An F1 DPR chip arrived on my doorstep to be fixed. It looked perfectly OK but would not run when connected to a chassis and tested on a powerbase. After a check of the components, I found that three of the four diodes were not working at all so I replaced them with 1A Schottkys .........
All still looking OK ....... time to test my handy work .......
Reassembled back onto the testbed, I pulled the throttle back and woomph!
... very bright flames & tons of acrid smoke.
As you can see, the dual MOSFETs, top left, has decided to expire. It is a dual MOSFET bereft of any magical smoke, it is an ex-MOSFET.
I wonder if Farnell can help me out in the morning? ......
The dual MOSFET chip has melted and released it's quota of smoke and vapours ....
Sometimes, the evidence for the chips failure is not always quite so easy ...
The larger arrow shows a degree of discolouration on the solder pad of a diode. A further clue to the problem is highlighted
by the smaller arrow, is an adjacent solder has turned orange from a normal silver. These features are caused by
excessive heat ....... note: the wires for the braid pickups have been removed for clarity.
The damage to the NO2 MOSFET is quite obvious and not unexpected but what about PO3, the MOSFET that controls braking?
If you look carefully, you will see a split running down through the surface marking. Only replacing NO2 would correct the power
to the track part but when you test it you would probably find that the motor would run continuously. Often the damage to
these MOSFETs can be a miniscule pin-prick mark on the surface and sometimes there are no visible signs at all.
Now that you are sitting comfortably gentle readers, I shall continue showing more photos of damaged chips .....
Obvious signs here ... destroyed NO2, stress blue wires on the right side and missing wires below the large yellow capacitor is and
directly above it, both on the outer edges of the PCB. The mark across the top diode is just a streak of glue.
Same chip, NO2 & PO3 both replaced, wires removed and added including the red & black ones. However another fault was picked up
in testing .... the blown resistor for the infra-red LED. Looking back at the previous picture shows that I missed it which is why you
should always check the functionality of the chip after repairs even in areas away from the ones you had worked in. Good job too as
this one was off The Netherlands.
Now a series of C7005s
The NO2 MOSFET (top right) had a miniscule, easy to miss pinprick hole found using a 20x microscope.
A more obviously blown NO2.
This one has a blown PO3 MOSFET. Look at the darkening around the top of the microprocessor legs. The large black blob on the bottom
right of the PO3 was the charred remains of the corner and the copper pad that it was soldered to.
In preparation for the forthcoming plethora of the F1 cars from the 60s, I wanted some chips that could be squeezed into tighter spaces ..
So I replaced them with much thinner, fast action Schottky diodes. The large silver capacitor looks so much bigger now but that
will be removed and attached to fly-leads make the board much thinner .......
Next up is not quite a repair as much as a refurbishment of a Saloon Chip - C7006 .........#
When I get a chip like this, I would usually remove the white plug entirely and just solder the wires directly to the PCB.
A digital module plate sitting on my desk gave me a inspiration so I removed the plug from the plate and found a DPR wiring loom
in one of my spares boxes.
After a little bit of cleaning up, the iron was switched on and quickly heating up .....
Those with sharp eyes will notice that the wires on the PCB do not quite match that of the wiring loom. Here I have switched the
Green and the Yellow wires over as it was the neatest way to add the plug. The chip's internal bridging diodes will always ensure
that the correct polarity of from the track is presented to the chip correctly, so the switching of pickup wires does not matter.
The finished article. You may ask why not just add extra wire to the existing bits? The wires were just a bit too short to make the
job as easy as doing it this way .....
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