Atari 7800 parts help!!!!

[CPUWIZ]

https://www.elprocus.com/capacitors-types-applications/

[Wolfrin]

The one next to the black one is burnt out due to me having the positive and negative power supply wires wrong. I watched a YouTube video and found out the right orientation of positive so I switched them to the right terminals. I think when power went through the board when it was wrong it blew some capacitors and the zener diode. Let me check this other thing out though.

[Wolfrin]

when there is no power connected i placed the red lead on the bottom lead of cr5 and the black lead on the top of the black resistor. I get -60.6 and when i put the black lead on the bottom of the black resistor i get -60.6.

When I put the red lead on the black resistor and the black lead on cr5 i get nothing on the meter. I try swapping to the top of the cr5 to get any reading and i get nothing on my meter.

Also when I put the red lead on the top and bottom of the black resistor and the black lead on the cr5 top lead i get nothing.

[ChildOfCv]

3 hours ago, Wolfrin said:

The one next to the black one is burnt out due to me having the positive and negative power supply wires wrong. I watched a YouTube video and found out the right orientation of positive so I switched them to the right terminals. I think when power went through the board when it was wrong it blew some capacitors and the zener diode. Let me check this other thing out though.

The capacitor in question doesn't care about polarity though.  Only a high voltage could cause that damage.  I don't know the voltage rating on that particular capacitor, but you'd have to exceed the normal operating voltage by A LOT to destroy it.  And if that's the case, I'd be worried about the entire system.

 

The large canister above those two components is the capacitor that hates reverse polarity.  Typically they show some sign of damage, which can range from a slight bulge to literally launching themselves into the ceiling, depending on the level of abuse.  Same also applies to over-voltage situations.

 

3 hours ago, Wolfrin said:

when there is no power connected i placed the red lead on the bottom lead of cr5 and the black lead on the top of the black resistor. I get -60.6 and when i put the black lead on the bottom of the black resistor i get -60.6.

When I put the red lead on the black resistor and the black lead on cr5 i get nothing on the meter. I try swapping to the top of the cr5 to get any reading and i get nothing on my meter.

Also when I put the red lead on the top and bottom of the black resistor and the black lead on the cr5 top lead i get nothing.

You're getting negative resistance on the ohms setting?  With no power?

 

Also, what is the orientation of CR5?  On mine, the band faces the power plug, not the controller plugs.  Also, perhaps you should use that terminology instead of "top" or "bottom" since they can be relative to how the board is facing.  Anyway, the side to measure faces the power plug.

 

If you're getting no reading at all from the black inductor to the power plug side of the CR5, then there's a bridge out somewhere on that road.  The electrical connection starts at the plug, goes through the black inductor, then over to R53 (see that diagonal arrangement of resistors to the left?  R53 is second from the top left.  Green, brown, brown, gold bands).  After going through R53 it reaches CR5.  If any components are dead on that path, or if the copper traces on the board are broken along that path, then you won't be getting power there.

[Wolfrin]

I have the system plugged in and the power port reads 9.12v.

 

When I put the black lead on negative terminal of power port and put the red lead on the black inductor leg facing the power plug i get 8.96v.

When I put the black lead on negative terminal of power port and put the red lead on the black inductor leg facing the controller ports i get 8.72v.

 

When I put the black lead on black inductor leg facing power port and put red lead on r53 leg facing controller port i get nothing

When I put the black lead on black inductor leg facing controller port and red lead on r53 leg facing controller port i get 8.35v.

When I put the black lead on r53 leg facing controller port and red lead on black inductor leg facing controller port i get -8.35v

 

With red lead on r53 leg facing controller port and black lead leg facing power port i get 8.32v

I get no reading trying the other way.

 

red lead cr5 leg facing power port with r53 legs facing power or controller port i get 0.23v

When I switch to use the black lead on cr5 leg facing controller port and r53 legs facing controller or power port i get -0.23v.

 

When i put red lead on cr5 leg facing controller port and black lead on cr5 leg facing power port i get 0.25v.

When i put red lead on cr5 leg facing power port and black lead on cr5 leg facing controller port i get -0.21v.

 

9.13v is going into cr5 via black lead on negative terminal of power port and red lead on cr5 leg facing controller port

 

My cr5 band faces the power plug too

 

I just double checked and the cr5 isnt reading properly at all in ohms and c57 is completely missing from the board and i dont know if the big capacitor is bad or not. There is no sings of bulging and no leaking fluid.

Edited July 3, 2019 by Wolfrin
forgot a word

[ChildOfCv]

53 minutes ago, Wolfrin said:

I have the system plugged in and the power port reads 9.12v.

 

When I put the black lead on negative terminal of power port and put the red lead on the black inductor leg facing the power plug i get 8.96v.

When I put the black lead on negative terminal of power port and put the red lead on the black inductor leg facing the controller ports i get 8.72v.

That is okay, but not great.  If the burned capacitor is now shorted out, it may be pulling power.  If you at least remove it, you may see some improvement.

53 minutes ago, Wolfrin said:

 

When I put the black lead on black inductor leg facing power port and put red lead on r53 leg facing controller port i get nothing

So that will be (almost) a direct connection.  An inductor is internally just a long string of wire wound around and connected on both ends.  So for low voltage DC measurement, it's just wire.  So the voltage reading between those two points should be about zero.

53 minutes ago, Wolfrin said:

When I put the black lead on black inductor leg facing controller port and red lead on r53 leg facing controller port i get 8.35v.

Okay, so everything after r53 is shorted to ground.  You have about 0.4V powering the 4013.  It needs more.

53 minutes ago, Wolfrin said:

When I put the black lead on r53 leg facing controller port and red lead on black inductor leg facing controller port i get -8.35v

Yeah, DC voltage read in reverse will just be the negative of the DC voltage read forward.

53 minutes ago, Wolfrin said:

 

With red lead on r53 leg facing controller port and black lead leg facing power port i get 8.32v

I get no reading trying the other way.

In this case, you should see -8.32V.  It may be that you just didn't get good contact.  Old solder joints can get an oxide film and can prevent conduction.

53 minutes ago, Wolfrin said:

 

red lead cr5 leg facing power port with r53 legs facing power or controller port i get 0.23v

For power side of cr5 to one side of r53 (I think it's the controller side), you should read very close to zero volts.  For power side of cr5 to the other side of r53, according to your other readings, it must read closer to 8.3V.  Could you retry this one?

 

53 minutes ago, Wolfrin said:

 

When i put red lead on cr5 leg facing controller port and black lead on cr5 leg facing power port i get 0.25v.

When i put red lead on cr5 leg facing power port and black lead on cr5 leg facing controller port i get -0.21v.

This reading is consistent with what we've determined so far.

53 minutes ago, Wolfrin said:

 

9.13v is going into cr5 via black lead on negative terminal of power port and red lead on cr5 leg facing controller port

I think you're still backwards on which is the negative terminal of the power port.  CR5's controller leg is connected directly to the negative side.  So the terminal you read must be the positive terminal.  In your picture, the positive one is on the left and has that thick trace going to the inductor.

 

Anyway, we have now confirmed through voltage measurements that there is a short to ground.  Most likely it's CR5.  But if the 4013 has been damaged, it may also be a problem.

 

An easy way to find out is to remove CR5 entirely and then see what the voltage is between the two holes where it was soldered in.  It should be close to the 8.3V you've been reading.  The 4013 can easily handle the voltage it's now going to get, so don't worry.  In fact, I'm not even sure why they though the zener was necessary.  Maybe they were planning to use a TTL chip but decided the CMOS was cheaper.

 

http://pdf.datasheetcatalog.com/datasheet_pdf/motorola/MC14013BCL_to_MC14013BD.pdf

 

Anyway, if the voltage is still too low at those solder points, then the 4013 is probably dead too.

[Wolfrin]

i have the power connectors backwards let me fix that first

[Wolfrin]

I switched the terminals.

I also found the leg of the conductor facing the power port had no solder and was moving freely. I added solder and plugged in power and it works now.

I wont be able to see it run until i get a kit to add av to it because i removed the old connection due to not having any way to connect it.

Thank you very much for your help i would have never learned any of this without your help.

Edited July 3, 2019 by Wolfrin

[ChildOfCv]

Oh, so you didn't have to remove CR5 then?  Cool.  Well, still remove that burnt capacitor and replace it.

[Wolfrin]

I will I ordered a lot of 20 for them for 3 bucks so i will put the c57 back and replace the other one.

Thank you again.

Edited July 4, 2019 by Wolfrin

[Wolfrin]

I got the av mod installed and I hooked up a controller. The joystick works like in space invaders i can move left to right but the fire button does not work. Any suggestions on what it could be?

[ChildOfCv]

Well, first check continuity of pin 6 to 8 on the joystick input while holding the fire button.  If that passes, make sure pin 6 (player 1) reaches pin 35 of the TIA chip, and pin 6 (player 2) reaches pin 36.

[Wolfrin]

I found out it was a bad solder joint i re flowed the solder and added more and it works fine now. Thank you for this advice I checked this first and I had continuity just a loose solder joint. It is kinda weird this is the second time I have found a spot on the board that didn't have much solder or there was missing solder. Could it have come from the manufacturer like this?

[+DrVenkman]

14 hours ago, Wolfrin said:

Could it have come from the manufacturer like this?

Sure. These boards were by and large wave-soldered by machines. Stuff happens. It was probably fine for a couple of decades before the solder joint loosened and turned into a cold joint. 

[Wolfrin]

Yeah that makes sense I didn't really think of it that way.

[+-^CrossBow^-]

Yes, many of the 7800s I've worked on especially the later made units don't even have much solder if any that comes up through the via on the top layer of the PCB where the components are. Now granted if they are actually through hole vias, then solder on bottom and top might not be absolutely needed, but I feel better about the component connection when it does. In many cases, I will look over the entire board when I see one or two and touch up bothersome looking ones just to avoid issues in the future for my clients.

 

[+DrVenkman]

6 hours ago, -^Cro§Bow^- said:

Yes, many of the 7800s I've worked on especially the later made units don't even have much solder if any that comes up through the via on the top layer of the PCB where the components are. Now granted if they are actually through hole vias, then solder on bottom and top might not be absolutely needed, but I feel better about the component connection when it does. In many cases, I will look over the entire board when I see one or two and touch up bothersome looking ones just to avoid issues in the future for my clients.

 

No matter how bad it is, will it ever be as terrible a soldering job as this poor Breadbin C64 I've got? Look at the factory "soldering" on this board! This is one of the CIA peripheral interface chips but it's the same for nearly every chip and component on the board. Ugh.

 

[+-^CrossBow^-]

Yeah that is scary! But I've seen a few resistors and other passives off the 7800 with a via or two looking like that. I even install caps a tad highwater so that I can get plenty of solder on the bottom of the PCB, and then use my iron on the legs and pads on the top of the PCB to draw the solder back through the via to both sides.

[+DrVenkman]

Ironically enough, that Breadbin worked fine for 20 years after it was made and when it went into storage 12 - 15 years ago. I accidentally killed it with a bad PSU two years ago when I pulled it out to mess with it. Silly me, I had no idea C64 bricks were deadlier than Atari "Ingot" PSUs. One flick of the switch killed two 4164 DRAM chips, the SID (sound chip) and the PLA (basically the MMU + glue logic). It might also have killed two logic chips I removed, socketed and replaced just to be safe as part of my troubleshooting. On the plus side, the lack of good soldering made removal of the dead chips pretty easy, lol. 

[Wolfrin]

I have one last question about my Atari 7800. I have the av mod installed and everything hooked up the way multiple manuals have told me to hook them up. When I turn on the system it takes about 5 seconds for the display to show. Then when I turn it off you can see the screen for about two then it goes black. Is this normal or do I have to look for something? I have watched a couple YouTube videos and when they turn the system on the screen displays immediately. I don't know if its a faulty av board or some capacitor.

[ChildOfCv]

Hmmm... did you replace any of the polarized capacitors?  The smaller one should be 220uF.  It comes just after the voltage regulator.  If you, maybe, upped its capacitance by a bunch, it could potentially cause a longer turn-on time as well as a hold time that delays turning off.

 

But no, it's not normal to see those kinds of delays.  When I hit the on/off button on mine, it's almost instant.

 

Modern TVs can take longer to show a picture though, since they are completely digital and have to play with the signal until they're happy with what they see.

[Wolfrin]

I checked the schematic of the Atari 7800 and all the capacitors are at the right values.

[ChildOfCv]

Does the LED turn on or off right away after hitting the on/off button?

[Wolfrin]

Honestly never tried this but I'm going to check it now. I checked it out and the led turns on and off instantly.

[+DrVenkman]

31 minutes ago, Wolfrin said:

Honestly never tried this but I'm going to check it now. I checked it out and the led turns on and off instantly.

As suggested above, it almost sounds like your TV isn't locking onto the signal immediately. What sort of display are you using currently? An old-school CRT should give you a picture in a second or less, I'd say. An LCD or LED, especially going through any kind of HDMI converter, will take longer.