If the main power line zener has been damaged, the voltage on the pot will be lower than 8.5v. However, this voltage being low can also indicated that the pot fault transistor has been damaged.
Before using the information on this page, you should read and understand the page Pro, Scoota and NCC series controllers – Internal power supply and protection circuitry.
The power supply arrangement in almost all controllers made by 4QD is designed to fail safe in the event of a fault. This is done by means of a current source (explained in Current Sources and mirrors) feeding a 9.1v zener diode.
In the event of a fault causing excessive voltage on the 9.1v rail, the zener over-dissipates and fails usually to a short circuit, protecting MOSFETs and other components.
If the overload is relatively minor, the diode can partially fail to a lower voltage than 9.1. If the fault current is high, the short circuit diode can get very hot and fail open-circuit, when other components can be damaged.
Probably the commonest cause of zener failure on the NCC series is handling the board when the main capacitor is still charged. If it contacts any conducting object the charge in the main capacitor can discharge through the foreign body.
Another cause of failure is the use of uninsulated push-on connectors. If these are removed while the capacitor is charged, and they contact a solder joint, fault currents cam flow. This is most likely to happen with the motor terminals and the obvious joints to touch are the relay drive transistors – which can therefore be damaged.
The whereabouts of the zener is indicated by the red arrow marked Z on the diagram below. Type used is BZX55 C9v1 which is 500mW rating, but in fact any zener of 9.1v will do.
Pot Fault detect transistor
If the +24v line shorts to the top of the speed pot, not only can the zener be destroyed, but the pot fault detect transistor can also be damaged. This is indicated by the red arrow PF. Original type was BC557, but virtually any PNP transistor can be used.