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4QD controllers
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Systems using the expansion connector to double head, either with a pair of Pro-120s or with a pair of VTX controllers simply work when wired correctly.
It is important that the correct size cable is used: wrong wires into the IDC connector can distort the connections causing them to short to each to other. Use an ohm mete buzzer to check for end to end continuity and that there are no shorts between adjacent pins.
Most problems are then down to wrong wiring. However occasionally we see a case where there is a bad connection in the power wiring, specifically in the battery - connection.
Consider the following:
The diagram shows a bad connection in the master battery negative earth. The same thing will apply to a bad earth in the slave.
As current cannot now so easily flow from controller B- connection to battery negative it will try to find alternative paths. One such path is as shown by the pecked arrowed line, through the internal connections of the master controller, through the 0v connection in the multicore, through the slave controller and so back to battery negative.
It doesn't matter if the controllers are fed from separate batteries or the same: battery negatives are usually joined.