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The 7800 Mod Mess Mixup Fixup - pt. 2

Nathan Strum


So when we last left "John's" 7800 ;) , I still needed to test the audio, and look into the joystick problem.


To hear audio on my Sony monitor, I have to munge any stereo signals together into mono (each input has only a single audio connector). Since this 7800 has a stereo mod (or two audio outputs anyway), I needed such an adapter. My other two inputs on the monitor already have adapter cables plugged into them that I made, but now I'm using the third input, and I want to minimize cable swapping, so it's time to make another one.


You shouldn't just use a stock "Y" cable though, since you should attenuate the two signals when you mix them together. "Y" cables are meant to split, not combine. A simple mixer circuit can be found in this article: https://www.epanorama.net/circuits/linemixer.html


I've used this to make my own simple AtariVox mixer for my 2600: https://www.cheeptech.com/atarivox.html


Effectively, I'm doing the same thing here. But instead of mixing four channels down to two, I only need to mix two into one. So I only need half the circuit. And I'll just make it inline, using a "Y" cable and a couple of 10K Ohm resistors:



Now, I just had these sitting around in a box of parts. So how did I know these were 10K Ohm? Why, because I've memorized resistor color-coding, of course!

Not really. I used my recently-purchased Fluke multimeter on them. Then I looked up the color coding to confirm it. :roll: (I bought the Fluke multimeter because my ancient Radio Shack one was literally falling apart.)


Man, I miss the days where you could just walk into your local Radio Shack and buy a pack of resistors. :sad: 


So, let's make a cable! First, chop it in half, and strip the ends. I use a stripper similar to this. But mine came from Radio Shack. :sad: 


The two ends on the left are sitting on top of each other - but there are two wires there. Bad photography.


Twist the ground (bare) conductors so you can solder to them.


Next, strip the ends of the center conductors (the red and white wires), and re-strip two of the cables to expose more wire because you realized after-the-fact that you didn't leave enough room to put on the heat-shrink-tubing :roll: :


Before getting too far, be sure to slide on larger diameter pieces of heat shrink tubing (blue, in this photo) that are going to cover up everything later (and yes... I've had to desolder things because I've forgotten):


Solder the resistors to one side of each center conductor. Leave one of the resistor leads intact so it's easier to work with (I use some "helping hands", also from Radio Shack):


Before soldering the other side, be sure to slide on the smaller heat-shrink tubing (yellow here) that will cover the resistor and exposed wire:


Then shrink it into place. Nice and neat.


Since the bare conductors will be too short to meet, bridge the gap by soldering one side of each conductor onto some spare wire. I stripped off the insulation, since it's all going to be covered anyway and it's easier to work with:


Trim it to length, and solder the other ends:


Slide the heat-shrink tubing over, and shrink it into place! Job done. You now have a proper stereo-to-mono-munging-mix cable! They make great Christmas gifts!


So, time to plug the 7800 back in and test the audio. Just one little problem... with 7800 Pro-Line controllers plugged in, I can't get past the Concerto's start-up menu. Pressing the fire button does nothing:

To get it to work, I had to plug in a CX-40 joystick, which works fine.


Once I got past the menu and was able to load the 7800 Utility Cart binary, I plugged back in my 7800 Pro-Lame controllers. Apart from a sticky right button on the right controller (it is the controller - not the console), both controllers worked fine.

That said, both joysticks are terribly sloppy. I really need to get rebuild kits from Best. I'm also thinking of converting my old Gravis Mac GamePads over to a switchable Sega/7800 controller. :ponder: 


Anyway, the rest of the switches and buttons all worked fine.


And the whole point of all this was to test the audio. So I did, and it worked:

Just pretend that's a video, and that you can hear sounds.


Now that I knew the audio worked, it was time to dig further into the joystick issue. Weirdly, while the Concerto still refused to recognize the Pro-Line fire button at startup, my Pole Position cart did. It worked just fine.


So maybe it was the Concerto? Well, most of my carts are still packed away, so I tried my Harmony Cart.


Same results - Pro-Line buttons wouldn't work, but the CX-40 would. (As an aside, With the Concerto, I can't navigate without a working fire button. But I can with the Harmony cart, using Select and Reset. But it's being worked on... just not finalized yet.)



Since I had the Harmony plugged in, here's the 7800 showing the 2600 Colors binary. Yeah... that's really dark...


Since the Harmony had the same issue, I thought, "Okay... maybe it doesn't like multi-carts." Some weird issue with the HDMI converter and the cart drawing too much power or something?


So I plugged in my go-to lowest-common-denominator: 2600 Pac-Man. But the Pro-Line button wouldn't work there either. I could start the game with Reset, and found that the directions worked just fine, but the fire button was a no-show.


Yet again, with a CX-40 plugged in, the fire button worked fine.

To try and further troubleshoot this, I popped in the 2600 Testcart. Since I own the cart (rather than loading it from a multi-cart), that would remove the multi-cart variable, and I could leave the Pro-Line plugged in, since after just a few seconds, the Testcart switches to the joystick test screen.


The left Pro-Line, left fire button reads as: Left Keyboard * and Paddle 2 far-right:


It reads the left Pro-Line, right fire button as Paddle 1 far-right:


For the right Pro-Line, it reads both buttons as it should: Left fire button:


And right fire button:


It shows a CX-40 fire button correctly in both ports, left:



And right:


On my 7800, both Pro-Lines read as they should. left Pro-Line, left button:


Left Pro-Line, right button:


Right Pro-Line, left button:


Right Pro-Line, right button:


So John's 7800 is reading Pro-Lines plugged into the left port all wonky. Could be the RIOT, but I think before I go ripping that out, I'm going to completely remove the HDMI-converter mess, leaving just the AV mod in place, and see what that does. It's got to go away sometime, and now seems as good of a time as any.

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To try and further troubleshoot this, I popped in the 2600 Testcart.


The left Pro-Line, left fire button reads as: Left Keyboard * and Paddle 2 far-right:

It reads the left Pro-Line, right fire button as Paddle 1 far-right:

For the right Pro-Line, it reads both buttons as it should


It shows a CX-40 fire button correctly in both ports



That's weird. Seems like the "two buttons" joystick functionality for the left controller is enabled in 2600 mode, which shouldn't be possible normally.
Two button mode is achieved by strongly pulling up pin 6 of the joystick port. To do that, for the left port, both the TIAEN signal (green) and pin 22 of the RIOT (PB2, blue) need to be LOW, to switch on the two transistors (red and yellow).
I'd check if those two transistors are good, and if there is any short in the areas near to those components before trying to replace the RIOT.



A faulty RIOT alone (e.g. pin 22 stuck LOW) would only affect 7800 mode, 2600 games should work normally. (That's because TIAEN is HIGH in 2600 mode and turns off the first transistor).

Edited by alex_79
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I plan to solder up some more of my 7800 UAV mount boards this weekend Nathan and will let you know when I've got them ready. I know you are waiting on me so you can install the UAV into this 7800. BTW..on that LED, they likely didn't replace out the original 150Ω resistor that fed the original. One of the things I do with LED swapouts these days is install a 460Ω resistor to drop that current way down and make the LED not laser blinding bright anymore. On some of them, I will actually take the LED and run it on sandpaper on the top of it to flatten it and make it diffuse the light to also lessen that laser point brightness. 


BTW, I'm pretty sure I know who did that HDMI mod work and I've criticized it before. Mainly because of the fact that they were powering it from the 7800s 7805 which, isn't designed to drive that much stuff since those converters want a 1A output supply by themselves. But I also criticized the cost. I now that on the one I ripped out and replaced with a UAV several years ago, that they paid about $250 for their HDMI modded 7800. 


So yeah...I will be sure to shoot you a PM when I've got those mount boards done.


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