Saturday, December 14, 2019

Icom IC-746 DC Filter Board Repairs

This post is about the repair of the DC power input filter board B5143D in an Icom IC-746. This is an uncommon failure in some of Icom's amateur and commercial gear of this era. The metal used in the plating of some parts will grow "dendrites" (metal whiskers) over time and cause arc-overs of some of the higher power portions of the PC boards. In this case, there was only a small distance between the high current DC positive voltage input terminal and the PCB ground trace of the power supply filter board B5143... one the metal whiskers got close enough to ground, and BOOM!! ... A short circuit appeared, and the high current capability of the power supply turned this edge of the board into an arc welder. 

Upon initial inspection it looked bad, but it wasn't too difficult to rebuild. These boards are no longer available, so replacement wasn't a simple option. I cleaned the board to determine how much of the board the fire had consumed, then used the scroll saw and cut that part of the board away. Since the damaged part was now carbon impregnated from the fire, it was conductive and had to be removed. 

Next, I cut a piece of perfboard, and bonded it to the original board with fiberglass resin. Another coat of resin was applied to the top and the bottom of the new part to better bond it to the OEM board. The new piece was then drilled and fitted with the power connector and the discrete components that were burned away. 

The repaired board was reinstalled, reconnected, and the radio was as good as new. Here's a few pics of the work...

C1 & C2 are the discrete capacitors that were blown away from the OEM board...

Finished and ready to go!