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ChmA2678

Atari 7800 Audio Video Issues

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Hello everyone.  I am having some difficulty with an Atari 7800 that I picked up that I hope, with the help of this community, we can figure out.  The unit didn't have a power supply so I picked up a new switcher from best and it checks out fine and powers up the unit.  The problem that I am having is the picture has a lot of static, you can barely make out the image but it has color and responds to control input.  The second problem is the sound which is nothing but static, no game sounds what so ever.  I did clean out the cartridge receiver on the board with 91% alcohol and a cloth over a credit card.  I'm connected through an rf coax adapter using an rf cable.  I've tried different cables and adapters, tried different cleaned 2600 games since I don't have any 7800 games yet, switching through channels on both the console and the tv, re-scanning channels on the tv with a game running on the console, running through a vcr to tv using the composite and then the coaxial output of the vcr, and lastly using an old tube tv.  The ohm resistance on the rf output is 1 with a the meter set at 200 while my 2600 has an ohm reading of 100 with the same setting.  I use to get a reading at .18 with it set above 2k but now it's just reading 1.  Best electronics says it sounds like a motherboard issue and the av mod likely wouldn't make a difference.  I'll try to upload an image of both sides of the board.

20190815_174129.jpg

20190815_174156.jpg

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Posted (edited)

First major test:  See if wiggling the coax on the Atari end does anything at all.  One problem can be that coax connection is old and has bad contact with the connector.  If you don't have a solid connection to the electronics, your coax is just acting as a meager antenna that gets what little signal is radiating to it.  It may be usable by the TV, but won't be strong.  Second, are you using an RF switch box, or a direct connection?  Direct connection is best because it's one less aging component to go wrong.

 

The DC resistance of the 7800 RF box will be close to zero.  So no issue there.  And unfortunately, you only see about a millivolt at best even on a good signal, so the voltmeter is pretty much useless all around.

 

If nothing above helps, next step is to adjust the trim in the RF modulator.  It's accessible through that hole on the side.  It may be a hex head, so try that before jamming a flat screwdriver in and shredding the metal slug by using the wrong tool.  That would be a sure way to make sure it's never usable again.

 

If you manage to get a good screen but there's still no sound, somebody probably screwed up the adjustment there too.  That would be the red trimmer near the cartridge slot.

 

Last resort is a composite mod.  UAV seems to be the cadillac of composite mods.

 

post-66741-0-54705800-1558843166_thumb.png

Edited by ChildOfCv

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Thanks for the troubleshooting ideas.  I am using a direct connection with an adapter so there is no switch in the mix.  I tried wiggling and turning the cable but that didn't do anything for the picture.  One thing I did notice is that simply touching the cable near the rf out caused me to lose color and just be in black and white.  All it took was a light touch to change and I know it's not the connection since I held it further down the cable to move it without losing the color.  I tried adjusting the rf modulator but I lack the correct tool.  I did try to use a plastic qtip minus the ends and also a toothpick but I don't believe either one of those were able to turn the adjuster.  I don't want to use anything metal since that will likely break it and cause interference.  Any ideas on where to get the plastic hex wrench?  I've attached a picture as to what the screen looks like.  Another minor detail is that I only get color on channel 2 and not channel 3.  Thanks again for the help.

20190816_000525.jpg

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Also if you are able to see something appears to interact but just isn't clear, then it is very possible that an AV mod could work since you wouldn't need the RF modulator at all and at least with the Magic Knight and UAV boards, they tap the video signals before the RF modulator so should be good to use either of those.

 

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Thank You both for the links, I should have more information tomorrow on whether or not the adjustments did anything.  Bradley at Best Electronics doesn't believe an AV mod would make any difference and believes it's a motherboard issue.  That being said I believe the mod would be a reasonable step if the adjustments fail to do anything.  Are there video components that can be tested with a multimeter that feed into the RF Modulator?

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Posted (edited)

The only way I could see it being a motherboard issue is if you're not getting +5V out of the regulator.  Find one of the small logic ICs and measure from the bottom right to the upper left to see what voltage the system is getting.

Edited by ChildOfCv

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Can likely just measure the voltage along the main +5 rail and grounds down the middle of the board to see if the regulator is doing its job.

 

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17 hours ago, ChmA2678 said:

Thanks for the troubleshooting ideas.  I am using a direct connection with an adapter so there is no switch in the mix.  I tried wiggling and turning the cable but that didn't do anything for the picture.  One thing I did notice is that simply touching the cable near the rf out caused me to lose color and just be in black and white.  All it took was a light touch to change and I know it's not the connection since I held it further down the cable to move it without losing the color.  I tried adjusting the rf modulator but I lack the correct tool.  I did try to use a plastic qtip minus the ends and also a toothpick but I don't believe either one of those were able to turn the adjuster.  I don't want to use anything metal since that will likely break it and cause interference.  Any ideas on where to get the plastic hex wrench?  I've attached a picture as to what the screen looks like.  Another minor detail is that I only get color on channel 2 and not channel 3.  Thanks again for the help.

20190816_000525.jpg

What game was plugged in when you captured this? Battlezone?

 

 

Edited by -^Cro§Bow^-

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Well... that's too low.  For Maria, the power is on pins 1 and 25, just to make sure you measured that correctly.  It's only the smaller 14-pin and 16-pin chips that use the standard lower-right and upper-left arrangement.

 

Anyway, it's probably time to replace the regulator IC.  7805L.  You can get it from console5.com, among other places.

Edited by ChildOfCv

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13 hours ago, ChmA2678 said:

I'm getting 4.16 volts from the MARIA chip and yes the game is BattleZone.

As @ChildOfCv stated that voltage it too low. But more importantly...and the reason why I asked if that was Battlezone is because it is proof that the logics are fine. The fact that I could still see that was battlezone is proof that the rest of the game logics are fine and the mainboard is technically good. Just need to get those voltages sorted and the 7805 does wearout on the 7800 commonly.

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Thanks for all the great info.  I did measure Maria wrong, had them backwards. So I'm getting 4.96v from Maria, 4.97v from the 7805, the two smaller IC chips on the bottom right are reading 4.96v but the small chip on the top right (tc40138p) is reading 4.5v

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Hmmm.  Well, the D flip-flops are separately regulated and will work as low as 3V, and might be using the "4.7V" zener that the schematic allows for.  So it sounds like power is good, and all indications point to the RF modulator itself.  As noted, you have a playable system and the power readings are good, only the final picture output is bad.

 

Once you get some alignment tools, see if adjusting the tuner helps.

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I got the alignment tools and it didn't help clear up the picture or sound.  The picture instantly lost color when adjusted and I had to power off and back on to get the color back.  The adjustment did make a change but it was just for the worse.  Looks like it's on to replacing the RF Modulator with a mod.  It'll probably be later in the week until I'm able to get that done.  Meanwhile thanks again to both of you and I'll update when I have some different results.

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While I'm waiting for the mod to arrive I did manage to pickup a few 7800 games.  Cleaned each one and get absolutely nothing, just snow on the screen with no static sounds.  I even let it warm up for 15 minutes.  Initially I got one faded snowy Atari 7800 logo and that was it and now not even that shows up.  The 2600 game still loads with the problems previously listed.  I did take a closer look at the MARIA IC and pins 21-24 look a bit odd, the pins 25-28 on the other side look similar as well.

20190818_122209.jpg

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It's possible that MARIA is bad, but that won't cause straight up static.  With a working RF modulator, the static would be replaced by incoherent "stuff" but it would still be a noticeable difference.  So I'd treat that as two separate problems.

 

I can't tell if that's just reflections or actual corrosion on the pins, but what I can tell is that those pins didn't get enough solder (not uncommon).  It probably does at least make contact with the traces though.

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4.5v seems too low. You might try replacing the voltage regulator and see if it gets it up to a solid 5.0V. Voltage regulators are pretty cheap so it is probably worth a try.

 

Mitch

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1 minute ago, Mitch said:

4.5v seems too low. You might try replacing the voltage regulator and see if it gets it up to a solid 5.0V. Voltage regulators are pretty cheap so it is probably worth a try.

 

Mitch

He's getting 4.9 after the regulator.  The D flip flops come before the regulator, and are weakly regulated by a single zener diode.  So replacing the regulator IC won't make a difference on that one chip.

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3 hours ago, ChildOfCv said:

It's possible that MARIA is bad, but that won't cause straight up static.  With a working RF modulator, the static would be replaced by incoherent "stuff" but it would still be a noticeable difference.  So I'd treat that as two separate problems.

 

I can't tell if that's just reflections or actual corrosion on the pins, but what I can tell is that those pins didn't get enough solder (not uncommon).  It probably does at least make contact with the traces though.

With the 7800 cartridge it's not complete static snow, I suppose it is static with varying degrees of lines running in multiple directions without any sound or identifiable images.  I can adjust the RF Modulator to make it complete snow if that helps at all.

I believe it may be light corrosion on the pins since they feel a little rough.  The 20-29 pins also look like they were bent closer to the chip than the rest of them.  As far as I can tell they all make contact with their traces.  I was picking up .4 on a diode reading on one of the questionable pins.  

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Yeah those pins could use some more solder, but the way they look isn't unusual at all for a 7800. The discoloration at the base of those pins, I'm pretty sure if just old leftover flux and could be cleaned away with some 90%+ IPA and a toothbrush.

 

Which mod board did you end up ordering out of curiosity?

 

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I'm trying to order the mod from Best Electronics but he says the video and system needs to be working before the mod is installed so he hasn't filled the order.  Apparently any one of the chips could be bad and since they're on the same buss would have to be removed to be tested which could lead to trace damage further complicating the problem.  So I'm probably going to just find a parts board and see which one is easier to fix and use the other for parts and then mod it.

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The Best mod isn't a DIY kit I didn't think? I thought it was a service whereby you send in your 7800 to be modded with composite output? At least that is what it used to be?

 

But since Best was brought up on this...I have something to share. I literally just placed an order from some stuff with Best on Friday. Got the notice from Bradley what my total was plus the shipping and insurance and paid for it Friday fairly late evening. I was shocked when at about 11:30pm I get an email from Bradley stating everything was boxed up and would get be getting shipped out asap. This was very late Friday night... 

 

My items were in my mailbox this afternoon...

 

This is how my experiences with Best have always been in the past and continue to be. So I'm not sure why some people seem to have difficulties ordering from Best because I've not had any issues myself and this latest order is testament to that.

 

On the subject of AV mods, if you just want composite, then you can either put in a 2600 simple composite mod kit or get more options going with the UAV that @ChildOfCv linked to above or contacting the AA user Magic Knight and asking him about his 7800 s-video boards. While his are s-video only, you can get a cheap s-video to composite adapter to plug off the s-video port to get composite.

 

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