Hey guys,
Sadly my 2600 heavy sixer (Sunnyvale ca) randomly started having the fire button always on issue. It worked totally fine a few months ago and has been sitting in a box. I was getting ready to give it to a friend and I wonder if static shock killed the input chip. I did also find an iffy cap in there, does it look bad to y’all?
I read about testing pin 6, I need to get around to that…
This thing has been the most reliable console I’ve ever had other than this!
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I’ve never worked on Atari consoles but you got me curious.
I did a Google search for schematics, not surprising, found many variants. So I don’t know if this one is your board, but here’s the schematic for one with some of my colored markup:
In working operation the Red arrow is apparently the “fire” button on the joystick and to activate the function, pressing the fire button ties Pin 6 to Pin 8 (blue arrow). Pin 6 is normally pulled down (to ground) by that circuit I have circled in dark red. Pin 8 has 5v+ generated by part I have circled in magenta. So pressing the button sends 5v+ first through that dark blue circled area which I think its doing some debouncing (cleaning up noise preventing accidental quick/up/down/up/down in the micro seconds of the fire button is pressed). If any of those capacitors or that diode is shorted, it would send 5v+ constantly “holding down” the fire button.
Assuming all of that is fine, the next area I’d look at would be that dark red circled area. This is where the pull down to ground comes from making sure pin 6 is low and the fire button is “off” or “not pressed” if any of this is floating, it could show up as “not ground” and the main IC would think the button is pressed.
Next would be the those 4050 ICs circled in green. These are CMOS buffers and CMOS ICs ARE EXTREMELY VULNERABLE TO STATIC DISCHARGE. Their job is just to take an input of some voltage and output a single clean digital signal of either 1 or 0. There is one buffer for each fire button (left and right joysticks).
Finally the fire button output of that 4050 buffer is delivered in to the main CPU that A201 TIA PAL (my schematic may be from a European model).
If you had this disassembled on a bench and had a voltmeter, you could get a good idea of where the problem is in about 10 minutes.
Getting to that! I have a multimeter and an o scope. I just need to know the test points which I think now I do. Thanks immensely for the help! Hoping JTS that green cap and not the 4050 chip…
Its just odd to me that 6 months ago it wormed perfect when i put it in the box. That’s what makes me think the cap finally gave out.
Also interesting, when using paddle controllers the fire button works fine. They use a different pin I think since those are 2 controllers on 1 port.
Hoping JTS that green cap and not the 4050 chip…
That green cap I think is a mylar capacitor and will cost you maybe 5 cents at retail (and .00001 cents in bulk).
That 4050 is also dirt cheap. Maybe 50 cents to $1 USD at retail. You’ll pay more in shipping costs than for the part. Today’s CMOS ICs are a bit more robust against static discharge than those made in the 1980s, but don’t risk it when you do the replacement. Make sure you use a grounding wrist strap or the like when you desolder the old 4050 and put in the new 4050, partially to protect the 4050 but really to protect that CPU which will probably cost you closer to $11-$20 (just a guess) to replace if it dies.