7800 RF Modulator Help

[OldAtAtari]

I picked up a $20 7800 at a flea market Saturday. Really filthy, but it cleaned up nice. It powered up right away, but the display was black and white and very low quality. Then today, there's no display at all. So I took it apart, cleaned the gunk off the motherboard, and tried again. Same deal. Probing around with a multimeter, I found that there is less than 1 ohm restance between the signal output and the shield of the modulator. My working 7800 shows 50 ohms there. So I removed the modulator. It looks good inside (visually), but the signal and ground are still virtually shorted in there. So my assumption is that the modulator itself has a problem.

 

Does anyone have any thoughts on this? Is 1 ohm a bad resistance here? For that matter, is 50 ohms a good resistance? Does anyone have any idea as to a possible cause?

 

Thanks.

 

[DrVenkman]

I’m not an expert on the inner workings of modulators, but you might want to measure the capacitance of those caps or see if any of them are internally shorted, and replace them if so. Better yet, forget RF and do a

video mod instead.

[OldAtAtari]

Thanks, DrVenkman. I was thinking exactly the same thing. There might be a decoupling cap between the signal and ground. If that cap went bad, it could short the two together. Fixing the modulator is my first choice. It would be neat to restore this thing to original working order. Second choice is the video mod. I wouldn't spend a lot of money on it like I did with my good 7800 (UAV plus a good refurbishing by Crossbow). This is a $20 beater Atari, and I want to keep it cheap. I've got the parts to do a Soundby S-Video mod. I could go that route with it if I want to make it a real project. We'll see.

 

Thanks again.

[+-^CrossBow^-]

I will have to dig around and look but I might have the original RF I pulled from my personal AV modded 7800? If I do, you can have it for cost of shipping?

[OldAtAtari]

Or you can ship me your personal AV modded 7800 instead if you like. :-) But seriously, if you find one, let me know. Shipping should be cheap, so it would be a good deal for me. I'm still gonna see if I can get this one working, though. Thank you, Crossbow!

[+-^CrossBow^-]

Or you can ship me your personal AV modded 7800 instead if you like. :-) But seriously, if you find one, let me know. Shipping should be cheap, so it would be a good deal for me. I'm still gonna see if I can get this one working, though. Thank you, Crossbow!

 

You don't want my personal 7800. It is a mess honestly. It has lived through three other AV mods in its life and had components replaced...etc. Not pretty inside at all which is why I keep it and don't sell it out LOL. I know what I've done to the thing and know its history...

 

Sordid tales...sordid tales indeed...

 

I actually think I know where that old RF is and if I do that will amaze me but I've a pretty good idea. (Could also be the RF from my 5200 I'm thinking about though...)

[OldAtAtari]

Ok, then, you can keep your personal 7800 and your sordid tales. But if you really want to do me a favor, when and if you dig out that RF modulator, can you please stick an ohm meter on it and see what resistance you have between the output signal and the ground shield? I don't even know for sure that mine is abnormal.

[+-^CrossBow^-]

Ok, then, you can keep your personal 7800 and your sordid tales. But if you really want to do me a favor, when and if you dig out that RF modulator, can you please stick an ohm meter on it and see what resistance you have between the output signal and the ground shield? I don't even know for sure that mine is abnormal.

Yes I can do that. If i can't find it, I can test that with the 7800 I have on my work space now getting a UAV. Wonder if I have to remove it though as I don't really want to do that on this 7800.

[OldAtAtari]

No, don't remove the RF modulator from any 7800! Especially one that doesn't belong to you! I just stuck one probe into the AV-style output and put the other probe on the outside of that output. No need to disassemble anything. My fully assembled good 7800 was about 50 ohms. My busted flea market 7800 was approximately 0 ohms. And it's still 0 ohms after I removed it.

[+-^CrossBow^-]

I just making sure that the reading didn't need to be made with the RF modulator out of circuit is all. But sure thing I can check the ohms from the RF output to RF casing.

[OldAtAtari]

Yep. I had the same thought about a modulator that's connected to the main board versus a modulated that's sitting on my kitchen table. But since the resistance on this one was 0 in the Atari and 0 out of the Atari, I'm assuming it doesn't matter whether it's connected or not. But I'm totally guessing at all of this. :-)

[OldAtAtari]

I removed the board from the modulator, hoping to find an obvious bad component.

 

My first guess was a cap that is between signal and ground. I assumed that this cap was bad, so I removed it from the board. But the cap tested fine, and I found that the short still exists on the board, so I kept looking around and found this black component labeled T1. It has orange and red wires running through it, and I'm pretty sure it's a tiny transformer. I'm hoping someone can confirm this for me. But the interesting part is that this component also has a plain metal wire running through it. One end of the wire runs down through the board and solders to the ground plane, and the other end of the wire solders to the signal plane. It seems to be a direct short between signal and ground.

 

My mind is blown by this. Why would a wire short signal to ground, in parallel with a cap? I don't understand RF modulation. I'm hoping some electrical expert out there can help me out with this. And there is the possibility that it's really two separate wires that happen to be touching internally in the black component (but shouldn't be). But it sure does seem like one piece when I wiggle it around.

 

Thanks.

 

 

[OldAtAtari]

Just found this AtariAge schematic of a 2600 RF modulator. It appears to show a transformer with a single coil between ground and the signal (which then has a resistor before the output jack). It's also in parallel with a cap.

 

So that looks like my 7800 modulator, except that mine has an inductor instead of a resistor. So I took the same resistance measurement (between the RF signal and the casing) off my 2600 output and found it to be 100 ohms.

 

So I have:

Good working 7800: 50 ohms

Broken 7800: 0 ohms

Good working 2600: 100 ohms

 

But still with that wire between signal and ground, it seems like the resistance would always be 0. Maybe I should study how transformers and RF modulators work and then report here later. :-)

Edited June 29, 2018 by OldAtAtari

[+-^CrossBow^-]

Well, I don't have any good answers for you.

 

Measure with the inside of the RF modulator output RCA jack to the modulator casing only yields .5 ohms... yeap... .5

 

But when I first touch the probe to the inside, I do see it show 45ohms or so but only for a second and then it drops to .5 and stays there steady. This was done with an rf modulator still attached in circuit.

[OldAtAtari]

Hmmm... No that makes sense. Low resistance because it's shorted. Decreasing from 45 because there's a cap across it. Apparently a cap does that when the current from multimeter probes passes through it. So says the internet.

 

So given that what you're seeing matches what I'm seeing, maybe my modulator is fine. Or at least the 0 ohms thing isn't a problem. Thank you for letting me know, Crossbow.

 

I might reassemble it and keep poking around the board.

[+-^CrossBow^-]

Well...have you tried a 2600 game in it and still get only B&W? I've also seen this happen when the 74LS chip by the Audio resistors goes bad and doesn't provide proper sync signal. And the last 7800 I just UAV modded, had a bad Maria that was producing a b&W like image. There was still some color, but blue was missing for sure from it and so the images mainly looked mostly b&w. But, 2600 games looked fine. That made sense since that is all the TIA doing the work there.

 

So yeah, to try and isolate an IC issue as the cause try both 2600 and 7800 games to see if the issues persists and then try changing out the 74LS.

 

Also and I'm going to assume you have done this since you are using the RF...

 

But put a game in, turn on the system making sure the channel switch is on the right channel you want. Then have you TV do an auto scan on the analog channels. I've found that many TVs will not latch onto the Atari RF signal very well unless you have it turned on with a game going and then run the scan on your TV. Additionally, you can use a plastic hex tool to adjust the RF modulator incase that is the issue.

 

And I forgot to look for the old 7800 RF modulator I was talking about, but I think the issue is something else at this point.

[OldAtAtari]

Yeah, I don't understand the difference between the modulators on my two 7800's, but I agree that the modulator probably isn't the problem.

 

I definitely tried a 2600 game at the start when it was black and white (It later lost the image completely). I'm not sure I ever tried a 7800 until after the picture was completely gone. I'll try the thing about having the game running first before powering on the TV. My other 7800 doesn't require that for the RF to work, but it's a different beast. My 2600 RF works fine as well. But it's worth a try for the junker 7800.

 

And I'm considering taking DrVenkman's advice, since the RF Modulator is already out, and just seeing what the simplest video mod available is. Just enough to get a picture that I can test with. And if I get a decent modded picture, then maybe the problem is the modulator after all.

 

Crossbow, thanks again for your help.

[+-^CrossBow^-]

No problem!

 

BTW...cherish that UAV modded 7800 you have. Word on the 8-bit forum is that Bryan doesn't have any more and isn't having any more made up until he has the next revision ready. There is naturally no ETA on that... so I guess that just leaves using something like Osgeld does on his 2600s for composite.

 

Although it looked like crap at the time, I believe I at one time was able to tie all the luma lines together off the resistor ladder and took the chroma from the maria and actually run them through the same s-video circuit you use to amp up a sega genesis. It did produce a picture but but it didn't look right colorwise etc. Still... I do know I was able to pull it off.

 

You can also give yourself a pseudo s-video signal by putting a cap on the composite to filter the Y and C separately. But I've never had very good results doing that. The first TG-16 s-vid mod was doing just that basically.

Edited June 29, 2018 by -^Cro§Bow^-

[OldAtAtari]

Yep, I'm grateful for my UAV, for sure. It's working great.

 

I think I still have the board I worked up for the Saundby video mod. It was full of bad solder joints. Just poorly done, but it did produce an output. Perhaps I can clean it up and wire it in, and that will be good enough for now.

[OldAtAtari]

Ok. It's not pretty, but I cleaned up my Saundby mod board and got it installed.

 

 

Initial test showed the typical unclean cartridge effect. Stripes and such. Most of the time the screen was black and white, but occasionally, I'd get some color stripes, along with a beep or a screech, both of which were encouraging in that the machine and mod are capable of displaying color and outputting sound.

 

 

I tried different games, and then I realized something: These games were sliding in and out of the cartridge port like butter. There was no force at all required to get them in, except for a few where the cartridge casing was rubbing up against the sleeve.

 

So I started looking at carts and found that some have noticeably thicker cards inside. So I stuck a thick one in there, and it booted right up!

 

 

That was a 2600 game, so I tried a 7800 game. Worked fine.

 

 

Apparently the problem now (and maybe before) is that the pins inside the cartridge port have spread apart, and they aren't making good contact. I know they're clean, as I've cleaned them several times, and they look shiny down in there. I think they've just spread. I need to find a way to bend those pins a little such that they'll make good contact with the cart contacts.

 

While I had F-18 hornet in there, I tested the player 1 joystick. Directions and both fire buttons work fine. As for the console itself, the buttons are shot. The power button will work with some effort, but the only three buttons don't do anything, though I can short the button leads with a wire and get Pause, Select, and Reset to work.

 

All in all, I'm happy. There's great potential to fix this thing. And if nothing else, it's got some good parts that I can use later on if needed, including the power supply.

 

And as for the modulator, I bet it's fine. I bet it was never the problem. I might solder it back on and try it out. My implementation of the Saundby S-Video mod is better than my last attempt, but it's not great. The whole screen is kind of wavy. If this is ever going to be a fun console again, I either need a proper AV mod or at least get the RF in working order again.

 

So thank you DrVenkman and especially Crossbow! I really appreciate your inputs in this thread about my RF Modulator, which is probably not a problem after all (except that's it's now disassembled in a Zip-Lock bag).

 

 

[DrVenkman]

I've tightened up the cartridge pins on one of my 7800 by using a wooden toothpick, of all things. It's just about the perfect size to slide behind the copper contacts and bend them back to the center of the cartridge slot. One caution, however. The two outermost sets of pins on each side are 7800-specific. So if they get bent too far inwards and make contact, you won't be able to play 2600 games; the 7800 cartridge edge connector uses all those pins so when you insert a 7800 cart, they'll make contact with the pads on the cart edge and everything will be fine. But if you insert a 2600 game, those four pins will be touching each other and the system won't load the game.

Edited June 29, 2018 by DrVenkman

[OldAtAtari]

I've tightened up the cartridge pins on one of my 7800 by using a wooden toothpick, of all things. It's just about the perfect size to slide behind the copper contacts and bend them back to the center of the cartridge slot. One caution, however. The two outermost sets of pins on each side are 7800-specific. So if they get bent too far inwards and make contact, you won't be able to play 2600 games; the 7800 cartridge edge connector uses all those pins so when you insert a 7800 cart, they'll make contact with the pads on the cart edge and everything will be fine. But if you insert a 2600 game, those four pins will be touching each other and the system won't load the game.

 

Cool. It's good to know that this is a thing that can be fixed. And thanks for the cautions. I'll try to go slow and not over-bend the pins. I'm excited to see the difference this makes.

 

It's funny that this $20 Atari (came with paddles, 2600 joystick, and 5 games, by the way) that I intended to be a parts donor, has become a full restoration attempt. I'm a sucker when it comes to restoration opportunities, particularly things originating in the 80's and prior.

[+-^CrossBow^-]

It's funny that this $20 Atari (came with paddles, 2600 joystick, and 5 games, by the way) that I intended to be a parts donor, has become a full restoration attempt. I'm a sucker when it comes to restoration opportunities, particularly things originating in the 80's and prior.

 

No so much, I recently bought both a beater 7800 and spare 7800 main board from an AA member through the marketplace forum just over a month ago for like $25 shipped. I just wanted them for parts. Turns out all I had to do was to replace the power jack on the beater that I received and it was fully working without issues! I used the power jack from the spare main board that was sent along with it. So while I might not have gotten paddles and joysticks or games with it, I did get a bare 7800 in decent shape up and running along with a spare parts board for $25. So that was quite a deal in my opinion as well.

 

Congrats on bringing another 7800 to life in the world!

[OldAtAtari]

Thanks, Crossbow. $25 for 1.5 Atari 7800's sounds great to me! And you got it working with little effort. That's terrific. Yes, it's good to get something working again.

 

As for my beater 7800, I bent the pins in the cart port a bit such that all the games seem to make firm contact, and I refurbished the buttons using a method I found elsewhere in these forums. I appreciate the better quality buttons you put in my good 7800, but for a good quick fix, this worked pretty well to get the beater back in working order without much time and without any money invested.

 

That's when things went south.

 

I removed the Saundby mod and reinstalled the modulator. No image. I tried the order of events described in your previous post, but to no avail, on channel 3 or channel 4. I get a voltage on the inputs to the modulator, but absolutely zero volts on the output. I'm back to thinking the modulator is dead. And I'm tired of taking that little bugger apart and trying to figure it out. So I think that 7800 is going to sit in a box with the knowledge that it can be fixed possibly with a working modulator or with a video mod.

 

But I'm happy to have an Atari with a known problem which seems to work in every other way.

[+-^CrossBow^-]

You know I remembered once long ago about the Saundby video mod, but at the time it was well above my skill to understand and attempt. (This was like 15+ years ago now). In looking at it again last night I figured something out.

 

I believe that the Magic Knight S-video only board is basically the Saundby mod but perhaps with some different values on the resistors used and the like. Sadly I haven't that Magic Knight S-video board any longer as I sold it a few weeks back for like $20 shipped to someone. But in looking at the way his PCB came out, and yours and the schemes...it all looks awefully familiar.

 

Since I've not actually created a Saundby S-video board, how does it compare with the s-video output of the UAV? In your pics the title shot of F-18 looks pretty washed out, but I'm thinking that is just the camera and LCD not playing well together?