Tron MCR reset circuit doesn't (purple wire).

supertechnoboy

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My Tron won't boot, and the problem is an inoperative reset circuit. It was working a couple of weeks ago, and now does not. The purple wire does not go from low to high on boot. If I ground the base of the last transistor in the reset circuit, the output is pulled high by the pull-up resistor, and the board boots fine. I tried shotgunning the LM3900 since I had one laying around, but no change. I also checked all the resistors and diodes in the reset circuit.

Anyone know the most common cause of reset circuit failures besides the LM3900? (and please don't tell me to cut the purple wire:))
 
are you sure you were grounding the base? Grounding the base should pull the collector .5~.7v higher than the base (low), causing the reset to go low, not high.
Did you check q201 itself, and the 4n28 optocoupler, and all the caps?

That's not really a pull up resistor, its a current limiting resistor, so when the transistor is conducting (base grounded), it limits the current that it can draw. When the base is low or 0v, you should see approximately .5v at the collector (aka reset-not).
 
Pretty sure I grounded the base to make it boot, I'll be a little more precise in my troubleshooting documentation next time:) I did check Q201. I have not checked the caps--about all I can do there is look for shorts. I have not checked the optocoupler. What should the output look like?
 
either the same, or opposite of the input.... I know, not very precise. But, i think you should see a change as the input changes.
 
Looks like C201 failed open, blocking the output from the optocoupler to the pump charger circuit. If I short it, everything works fine. I'll pick up another and report back.
 
Looks like C201 failed open, blocking the output from the optocoupler to the pump charger circuit. If I short it, everything works fine. I'll pick up another and report back.
OK, bad troubleshooting on my part. C201 was just fine.

Once I dug out my scope, the problem became pretty apparent. The optocoupler should be putting out a 5-volt square wave, and my output was a solid 5 volts. The issue was actually a burnt resistor on the input side that was still conducting, just not at the right resistance. I replaced R201 and everything works great again.

In the middle of all of this, I found this manual: http://arcarc.xmission.com/PDF_Arcade_Bally_Midway/Bally_Midways_MCR_II_System_(0986-0300-0000).pdf

The entire reset circuit is explained in great detail on page 15 (pdf page 16), and if that wasn't enough, waveforms for each section of the circuit are shown on the last page. I shall never worry about MCR reset issues again.
 
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