VTX / NCC Fault Finding

Introduction

Common faults on the NCC & VTX  series are – not common.  But there are a few faults which occur, mainly because the ‘bare board’ style of the controller makes it vulnerable to damage.

You will probably need to identify the issue number of the controller.


 

Blown MOSFETs

When MOSFETs fail, they can usually be seen to be visibly damaged.

Controller is dead

The usual cause of this is either faulty ignition wiring or a faulty pot or wiring to the pot, which may have blown an on-board fuse track..

Controllers generally fail in a visible way: if the control really is at fault, usually the MOSFETs will have blown and will be visibly damaged. Also the 10 Ohm gate resistors are likely to be burnt. These components can be identified on the diagram of the VTX board. There is often a distinct smell of burning.

  • Check the three fuse tracks on the VTX. These are explained at the back of the instruction manual.
  • Read Pro, Scoota and NCC series controllers internal power supply and protection circuitry.
  • Is the ignition switch fitted and working? It should apply battery positive (or any voltage greater than about 9v) to the ignition connection, pin E, white wire.
  • Check the voltage between pins D and F on the 6 way connector (i.e. across the pot) when the ignition is on: it should be about 8.5v. If zero, check that the current source is working. To check, measure the current draw from the battery positive with the speed pot at zero (i.e. with both relays de-energised) and the ignition on. If should be around 30mA.If any accident occurs to the board, it is designed to fail safe. On earlier boards (NCC and early Pro-120), the zener diode may fail short circuit if battery voltage is applied to the pot or something contacts a live part of the board while power is still present (i.e. before main capacitors have fully discharged). There will then be zero voltage across the pot although the battery draw is correct.See NCC series controllers, key components to identify the zener. The zener on the VTX rarely fails as it is much harder to contact any live part of the circuitry.
  • If this is all OK, check the pot resistance. The controllers are designed to work with a 10K pot (2K to 20K is acceptable) and will not work with a higher resistance pot.
  • Check the pot wiper voltage varies correctly between 0v (min speed) and 8.5v (or whatever voltage you have set with the gain control).

Controller will only go in one direction.

Does the reversing switch have any effect?

  • Reverse switch has no effect
  • Reverse switch has an 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 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.

Check the voltage on the controller pin (i.e. the soldered connection) – the IDC connectors are reliable, but we have seen problems when the wrong wire has been used.

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 either

  • there is a short in the wiring.
  • you have used the wrong size (too big) wire in the connector – that can cause shorts.
  • (older NCCs only) you have blown the reverse-ignition diode – between ignition (pin B) and reverse (pin C). See NCC Series: Key components. This diode was fitted on NCC issue 16 (See wiring for use with push-buttons for reason) and later removed as some customers were blowing it! On the VTX it can be fitted as a factory option on volume orders. This diode may be safely removed unless you are using push-buttons but be careful not to disconnect the through-board connection. best snip out the diode rather than unsolder it!

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.

  • Same relay clicks, both directions
  • One relay clicks, other does not
  • Relays click properly

Relays click properly

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 tom 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.

One relay clicks, other does not

  • The commonest fault is blown relay drivers, usually caused by not using fully insulated crimp connectors. If these touch the board during removal or fitting, the relay driver transistor can be damaged.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.See NCC Series: Key components
  • Another possibility is that the deceleration ramp has been set too fast. This can cause the motor to be braked by the relay contacts rather than by the MOSFETs. If the relays change state while there is significant motor back emf (i.e. while the motor is still running too fast) there is an arc at the relay contacts and this can weld the relay contacts together.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.
  • Another possibility, rare, but we have seen it, is that there is a small particle of solder lodged in the armature gap. These relays are hand soldered during manufacture and on rare occasion solder particles can spatter and stick. If they get dislodged, sod’s law says they are bound to get stuck in the worst possible place.

Same relay clicks, both directions

This would imply that there is a fault in the reversing switch or the reversing logic on the board.


Dual ramp reversing goes to zero, but won’t go through zero sped

You do have a motor connected, don’t you? The regen current limit gets confused if there is no motor and stops the demand speed going to zero. To some extent commutation of current from drive to regen depends on the motor’s inductance.

The action of the dual ramp reversing is explained in the circuit dual ramp description.

If the dual ramp reversing ‘latches’ with the controller at zero speed, there are two possibilities:

  • The ramp clamp is not pulling the demand speed low enough. Could be a low gain Tr25. On earlier boards (the change was around issue 17) the base emitter resistor of Tr25 was 22K. Change it to 47K.
  • The threshold on IC2d pin 8 pin is not correct. The implication is that D21 is short circuit: this type of fault can only happen because of bad handling: the board has contacted something metallic whilst live – i.e. while there is still charge in the main capacitor.

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.


Pot doesn’t control speed

Does the reverse switch operate? When it is changed while the motor is running, the controller should ramp the motor speed down to zero, reverse the direction and ramp up to speed again.

If the reverse switch operates as above then the controller is working properly. So there is a fault in the pot, or its wiring. Most likely the earth fuse track has been blown by a wiring fault. See instruction manuals for details of this fuse track.

Machine wiring: Good and Bad practises is very relevant of such a wiring fault exists.


Speed pot won’t turn speed to zero, but does have some effect

The electronics of the controller simply does not develop this fault. So there are two possibilities:

  1. The speed pot is not turning to zero. Check the pot and check the voltage on the wiper: this should be zero for zero speed.
  2. The board is contaminated. We have seen fuse tracks (specifically the main fuse) repaired using plumber’s paste flux (which should never be used for electronics) which has spread over the board causing this problem.Wash the board with washing-up liquid and water and dry it carefully before reusing.

Other relevant pages

  • Service Manuals – Index lists the NCC series service Manual.
  • NCC Series: Key components