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DNO Fault Finding Guide

This is the fault-finding guide for the DNO range of controllers.

The DNO is the end result of a long line of 4QD analogue controllers including the NCC and VTXs. It is therefore a well proven and reliable circuit and many thousands have been in use for many years. This page is to help identify and solve any problems that may occur.

There is a fault-finding sheet here which lists a number of key voltages to measure. We recommend printing this off and recording the voltages listed as this often gives a good indication of where problems are.

Other issues that we know about are…..

1. Tripping out due to condensation. The DNO has a current limit trip circuit, when the current exceeds a set limit for 17 seconds the controller will switch off and require an off / on reset to restart operation. The current trip circuit uses a pair of transistors in a high gain configuration to perform the trip operation, and this circuit can be susceptible to moisture or condensation on the circuit board. When this occurs the DNO will run for a short period but then stop responding to the throttle, and will need a power reset to restore operation. The problem occurs when the DNO is subjected to a sudden temperature change with high humidity levels. It can be cured by applying conformal coating to the components outlined in red below.  RS supply 15ml bottles [part number 321-7324]. This was included in production from March 2021.

condensation protection








2. Tripping out at high temperature. The current limit trip circuit can also give a false positive operation if the PCB is allowed to reach high temperatures. In tests the PCB has to reach over 50’C for this to occur, for this to happen the heatsink and mosfets will need to be over 80’C. If this problem is encountered, improving the cooling of the DNO will restore correct operation. This page gives more details on improving heat dissipation.

We came across this problem recently with a Toylander scale Landrover, after about 30 minutes running under heavy load the DNO started tripping out. We used a thermal imaging camera to examine the inside of the motor / controller compartment, this showed that the inside of the motors were reaching over 75’C, the hot air was then circulating and slowly overheating the DNO causing it to trip. The solution in this case was to fit a 24V, 80mm fan over one of the vents inside the motor compartment and wire it up to the ignition switch, this will extract the hot air whenever the Toylander is running.

3. Tripping out due to wrong connection. We have seen a few cases where the B+ wire has been accidentally connected to the adjacent M- terminal. When this is done, although the DNO appears to run correctly, the main capacitors rapidly overheat and cause the high temperature tripping described in 2 above.

4. Not switching from forward to reverse [or vice versa], this can occur if the acceleration and deceleration adjustments are set to their minimum values. There is a small interaction between these circuits and the dual ramp reversing circuit which can prevent the reversing circuit from activating. Increasing the ramp time slightly will cure the issue. In any case, having the deceleration ramp set to minimum is not a good idea as it can cause arcing at the relay contacts.