen with it, since it seemed to be worse on that side.

 

I can sorta see Best's side of it:  They don't want to do the work just to find that it's still not working, and now either you don't want it back, don't want to pay for the mod they did, or want to BS about them ruining it.  But still, there should be a point where you can say "Look, it's the RF.  Just shut up and take my money."

 

Most likely one of the transistors went bad.  Since you get "a" picture, it's likely Q1 or Q2 from the earlier schematic.

[ChmA2678]

I checked a 7800 game earlier and it works fine.  The pic is the same as the 2600 game with the bypass.  Much improved with the bypass instead of going through the RF.  So you're saying that a transistor might be why the picture isn't super clean?  I'll check them a bit later.  If Best was offering to do the work I can see their point but I'm just trying to order it with a few other parts to do myself.  I'll try to order again and see how it goes.  Thanks again for all the help.