This page was originally written to cover the original NCC series controllers which evolved to become the modern VTX. The information therefore applies to both.
Common faults on the NCC series were - not common. The VTX is surface mount, so slightly less prone to some of the accidents that befell the earlier NCC. So a full fault finding chart is not a practical proposition. But there are a few faults which occur, mainly because the 'bare board' style of the controller makes it susceptible to certain abuses.
This page will lists only the most usual faults: additional fault finding information present in the electronic service manual.
If it's an NCC, you will probably need to identify the issue number of the controller. There have been fewer changes on the VTX, so there is no similar history. However there is a date code (a number such as 08a, written on the board in ink. In this case 08 is the year and 'a' is January). This date code can be important if you ask for help.
See NCC series controllers, key components to identify the zener.
Does the reversing switch have any effect?
Reverse switch has no effect
I.e. the controller goes in the same direction whether or not you operate the reverse switch. In this case then the switch itself, or the wiring to it, is (or has been) faulty. The reversing switch should apply (battery) voltage (from pin A) to pin C. When the switch is open, Pin C should have no voltage present on it. A voltage higher than about 5v will engage reverse.
If there is no voltage here regardless of the switch - trace back the wiring through the reverse switch to find the reason.
If there is full battery voltage present, regardless of the switch, then there is a short in the wiring. Maybe you have used the wrong size (too big) wire in the connector - that can cause shorts.
Dome NCC series have a diode fitted between ignition (pin b) and reverse (pin c). If present, check this has not been blown. It was fitted to Issue 16 NCCs but not on Mk 2 NCC or VTX, because some customers blew it!
This diode is only useful when the controller is wired for push-button operation. In most applications, it can be removed - but make sure the lead which is top soldered is left in situ or reversing won't work.
Reverse switch works, but controller is dead in one direction.
Here, the reverse switch has an effect - probably the controller is dead in one direction. If the reverse switch has an effect, then it's working and some signal is getting through.
As you advance the controller from zero speed, one relay should click in forward direction and the other relay should click if reverse is selected.
Note: the controller will not reverse properly if the output is open circuit: it requires some sort of load (resistor or motor) or the highside current limit engages and prevents correct operation.
Usually this means the controller is working but the gain is set too low. On a standard controller, reverse is at half of forward speed. If the gain is set low, forward speed is low and half of this low forward speed may not be enough to get the motor moving.
Another theoretical possibility is that there the relay contacts are dirty. However these relays used do have high contact pressure and tend to be self cleaning as a bad contact will, initially, get hot - which clears the problem before it shows up.
The main capacitor can hold charge for more than an hour when the NCC is disconnected from the battery, so this can happen even when you think the system is dead.
Usually such welding is minor and a sharp tap, with a screwdriver handle, on the cover of the affected relay is sufficient to release the weld.
This would imply that there is a fault in the reversing switch or the reversing logic on the board.
Half speed reverse will not disengage
Sorry about this - you've spotted a manufacturing defect. We don't make many, but we are only human! The half speed reverse must disengage if the link is properly broken. It follows that removing the header is not breaking the link. So look for a solder bridge across the appropriate pins.
Other pages with relevance to this subject