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!

Dave WB4IUY
http://www.WB4IUY.net

Backlight Repairs, Icom IC-746Pro

This is a common problem for what is otherwise, a wonderful radio. This rig uses a CCFL back light behind the LCD display, similar to that used in the IC-756. The PNP transistor in the back light inverter circuit has practically NO heat sink, so the lower level the light is operated at, the hotter the device operates. In most of the 746's, it's not a matter of "if" it will fail, it's "when" it will fail. 

No worries, it's not a complicated repair, it just requires good eyes and patience :-) Here's a few pics of one I did for a friend locally. I replaced the device with a transistor with higher ratings, and heat sinked it to the metal inverter shield. 

You have to really dig into the rig...here, I'm in the process of removing the front panel assembly...

Front panel removed from the rig...

Back of the front panel...

Started disconnecting cables between the front panel and sub-logic boards...

Outer front panel (face of radio) removed...

Front panel with radio face removed...

PCB with back light inverter cover removed (upper RH side of board)...

The device in the lower RH side is the failed PNP transistor. It is soldered to a tiny pad of copper board, isn't even sinked to the rest of the copper. I remove that, remove the shield, drill a hole in the shield, attach a new device to the shield via thermal grease, and reinstall...

Here, you can see the device attached to the wall of the shield, with the shield re-installed. I use leads to extend the device back to the circuit board pads where the OEM part was soldered...

A view of the attaching screw for the new device, through the shield. The shield makes quite a nice heat sink for the small amount of heat generated.

A view inside of the inverter compartment, exposing the new output device. 

Power up testing on the new CCFL back light regulator. I set the internal range for very little adjustment, with the lamp at about 50% output.

Back in the shack of my friend and local ham. Looks great!