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The 7800 Mod Mess Mixup Fixup - pt. 3

Nathan Strum


When we last left the 7800 Mod Mess Mixup Fixup™, alex_79 helpfully suggested:

On 12/23/2022 at 1:12 AM, alex_79 said:

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.

We'll get back to the transistors in a minute. Meanwhile, I decided to rip out the HDMI "mod" as a first step, because: 1) It's horrible, and 2) I did find a short between ground and the composite video output that I traced back to that mess. I didn't take pics of the process, suffice to say it involved cutting wires, desoldering and re-soldering, chewing through a glued-down HDMI connector with side-cutters, and lots and lots of prying to remove epoxied-on crapola. Here's the debris field:


Good riddance to bad rubbish!


The "mod" was stuck down to the RF shield with some sort of clear, sticky goop. Still haven't cleaned it all off yet. They also soldered ground wires to at least four places on the RF shield. For some reason. Still have to de-glob those as well.


All of the prying resulted in some minor denting. I'll try to smooth them back out before final reassembly. And maybe try to remove those fingerprints.


They'd replaced the stock 2200µF cap with a big, honkin' 4700µF one. They'd also screwed a second voltage regulator to the RF shield to supply power to the HDMI "mod".


I reconnected the basic composite + audio mod, just enough to make sure the 7800 still worked (to the degree that it did before).


Here's the bottom of the 7800, where they attached the TIA audio:


I still need to clean off the snot (or whatever) they used to glue down the HDMI cable. There are also a few components they removed when they ripped out the RF module. Jesse (aka -^CrossBow^-) has been kindly answering a bunch of my questions and made some helpful suggestions about what needs to go back and what doesn't, in preparation for the UAV mod that will be going in there.

Unfortunately, even though removing the HDMI "mod" did fix the short, it didn't fix the joystick problem.


After this, I also installed a recap kit from Console5. I didn't expect it to do anything about the joystick issue, but it was on the "to-do" list.



Yes... the whole case needs a serious cleaning. Also patching - there are a lot of holes in it.

Also on the "to-do" list, was getting rid of that obnoxiously bright "rainbow" LED:



By comparison, here's the original LED in my stock 7800 (I had to adjust my iPhone's exposure to make it appear as dim as it does in real-life):



Here's the replacement red LED. Better, but still too bright:


-^CrossBow^- said he usually adds a 460Ω resistor, in place of the stock one to drop the brightness down. I tried that, but felt it was still too bright:



So I stepped it down further, with an 800Ω resistor:

Much better! More like a power indicator, less like Laser Pink Floyd.


Now then, onto the transistors. Both of the ones in question (labeled Q6 and Q8 on the board) are 2N3906 PNP transistors, but I didn't have any spares lying around, so I couldn't just replace them.


So I desoldered those two transistors from my own 7800, so I could test and compare them with the ones from John's console.


I marked them to keep track of which was which. John's are on the left (white dots), mine are on the right (orange dots):

It's been awhile since basic high school electronics :ponder: , so I had to look up how to test transistors with a multimeter.

So here are the results from my 7800:

Q6	Collector to Emitter	OL
Q6	Emitter to Base		.691v
Q6	Collector to Base	.686v
Q8	Collector to Emitter	OL
Q8	Emitter to Base		.689v
Q8	Collector to Base	.682v


Well, that all looks good.


And from John's 7800:

Q6	Emitter to Collector	11.6Ω
Q6	Emitter to Base		.703v
Q6	Collector to Base	.701v
Q8	Collector to Emitter	67.9Ω
Q8	Emitter to Base		.659v
Q8	Collector to Base	.659v


Well, that ain't right.


So, I transplanted my transistors into John's 7800, and re-tested it:


And lo-and-behold, that fixed it! The 7800 Pro-Line buttons now worked properly in 2600 and 7800 modes. Much easier than having to desolder and swap a RIOT.


At least, it was finally working... for all of five minutes. Then I desoldered my transistors, and put them back in my 7800 where they came from. I've ordered some new ones from Mouser (along with some other parts), and will install those... next time.


Published 1/3/23 at 11:46PM

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Wow - thanks for the detailed description of the 7800 restoration process! :o  This thing was in much worse shape then I though! :P  


Glad to hear it's the transistors that are causing the joystick issue! :thumbsup:  


Thanks Nathan, looking forward to having this 7800 in my collection! :D 

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I will take note of those 3906s for the future. The only controller issues I've had from 7800s were always the TIA at fault with stuck trigger buttons since the TIA controls that in 7800 mode. But yours was sorta the opposite in that they were working in 7800 mode and not in 2600 mode properly. So yeah...I will keep track of that.


As for the resistor value, it also depends on how much actual current is coming into that LED. As LEDs 40 years ago weren't what they are today, they took in more current and looked more dim. Now, they require much less and most LED calculators will state that only a 150Ω is needed. All of the ones I buy tell me they require a 150Ω and that is actually was was installed originally in the 7800 and 5200 as well way back when. So I put the 460Ω in there to be sure it isn't going to dye out quickly with a different colored one in place.


Remove all of the old wiring from the audio stuff since you won't need it once the UAV mount board is in place and soldered down the south leg points of R5 and R6. Just make sure you jumper that cap they removed at C10 so you can restore the external audio trace.

WHOA!!! What the hell is that huge inductor choke doing there in place of the audio tuning coil?! That is NOT stock and I've never seen that before. It likely won't even be needed since again, with how the audio will ultimately be done, that won't even come into play, but yeah...what the hell?


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My Pro-Lines weren't working in either mode - the only way they'd work in 7800 mode is by booting with a CX-40 plugged in, then swapping it with a Pro-Line. Anyway (spoiler alert) the replacement transistors have arrived, are installed, and now the Pro-Lines work properly.


From my (admittedly limited) experience with LEDs, the 800Ω shouldn't do any harm, just make the LED dimmer (which is the sole intent here). If it causes things to 'splode, John has my address. ;) 


Yeah... that choke is funky. It was clearly installed when the mod was. I wasn't sure I should do anything about it since everything is (presently) working. I figured if I had audio issues post-mod-install, then I'd dig up a replacement. I'll be replacing C10 when I remove the old mod. I'm finally to the point now where everything else has been restored enough so I know the 7800 is working, so I can rip it out, and put in the UAV.

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