Pitfall II tone adjustment [pot mod]

[Andromeda Stardust]

I added a potentiometer to the RC circuit in Pitfall II:

http://www.youtube.com/watch?v=EWONwis4D9Q

 

The Pitfall II Atari game by Activision relies on an RC oscillator inside the cartridge to produce audio signal which is pumped into the TIA by manipulating the volume registers on every scanline. Due to slight inconsistencies in manufacturing (part tolerances can often vary by up to 20%), the exact pitch of audio varied from one Pitfall II cart to another. I have modded my Pitfall II cart by removing the original 560k resistor and replaced it with a 220k resistor in series with a 1M potentiometer. This yields a range of 220k-1.2M, or a little more than one octave higher and lower than the default sound. The potentiometer was too large to mount inside the cartridge, so I had to superglue it to the back of the cart. Watch as I rotate the dial, raising and lowering the pitch, bending the musical notes to my will, Mua-ha-ha...

 

Sorry if the video's a little dark; my room lighting is poor and the Atari system, cartridge, and control knob are all black.

 

The music sounds normal with the pot tuned about 2/3 of the way. Overall, the tempo seems to be unaffected. What do you guys think?

Edited March 6, 2013 by stardust4ever

[+Yurkie]

I would have tried different resistors till I felt the sound was correct, then closed up the cart.

[Andromeda Stardust]

I would have tried different resistors till I felt the sound was correct, then closed up the cart.

But that takes all the fun out of tweaking it! Twirl the dial and go nuts with it!

 

I do think it sounds best with the pot resistance at 330k or 2/3 rotation between low and high. 220k + 330k = 550k which is approximately what the original 560k pitch was. I've since marked the cart with flat (♭), sharp (♯), and natural (♮) pitch symbols using a silver sharpie.

 

EDIT: Apparently, Atariage doesn't like Unicode symbols...

Edited March 6, 2013 by stardust4ever

[karokoenig]

I'd say go even further. Attach a servo to the pot and make it RC. Looks more Borg when it moves all by itself.

[19rsn007]

I would have done the same thing as the person that reacted the first.

 

I would first measure resistance with the pot meter and after I got the right pitch (like this....sort of

)

After that I would just replace the resistor and put the thing back in it's case again.

Replace the label with a label which isn't messed up by the screws, and enjoy my pitfall 2 game again.

[Andromeda Stardust]

I would have done the same thing as the person that reacted the first.

 

I would first measure resistance with the pot meter and after I got the right pitch (like this....sort of http://www.youtube.com/watch?v=akKQfQ7ssiA )

After that I would just replace the resistor and put the thing back in it's case again.

Replace the label with a label which isn't messed up by the screws, and enjoy my pitfall 2 game again.

The game sounded fine at first. I'm not tone deaf, but I don't have perfect pitch either, so I could care less if the music in one cart is a half a semitone higher or lower than another. I had one of those "what if..." moments and wanted to see what it sounded like if the audio was tampered with. Two points here: I had to mount the pot outside the cart because there's not enough space inside to mount it internally. Then I tried to get the biggest knob I could find to conceal the pot. Atari carts cannot be opened without label damage as you well know; either you attempt to peel the label back and risk destroying it completely or you puncture a couple holes in it to access the screws. Yes, the mod is pointless from a gameplay perspective; I just modded it out of boredom really. It's fun to twirl it for a while, but then you set it back where it belongs and play the game.

[19rsn007]

I had to mount the pot outside the cart because there's not enough space inside to mount it internally.

 

That's why I won't do a pot meter as a final product in the first place.

Just a POT on the bare PCB, but when I found the sound I liked....I would have measured the resistance and placed a resistor with that (or as close as it gets) value so I could really close the cart again.

 

Atari carts cannot be opened without label damage as you well know; either you attempt to peel the label back and risk destroying it completely or you puncture a couple holes in it to access the screws

 

I know they can't be opened without damaging the label.

I allways damage the labels, but after finishing up, I just print a new label just to make it look good again

[Andromeda Stardust]

I didn't mention it earlier but that video you posted looks like the graphics are more detailed than the Atari is capable of. What version of Pitfall II is that in the video?

 

Another thought, I remembered when disassembling the cart, that there were only two components on the board, and I swear that the other half of the RC circuit looked like a diode and not a capacitor. I believe a reverse-biased diode has some infinitesimally small capacitance which can be adjusted by changing the voltage bias. I once built an FM transmitter kit which used a diode-resistor combo in the oscillator circuit and a potentiometer to change the frequency, and it sucked because the frequency drifted all over the place and I couldn't tune my digital radio to lock on the signal. Terrible design IMO. I finally bought a new kit from Ramsey Electronics which used an LC circuit with a plastic Allen wrench to adjust the inductor, and it was rock solid. Set it and leave it and the frequency never drifted. If the Pitfall II cart indeed was using a diode-resistor oscillator, then that would explain why such a large resistor was necessary to output audible frequencies, and why there is so much variation from one Pitfall II cart to the next.

 

Any reverse-biased semiconductor diode displays a measure of voltage-dependent capacitance and can be used to change the frequency of an oscillator by varying a control voltage applied to the diode.

http://en.wikipedia.org/wiki/Voltage-controlled_oscillator#Control_of_frequency_in_VCOs

Edited March 10, 2013 by stardust4ever

[Shinju]

Looks like the 5200 version, and you can take the lables off with a little effort with out damage if you are good with a hairdrier and patience.