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How do I replace the transistor in my AnR Ignition Amplifier?

An insulated-gate bipolar transistor (IGBT) is used in the A/R amplifier.

Important notes: For adequate heatsinking, the transistor must be bolted down to the baseplate BEFORE its connecting leads are soldered to the PCB.

A 'silpad' insulating thermally-conductive washer is employed between the transistor and baseplate and both the baseplate and metallised area on the transistor must be free of any foreign matter, as even tiny specks of dirt or metal may compromise the insulation. Pulses of 400V are present on the back of the transistor.

1) With baseplate on a flat surface, remove 5 M3 nuts to release the entire module. You will also have to slip a knife blade through a silicone blob supporting the adjacent diode away from the base plate. Leave the baseplate on the flat surface if you want to avoid having to put all the screws and pillars back in place later.

2) There's no point trying to take the transistor out in one go - you are certain to badly damage the PCB (which is the last thing you want to do). Snip through the three leads to get rid of the transistor body. You can now fish out the leads one at a time, finishing with a solder-sucker to clean out the holes.

3) Fit the new transistor to baseplate with serrated washer and nut, not forgetting the silpad insulator. Pay attention to cleanliness as noted above. If you have a suitable meter check resistance from baseplate to centre lead of new transistor is infinite to verify integrity of silpad.

4) Bend up the thin ends of the device's leads at 90 degrees. You should now be able to lower the module onto the transistor leads and engage the four PCB mounting bolts. Slip the nuts onto the mounting bolts and you can now pick it all up without anything falling out.

5) Tighten the four PCB mounting bolts and solder the new device's leads to the PCB. Make sure some solder runs down the holes rather than just sitting on top. If you cannot achieve this, you will need to try a more powerful iron. The centre connection needs most heat as it is thermally connected to the baseplate. Check that the large diode you cut free cannot short on the baseplate. You might want to use a small blob of fresh silicone to re-glue it in place.

 

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