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How to dismantle the 4QD series

Hopefully you’ll never have to do this, but if the need arises then here is how to do it.

Top board removal.

Remove the top board by loosening the four corner screws. Disconnect the 14 way ribbon that connects the two boards together.Top boards are, generally, interchangeable, but it is always best to keep the top board with its base board: this is how the two were tested! Also, there are small differences between the top board for the -300 model and the other models, some values change between the various voltage options and sometimes current limit values on the top board have changed for different types of MOSFET.

Heatsink removal.

The heatsink is attached by means of 8 off M3 x 16mm screws, marked 1 through 8 in the baseboard layout diagram.Each screw has a washer and a plastic insulating bush. Take care not to loose these. Especially, don’t drop metal washers in the works!

Once these 8 screws are removed. the aluminium base may be removed. On older controllers the silicone insulating sheet can form a slight bond and gentle persuasion may be required to separate baseplate and metalwork.

Take care not to damage the grey silicone rubber insulating sheet which should be saved carefully for re-use. It should be protected from mechanical damage. In particular, keep any metal swarf and similar particles away from it.

Some service work can be done at this stage – but a lot is obscured by the metal busbars, which may need removing.

Removing the bus-bars and MOSFETs

Undo the 6 remaining M3 screws, shown on the diagram. One each near holes 1, 3 4 and 6 and two on the bottom busbar (shown arrowed). These have spacers to separate the board and busbars: do not loose the spacers (two sizes, 3 off 10mm and 2 off 5mm on the thin busbar)!Undo the nuts (10mm box or deep socket spanner) and washers on the M6 studs.

The busbars are now held on only by the MOSFET gate connections, 16 on the -200- controllers and 13 on the -150- controllers.

These gate connections are shown in green on the drawing. Use a solder sucker or similar method to remove the solder. Make sure each MOSFET gate lead is lose within the hole and not stuck by adhering solder. This is slightly more onerous by the conformal coating on the boards which tends also to fill the lead holes through the board.

Once all leads are free of solder, the board and busbars may be separated. A certain amount of force may be needed, but take care not to bend any MOSFET leads.

The Busbar assembly with attached MOSFETs is now loose and attached to the board only by a 2 core ribbon to the thermal sensor. Unsolder the ribbon if needed.

What components to check for

This link has a page that shows the most common components that can get damaged on the 4QD series power boards.


This is the reverse of the assembly process but here are a few hints that will help.

Re-fitting MOSFET / busbar assembly to board

This is the most tricky bit. Make sure MOSFET gate leads are clean of solder.Locate the four M6 studs in the circuit board and take up most of the spare thread on the two outer studs with M6 nuts. Take care not to bend MOSFET leads. Turn the assembly so it sits on the four corner pillars and the busbar assembly is on top.

Gently lever/lower the assembly down, hinging it on the studs so the MOSFET leads locate one by one in their correct holes. If necessary persuade any recalcitrant leads into position with a screwdriver or similar tool.

Avoid excessive force which will buckle MOSFET leads. Once all MOSFET leads are inserted into their holes, hold boards and metalwork together and turn the assembly over. Using a thin screwdriver, a nail or a similar object, drop a spacer through hole 2 and slide it into position beneath the mounting hole. Insert a 16mm screw and take up most (but not all) of the spare thread.

You can now insert the other 3 long spacers into position (holes 1, 4 and 6) and fit the screws.

Now fit the two short spacers and screws into the thin busbar (negative).

Tighten all 6 screws and the four nuts on the four studs. Make sure the thermal sensor wire and link wires are re-connected and not shorting out anywhere – tightening up the ‘loose’ stud can rotate it and could cause the crimps to short out, so inspect them.

Once all metalwork is tight, then re-solder the 17 (13) gate leads.

If any significant work has been done, it is sensible to test the board at this stage with a motor and a suitable power supply.

Re-fitting baseplate.

Make sure there is no swarf or grit on the baseplate and no dirt or burrs on the busbars. Visually position the grey insulator sheet on the baseplate and place the board/busbar assembly so all holes line up.Position a washer and an insulating bush on each screw and drop one ‘clad’ screw through each of the holes 1 to 8, so the screw drops through the busbar hole and mates through the hole in the insulator, into the baseplate. Take care not to elongate the insulator holes or you can destroy the insulator.

Screw down all 8 screws loosely, allowing the plate and assembly to float into position. Once all are positioned, tighten all eight.

Now with an ohm meter, check for short-circuits between each of the three thick bars and the baseplate. There should be no connection.


These are detailed on a separate page.