+- Australian Ham Radio Discussion Forum ( AHRDF ) (https://www.ahrdf.net/forum)
+-- Forum: GENERAL (https://www.ahrdf.net/forum/forum-29.html)
+--- Forum: Transceivers, Transmitters & Receivers (https://www.ahrdf.net/forum/forum-18.html)
+--- Thread: IC-7400 / IC-746 Pro backlight failure/fix (/thread-1399.html)
IC-7400 / IC-746 Pro backlight failure/fix - VK4ADC - 20-03-2024
uMy trusty Icom IC-7400 had recently an intermittent backlight for a few weeks then finally failed...
Google searches produced a number of results but the most promising was at https://iz0egm.jimdofree.com/repairs-or-mods/icom-7400/
It became apparent that this is a common issue with several models that presumably use the same or similar circuitry.
In due course I opened up the 7400, took off the front panel assembly, disassembled that to get down to the area on the PCB where the backlight inverter was and, sure enough, the 2SB1202/Q302 (on logic unit) was discoloured as was the PCB around it. I used a heat gun to remove Q302 and even the adjacent PCB was showing signs of heat stress. I doubted that I could cleanly install another SMD power transistor.
I wasn't going to buy a "backlight repair kit" at an inflated price from eBay and I have lots of bits on hand so I replaced the 10uF electro (C302) with a standard single-ended leaded type, the 47uF (C303) was replaced with a low-ESR standard leaded variety. The replacement for Q302 - that 2SB1202 - opened up a few issues due to parts non-availability. The IZ0 article mentioned a BDX54C as an alternative but these days even those aren't available. You have to remember that many semis are no longer manufactured due to changes in appliances' manufacturing techniques, so it becomes a matter of cross-checking data sheets as well as suppliers' product availabilities..
I finally settled on a TIP42C TO-220 style PNP transistor, drilled a hole through the side of the tinplate shield, used a flat insulating washer plus a nylon isulator that the 3mm screw passed though and then retained with a nut inside the tinplate box. Oh, and did I mention that I used heat transfer silicon grease between the insulating washer and the back of the TIP42C and the washer and again between the washer and the tinplate. This process allows the large surface area of the tinplate box to act as a large (though thin) metal heatsink. The 'B-C-E' leads were jumpered across to the PCB pads with short lengths of single-strand copper wire.
So, in effect I did the IZ0EGM repair - but without using SMD parts.
The result was that I now have the 7400 with a functioning backlight.
Doug VK4ADC