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classic electronics troubleshooting :)

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hey guys, I recently got in the mail a rather unique controller for the NES that I have wanted for a while.  it is the "VAUS" or arkanoid controller for the NES.  it is fascinating that it works because it takes a rotary input and encodes it into the 8 bit pulse that the NES gets in from its controllers.  but the cool thing is it does this without any micro controller, and without any A/D Converter chip.


after reverse engineering the circuit today and mapping it out.  I took a photo of the front and back of the board, then superimposed the layout on the back onto the components on the front so i could easier see what was wired to what, kinda like working in eagle, but with photos. The schematic is wired right to keep track of stuff, but the components aren't named correctly, (aka R6, C4, whatnot)




It turns out to be a 556 timer chip (that threw me for a loop because the marking on the chip is "MC3456P", and I had to read hiding in its datasheet that it was a "pin for pin replacement for a NE556", although what is funny is once it was de-soldered, the silkscreen under the chip said "NE556" right on the board).  the 556 was running a fixed "fast" clock, and a slower variable clock based on the rotary potentiometer as the resistor for the timer.  there is a quad NAND chip for some logic, and a 12 bit Counter to count how many "fast" pulses happen within the "slow pulse",  this counter chip feeds a parallel input 8 bit shift register so that the NES can clock out the 8 bits of data about the position of the rotary pot.  it is brilliant in its brute forcing method.


however they are known to be finicky and just stop working for no reason.  but thankfully since it is a hard circuit, it is just down to verifying out each of the 4 chips as to their function.


Mine wasn't working when it got here, (even though it was advertised as good).  ahh well I enjoy a challenge.  The shift register was spitting out data, but it was garble that had no relation to the potentiometer.  testing back up the circuit it turned out the "fast" clock on the 556 was being unstable in its pulse length.  Everything Ive seen is that the 555 series are fairly rugged and bulletproof, so i assumed a cap had failed and wasn't oscillating the timer like it should.  so i wen about de-soldering all the caps belonging to the 556 and testing them out.  they looked good, so then i realized the 556 itself had bricked sometime in the last 20 some odd years.  luckily I had a 556 in my bits box, and replaced the chip (de-soldering a DIP16 package with just a regular soldering iron it an interesting task.


anyway the clock is much happier now, and I will complete the testing and reassembly tomorrow to see if its fixed,  you can see the before and after clock trace on the 556 output pin




I enjoy troubleshooting stuff like this, because there is no help, no place to lookup a solution, just a thing that doesn't work, your test equipment and you.   I'm getting griped at that it is too late, and I need to come to bed :smile:


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