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788T DE-15 Connector DC Out Power Problem

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I'm making a remote roll button for my 788T with a built in LED.  The LED flashes when the machine is rolling thanks to the DE-15's logic out pin hooked to an Arduino. 


The whole circuit works perfectly fine when tested on my breadboard and powering the Arduino with it's 3.5V externally.  I really wanted to power the remote roll button via the Ext. DC OUT pin on the 788's DE-15 connector so I can do everything with one VGA cable, but I'm having some trouble with the power that it outputs.  I am using a DC to DC converter to step down the 13V coming out of my 788T's Ext. DC OUT pin to a usable 3.5V for the Arduino.


When I connect the circuit to any power supply (including connecting it to my Meon directly) it works as intended and the voltage converter powers up and outputs the 3.5v.  When getting power from the 788T's External DC OUT pin, the voltage converter does not power on.


I thought the DC OUT pin was just a direct loop through of the external power that the machine is getting, but it looks like something is different about it.  I can power the circuit fine off of my Meon which is powering the 788, but it doesn't like something about the power that the 788 is giving out.


When I probe the DC output of the 788's DE-15 it reads 13.5 volts DC as expected.  But the voltage converter doesn't like something about it.


Anyone know what might be different? 


This is the voltage converter I'm using: https://www.amazon.com/gp/product/B076H3XHXP/ref=ppx_yo_dt_b_asin_title_o06_s00?ie=UTF8&psc=1

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I had trouble with one of those dc-dc converter modules before where it worked fine with an alkaline battery but didn’t like an iPower. The best I could come up with was related to the load provided by the internal impedance of the battery. Inserting a 10 ohm resistor in line with the iPower made it work but was obviously wasteful in terms of power efficiency. Maybe experimenting with the load the converter sees could be helpful. 

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