Help with Repair?

[dutch4fire]

Hello,

 

I am in a bind. I used to do a lot of mods and console repairs on older systems, but this was over 10 years ago. I picked up the hobby again and had some successful repairs and mods. As I am learning again, I still have have a lot to learn and several projects I completely messed up. My friend asked if I would take a look at his 7800 as it was no longer working. He provided me with a ton of controllers, games and the console itself. He had tried to repair the RF unit and eventually took it out but left it there. I successfully got it up and working with an AV mod with some sound issues still and not getting pokey sound to work. I worked with all games and no real issues. One day I pulled out the system to test out a multi-cart to work on that issue and try some other games before getting the system back to my friend and no power.

 

I tested and found the power button needed to be replaced so I did that. I also repaired the power jack and got an adapter that is lighter and worked with the system before it did not power on anymore. It still did not power on so I replaced the voltage regulator and it still did not power on. I finally found a video that bridged pins 1 & 2 for an IC chip on the top right corner of the board, and it stayed powered on when plugged in, so I tried that and it worked. A few days later I tried to turn it back on and the light came on, but now there was no sound and no video and then the power light slowly dimmed before dying after making a quiet high pitched tone until it dropped out and the light went completely dim. I unbridged the pins and for some reason now it would power on with the power button and the red light dimmed and powered off again. I then noticed the voltage regulator usually has a grounding plate (I assume) that was missing so I'm not sure what to do about that or the dimming light (is the VR getting too hot?). I replaced some of the caps and tested the rest not finding any issues with those. I tried to remove the IC chip (TC4013BP) and replace it but the ebay chip did not work even thought it did power on but no sound and video. I put the original chip back. I accidently bent the Voltage regulator and tried to remove it but was unsuccessful after getting too tired tonight. 

 

Basically I am at a loss and tired after a month or more of tinkering, digging in forums, relearning some techniques and learning more and more without success. I would just say it's a loss and maybe just not in the cards but here is the thing....I am emotionally destroyed. My plan was to get this system up and running and return it to my friend and his family, the thing is his dad has cancer and he grew up playing with his dad and his brother. I wanted so bad to get this up and running and it was working! BUT something happened and it stopped turning on and now all of this. I  am asking the community not to help me figure out what to do but who would be willing to look at the system and fix my bad work and see what they could do. 

 

I am begging to get some help. I know I could buy a "new" old system but it's not the families original board or system. Please help and I am glad to pay for any service costs. I just want to get this back to him so they can enjoy and have some good memories playing on their old system again. Feel free to offer up some thoughts on what could be some issues but I would really like to see if there is anyone out there. 

 

Thank you for reading. 

 

 

[john_q_atari]

Please post detailed pictures of the front and back of the 7800 pcb. That will give people an idea of what there is to work with.

[+-^CrossBow^-]

Okay... first of all the regulator has to have a heatsink on it or it will go into thermal shutdown. Especially the original ones as they are know to go bad. If you replaced it, then you have to make sure you replaced it with a 1A minimum output regulator. I only buy 2A variants because they aren't that much more expensive and I know I can use them in pretty much anything. If you stuck in a lower rated VR say 500mA rated, then the 7800 is drawing too much current from it and it can't keep up as the 7800 with a stock pokey game will draw at least 710mA most of the time and usually a tiny bit more.

 

The 4013 is a very common chip and console 5 sells them. It is the same flip flop used in the 5200s and does the exact same thing...just controls the on/off state of the console. It doesn't have anything to do with the audio or video really.

 

Now, the original reason why you didn't have pokey, was likely the way you did the audio. Most of the common composite AV kit instructions have you take the audio in a way that doesn't always work properly usually resulting in no Pokey or very very low pokey audio. While not super fancy, the way I've always routed the audio on the 7800 works for everything so far. Basically I'm guessing you removed R5 and the small capacitor at C10? If you did then here is the basics on what you need to do:

 

- replace the cap you removed at C10... or just jumper across the vias as it isn't that critical

- replace the resistor you removed at R5. It was a 6k8 (6.8k) 1/4w resistor. But only solder in the south lead of it back in. South being the side facing towards the front of the console and North being towards the back of the console in this case.

- Desolder the north leg of R6 (It is directly between where R5 was and the red audio tuning coil). Or clip the north leg as close to the base of the mainboard as possible if that is easier.

- You should now have the two resistors next to each other soldered onto the board at only the south ends of them. 

- twist the two north side leads of R5 and R6 together and apply a dab of solder to attach them together at that top.

- get a 10µf 16v there abouts electrolytic capacitor and solder the + lead of that cap to the two soldered together ends of R5 and R6.

- solder a length of wire from the - lead of the cap and solder the other end of the wire to your center posts on the RCA jacks.

 

That is how I do the audio on AV upgrades, except that most of my clients prefer to keep the RF intact so I actually solder on a second pair of resistors at the south ends of R5 and R6 ontop of the current ones and then run the cap and wire to my RCAs that way. Doing this allows the RF audio to remain intact should that be needed.

 

Now, to get back to the power issue a few things to help...

 

If you changed out the original power port for something else. Then I strongly urge you to install a 4000 series rectifier diode in place of the ferrite bead just south of the large filter cap. stripe side towards the cap. That will help with polarity protection incase someone in the future plugs in the wrong polarity power supply in the future. 

 

If you don't have the heatsink anymore for the VR, then you either need to get one, or spend a bit more and get a DC-DC switching regulator instead. I use the Traco branded 2-2450 as that is a 2A output 7805 replacement. Those Dc-DC switchers do NOT require a heatsink as they barely generate any heat at all from them.

 

[dutch4fire]

Here are the pictures I forgot to post in the community thread. 

 

[dutch4fire]

I am just waiting for approval for the pictures submitted, but posted those. 

 

Thanks for responding! Wow thank you for the detailed steps, I will be looking over everything here.

 

 

[+-^CrossBow^-]

Looks like whatever iron was used, the heat was set too high on it. I say this because not only due to the missing via pad on the bottom of the PCB for pin 8 off the 4013 chip but due to missing substrate around the vias. Make sure you still have continuity from Pin 8 off the 4013 to the North lead of R48. It is the resistor that is 3rd from the right along the row of resistors just below that 4013 IC.

 

Do you have pics of the replaced VR behind the cartridge port? I see the bottom side soldering (Against showing too high a heat was used as the top layer of the substrate is removed around those vias) and a pic of the top side that shows the regulator missing? Obviously those leads will have to be removed from the vias and a new regulator installed. Again replace it with a 78S05 as that will provide a 1A output to be on the safe side and you will need to source another large heatsink for it as it does get very hot and will go into thermal shutdown without one attached.

 

The audio in your pics was done exactly as I suspected. So follow the steps I provided on the audio to get that working. You can actually remove the light blue/grey wire that is attached between the two small 74LS chips and the Maria because if you do the audio the way I specified, that composite mod board will no longer be handling it. Also in the pics you provided, I can see two red wires attached at the North end of where C10 used to be by the channel select switch but I do NOT see where the other ends of those two wire went to? The pic you show of the AV jacks on the side, does NOT have those red wires attached. Those red wires were the pokey audio on the original modification. Again that original method still has problems with the way it is done and needs to be corrected as I specified above.

 

BTW where are you located roughly as that would help those of us know who might be closest to you to assist if that is needed.

 

[john_q_atari]

And he's gone. Hopefully he got it all sorted out. I ended up sending him really detailed instructions on how to get the power circuit repaired and working assuming that was the only real issue.