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+--- Thread: Tx Fault on IC-7400 / IC-746Pro (/thread-613.html)
Tx Fault on IC-7400 / IC-746Pro - VK4ADC - 08-08-2018
My IC-7400 has been on and off the workbench since late last year while trying to obtain replacement transistors (/FETS) for the PA driver area. One semiconductor at a time, I have replaced the pre-pre-driver, the pre-driver, and finally the driver devices before it is back in operation. The original fault was varying Tx power band by band - like 2W on 50MHz, 80W on 14MHz, 25W on 1.8MHz, so it wasn't entirely dead, just bad news.
A complication was that the driver PCB was damaged when one of the driver FETS caught fire late in the repair process - yes, it was like a candle - and accompanied by the normal bad smell. It was upside down on the bench with the bottom cover removed so I was able to see it directly. Fortunately I had looked at the power supply current too and it was up around 10A on receive (instead of 2A). The power switch was turned off quite promptly while I investigated, accompanied by some muttered bad words of course !!
Google showed a definite lack of B5703B driver PCBs to be able to replace my damaged PCB so armed with a micrometer and a PCB layout from the service manual zoomed to 600%, I did a will-fit PCB layout, etched and transferred the basic SMD capacitors and the couple of 150R resistors to it. The bias SMD components were replaced with new.
The issue is getting the driver FET's source connection to solder to the heatsink. The first time I did it, from the top with a hot air gun, and it wasn't solid / good. The second time, I placed the heatsink on an upside down clothes iron (on max heat) and once the solder on the 'source tongues' was melting, popped the driver PCB over the source slots using long nose pliers and then carefully slid it sideways off the flat of the iron onto a piece of wood to let it cool down.
The original driver devices were 2SK2975's but these don't seem to be available now. There are a few articles on the web that refer to using RD07MVS1 devices in lieu. These FETs are no longer readily available unless you want to buy fakes out of China or Hong Kong BUT the recommended Icom replacement seems to be the RD07MVS2-T112, Icom part number 1560001232. You won't find them listed too often either but at least there is a greater likelihood to find some available. Not super cheap, but better than discarding an otherwise good radio.
I used two of these RD07MVS2 devices (push-pull design) and now have 100W on all bands 1.8 to 50MHz, the RF power is down around 60-70W on 2M (should be 100W), so these devices do work. The bias was set for a driver current of 1.5A (see the other web articles re this setting), down from the original 2.5A in the service manual. I haven't gone back to try trimming the 2M power up or going through the full PA adjustment procedure pending seeing if the new driver FETs survive. They are running pretty cool at the 50% duty cycle of FT8 and 100W out on 14 MHz for part of most days over the last two weeks so it looks promising.
The reason for this post is to give others with similar problems a bit of light at the end of the tunnel. Yes, there is often a fix. Even if it not an easy fix!!
Doug