The aluminum rivets holding the transistors in place were carefully drilled out. I used a battery drill with a 5mm drillbit which is a lot larger than the actual hole diameter. I drilled from the back of the heatsink, kept the speed very slow and stopped as soon as the rivet head broke free. The rivets could then be pushed clear of the heatsink either by hand or by gently using a punch.
The rivets pushed out of the heatsink.
It also shows the discoloration of the pcb.
Continued on page 5
Then it was time to unsolder the transistors from the printed circuit.
The connections had a lot of solder and there were short lengths of wire embedded to carry the high currents. This meant my usual Weller 45w soldering iron did not give enough heat. A gas powered soldering iron also was not up to the task. I used a 150w iron with a heavy copper tip but a 100w one is sufficient. As my 150w iron can get the tip extremely hot I allowed it to get up to sufficient temperature then switched it off before soldering. This helped to reduce any leakage from the mains supply into the circuitry, the iron has no transformer to isolate it. The heavy tip retained a lot of heat so allowing plenty of working time.
I used a solder sucker to remove the excess solder and to clear the pins. Although the heat had discoloured both pcbs the board and copper tracks were still sound. If the copper track had lifted from the board during unsoldering I would have replaced the damaged track with lengths of wire.
Power Mosfets are relatively robust to leakage currents compared to standard cmos components.
They are however still susceptible to static damage so I took care to discharge myself and the tools before handling them.
I bent the pins of the replacements to fit into the pcb with the transistor flush to the heatsink, trying to fit the new ones in the same position on the heatsink as the surface underneath was clear of any smoke damage.
The transistors do not use insulating washers, they just needed a thin layer of heatsink compound.
I fixed the transistors with 3mm aluminum pop rivets, nuts and bolts could also have been used.
Once they were fixed the connections were soldered.
Remember the currents are high so make sure the soldered connections are good quality.
The heatsink contact area was cleaned by vigorous scrubbing with a rough cloth and solvent.
Some smoke had fused into the hot aluminum surface and remained on the non critical areas after cleaning.
I resisted the urge to clean the transistor contact area with abrasive paper, even fine grits would scratch the polished aluminum surface and reduce the heat transfer.
With the repairs completed the units were reassembled.
When fitting the pcb I made sure the star washer was fitted between the pcb and the metalwork below instead of the more usual position close to the screw head. This is because it provides an electrical contact between the pcb and the metalwork rather than a shakeproof function.
The second hand charger worked fine after the transistors were replaced so a similar repair was made to our original unit. When that one was tested there was still no charge. The fact that one was ok gave encouragement for further investigation into the still faulty unit.
Removing the rivet heads.
The old MOSFETS IRF3205
Marked connections left to right Gate Drain Source Gate Drain Source
One length of wire is visible, another is buried in the upper block of solder.
The rivets holding the transistors.
and the new.
The new transistors in place.
Smoke particles remain fused into the aluminum but this is mainly cosmetic.
The result of replacing the transistors
The transistor bypass test on Page 2 should be done before starting as it will show if there are other major faults, there is no point in replacing components if the rest of the charger is badly damaged.