Powerbase 6 Repair

The Scalextric six-car powerbase (PB6) is somewhat underdesigned, with an output driver stage which does not efficiently power the final transistors. This causes the driver transisitors to become overheated under heavy load, and then prone to catastrophic failure should a short-circuit occur.

Repairing a blown powerbase when the one or more output drivers have failed, is not a tricky task. However replacing the MOSFETs with the same part (which can be hard to find) and returning to the same environment can only mean another failure.

This repair up-rates the MOSFETs with commonly available components, by extending the headroom for the powerbase.

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Outcome

Original Fault - invalid square wave

The purpose of this is to repair a blown powerbase - there is little gain in making the change on a functioning powerbase, as the output gains do not justify the effort.

The repair replaces the existing (failed) transisitors with uprated parts, thereby improving the ability of the powerbase to handle both normal use and fault situations. The desire is to use commonly available (and cheap) replacement parts.

By making this change, you accept that you are doing this at your own risk. This change will void your warranty.

For those who are interested, this is the comparision in the performance between the standard parts and the replacement parts.

	FR024N	FR9024N	IRF540	IRF9540
	N	P	N	P
ID const	16A	11A	28A	19A
ID pulse	68A	44A	110A	76A
PD	38W	38W	120W	120W
RDS	0.07Ω	0.175Ω	0.06Ω	0.15Ω

As can be seen, the biggest advantage is higher maximum capacities, with the 'On Resistance' (RDS) which is similar to the existing parts.

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Items Required

Moderate level soldering skill is required, using appropriate tools

Part	Qty	Description
IRF9540	x2	P channel MOSFETs, TO220 package
IRF540	x2	N channel MOSFETs, TO220 package
0.1Ω ½W	x1	SMD 1812 resistor (optional)
Desolder braid	100mm length
Heatsink	see pix
short M3 screws	x2	flat heads
TO220 insulating washers/mica	x2

Tools Required
Philips-head screwdriver
Fine tipped soldering iron for electronics
Side Cutters
Fine nosed pliers
Metal saw (for trimming heatsink)
2.5mm bit and drill
M3 tapered tap

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Procedure

Before starting, validate that the powerbase is a v1.5 device. Connect the power-pack and turn the device on. Watch the display and ensure it states v1.5

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Disassemble Powerbase

Unscrew the 4 screws on the bottom of powerbase, to separate the top from the base. Separate the two by hinging the top where it meets the track. Carefully drop the main control board down, exposing the flat cable that joins the main board to the display board.

The button plastics may slip out - this can be easily replaced later.

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Unplug Display Board

Gently slide the cable latch, by moving the two ends upwards about 1mm. This will release the cable, allowing you to slide it from the main board. Place the lid to one side.

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Desolder Daughterboard

PB6 Daughterboard - click to enlarge

Locate the small up-right daughter board. If this board does not exist, it cannot be upgraded with these procedures discussed here. (Although other revisions of the board can be repaired, the instructions discussed here are not relevant.)

Desoldering Daughterboard - click to enlarge

Turn the main board over, and locate where the daughter board protudes from the bottom of the main board. Using the desolder braid and iron, remove all the solder which attaches the two boards. Constantly replace the desolder braid as soon as it is filled with solder - Hold the braid with pliers so as not to burn yourself.

Ensure you desolder from both edges of the daughter board.

Removing Daughterboard - click to enlarge

Once all the solder has been removed, the daughter board can be pushed easily from its slot. Any excessive force implies the solder has not been fully removed.

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Add the Sense Resistor

Installing Sense - click to enlarge

Optional. Adding the extra sense resistor will increase the available output from the powerbase.

Carefully add some solder to the ends of the sense resistor, then place in onto one of the two spare sets of pads next to the other large sense resistors. Apply heat to each end while pressing the resistor onto the board, and solder the resistor into place. If should seat flat, and the solder should wick properly onto the ends of the resistor.

Sense Resistor - click to enlarge

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Form the Transistor legs

Transistor leg Folding - click to enlarge

Sort the MOSFETs into their two types. Fold the legs of the MOSFETs as shown...

The P-Channel MOSFETS (IRF9540) have their legs folded backward, thereby pushing the body outwards.
The N-Channel MOSFETs (IRF540) have their legs folded forward, thereby pulling the body backwards.

The purpose is to bend the legs to allow the transistors to be staggered so that they can be positioned in the original daughterboard slot. When finally positioned, the P-channel devices will be 'in front' of the N-channel devices.

Once folded, squeeze the outer pins together slightly to align the legs with the solder pads on the main board.

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Place all Transistors

Transistors in position - click to enlarge

Place all the transistors into the original daughter board slot, matching the types and position with those of the daugther board. Make sure they all fit, that the transistor tabs do not touch, and that the legs are properly spaced.

Double check you have the correct transistor in the correct position, and remove all but the first transisitor noting their positions.

Looking from the 'front', from left to right, you must have N-P-N-P -

The order of transistors will match exactly the transistors as on the (removed) daughterboard.

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Solder the Transistors

Installing Transistors - click to enlarge

Turn the main board onto its back.

Moving through each transisitor consecutively, position it then lightly 'tack' solder the 3 legs of the transistors into place. The legs of the transistors electrically match the 12 pads on the daughter board slot exactly.

After tacking each transistor in place, cross check the legs are on the correct pads, and that you have the correct transisitor in the correct position. Once confirmed, trim the excess leg length, then carefully full solder the transistor on the board. The transistors must be soldered onto the pads on both sides of the daughterboard slot.

Soldering Completed - click to enlarge

Cross check to ensure there are no solder 'shorts' between either the legs of the transistors, or between the tracks of the main board. Take extra care here and double check your work.

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Visual Check and Test

Cross-check what you've done

check for solder mistakes and solder bridges
check correct transistors in correct locations
check transistor tabs are not touching

Quickly apply power to the powerbase. Normally the chokes will start humming after about 1 second. Quickly remove the power, and check the transistors are cold. If not performing as expected, validate the transistors, and recheck for solder bridges.

Place a car with lights on the track, and reapply power. The car lights should come on, and its likely the car will twitch for a moment as the power comes on.

If you have a car programmed, plug in a hand controller and test the car operates. Disconnect power and controller.

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Attach Heatsink

Cut heatsink to size, and drill and tap for M3 screws. Smear with thermal paste and attach heatsink directly to the first two transistors. Place insulating washers between heatsink and second two transistors, and screwdown.

The two pairs of transistors cannot be connected together, as it will short out the outputs

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Re-attach the Display

Ensure the slide-bars clip for the flat cable are fully extended, then align and slide in the cable. When fully seated, slide down both sides of the clip to lock the cable.

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Reassemble

Ensure all the button plastics are properly positioned, then 'close up' the main board into its position in the lid. Close the parts together and reseat the top onto the base.

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Final Test

Repower the powerbase, and check the display is operating. Cross check that all buttons are working as expected. Place a car on the track, attach a controller.

Have fun!Ian Harding, Christchurch, New Zealand