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Service section and
technical information

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4QD service, repair, technical information and application notes

Contents: this page


This section of the site will be a repository for service and other technical information pertaining to 4QD's range of motor speed controllers.

Much of the information here is posted in response to customer's requests. The fact that the information is on this part of the site and is publicly available does not imply that we are inviting anyone to utilise it willy-nilly. Much of the information here requires some technical expertise and is given 'as is' without any warranty. In particular, unless you have contacted 4QD about your own situation you may not even have any way of knowing whether it applies to your controller or not.

Factory repair service

Controllers found to be not faulty

Many controllers that are returned for repair prove not to be faulty. We charge a test/handling fee if we cannot find a fault with your controller, unless the fault has been reported to us by email and we have agreed that the controller may be at fault.

Controllers damaged by external fault

Unfortunately there are some failures that are not caused by a manufacturing defect but are due to motor and other problems outside the controller. Several of these are covered on the page about MOSFET failures. This type of failure is not caused by a fault in the controller and is therefore not under guarantee!

Furthermore, it is usually not possible to tell from an examination of the controller exactly what caused the failure. Any attempt at diagnosis relies as much on you, the customer, giving a full and accurate description of the exact circumstances preceding the failure.

Old controllers

Technology advances. controllers improve. If a controller is over 5 years old, it is generally uneconomical to repair it. If you return it for test/inspection, there will generally be a handling charge for assessing it.

Warranty repairs

All controllers are covered by a warranty against defects in manufacturing for one year from the date of purchase(in practise, we do not usually hold to a strict time-limit).

For any repair under warrenty we need:

DIY repairs and modifications

Please do not undertake modifications or repairs unless you are sure you are competent to do them and fully understand what you are doing. Be sure also that the information you are using does apply to the repair you are trying to do: controllers are updated over the course of time and (even is we could put all the information on this site) we have no way of assuring the info you are using does apply to your controller!

4QD will not be held responsible for any damage you do whilst attempting to repair or modify your controller or as a consequence of any attempt at modification. Since these modification details are available to anyone, their incorporation may invalidate all guarantees on your controller!

If is doubt, contact 4QD for assistance: however the amount of assistance we can give on older controllers is limited.

Circuit Diagrams

These are generally not available.

Furthermore, repairing high current controllers is a very specialist business. Few people have the required expertise. Also modern electronics are very reliable. With many thousands of controllers in the field, we do not even run to a proper service department ourselves as returns are few - and most of those are not faulty. We also modify controllers frequently if we find any pattern of failures, in order to 'customer-proof our designs.

Returning equipment

All Countries

European Economic Community

No special paperwork is needed - other than the green customs sticker.

Non EEC Countries

Customs and excise in all countries are a problem: UK is no better and if you are returning things, they will have to pass through customs inspection.

All goods imported into UK are subject to import duty, VAT and handling charges. If the British customs do not know that the equipment was manufactured in England and is being returned for repair - then they will make a charge.

Therefore if you are not in the EEC, it is very important to mark the parcel and any paperwork

'Faulty equipment of EEC manufacture, for repair. No commercial value

You will need to fill in a customs form: declare the goods truthfully as, for instance, Motor speed controllers. You will probably have to declare a value - but clearly as the goods are faulty, the value is nominal only, maybe 2% of their listed price.

DO NOT declare their value as being the price you paid: if they are dud, then clearly they are not worth this much. If you tell an untruth on the paperwork - UK customs may think this is the true actual value and may charge you appropriately!

Failure to do this may cause the UK customs to slap on a heavy charge - which we will have to pay and charge back to you.

Some countries may also have customs 'temporary export' forms. Certainly South Africa does (P.T. 619 - 1986/87). Filling one of these in should speed the incoming customs clearance of the equipment when it is returned to you.


Make sure the carrier you use is fully pre-paid. Some carriers slap on a customs clearance handling charge, even when no customs duty is levied. Currently only UPS (that we are aware of) do this, so please avoid them.

Return Address

The address to which to send any returns is:


Charges are levied on a fixed scale and a separate page lists the standard service charges. Note that these charges are for equipment that is as supplied by 4QD and which has not been altered in any way.


You may pay for repairs by cheque, credit card or cash. Payment must be in Pounds sterling.

Spare Parts

4QD are not a component distributor therefore we do not generally sell spares (though you are welcome to email sales to enquire) - other than MOSFETS (see Replacement MOSFETs) and relays

All of the components we use in our controllers are very common items and are available from many different sources.

Modified Equipment

If there is evidence that the equipment has been tampered with, repaired by non-approved or by inexpert persons or modified in a manner not agreed by 4QD, then it will be considered as being 'modified' and 4QD reserve the right to refuse to service it.

This proviso particularly applies to any mechanical modification such as removing power terminals or soldering to them: boards thus modified will not fit our standard test equipment and cannot be serviced.

Note that the presence of instructions or information on this site (or elsewhere) concerning the equipment or modifications to it, does not necessarily constitute 'permission' to modify it.


2QD Controllers

These controllers are effectively obsolete: they are still made for some customers but only as demand warrants. However, the circuit is similar to most of our controllers and the mechanical implementation is simple, so they are a good learning tool. A full circuit diagram, development history and description is therefore available to members of 4QD-TEC

Page Information

© 1998-2014 4QD/4QD Ltd.
Page's Author: Richard Torrens
Document URI: /serv/index.html
Last modified: 2014