IC7300 and Linear Amplifiers

I have had this rig as my main HF-6M-4M radio for quite a few years now. It is an easy rig to use and communicates well with my Apple MacBook Air via USB. Its performance is quite adequate for normal use, though RX is slightly less good than my new FTDX10 for serious contesting. The rig worked flawlessly for the first 5 years or so, but then developed a habit of blowing the internal 5 amp fuse. The reason for this remains a mystery as when returned to Martin Lynch and Son (MLS) they could find no problem and having replaced the fuse it has not blown since! Possibly if the rig has been turned off by cutting the power rather than using the power button and then turning back on may have been the cause. These days I make sure I have turned the rig off properly before switching the PSU off.

A further fault developed when using my linears with the rig. The PTT switching of the linear can be implemented either using the RCA Send socket or pin 3 of the ACC socket on the rear panel. Both have current limitations – 500mA and 200 mA respectively. The ACC socket also has a (permanently on) 13.8V output at pin 8, which can be used for powering an external relay to trigger any linear which takes more than their prescribed current limits. I had built an external relay box and suitable lead to do this, using the little circuit recommended in section 18.2 of the full ICOM IC7300 manual. It worked fine for some years but recently gave up. Both ACC and RCA phono connections became open circuit so (being a back box operator) I took the rig back into MLS for repair. Having been warned there was a 4-5 week delay on repairs I was delighted to get a message about a week later to say that they had done a mod to the main board to fix the problem. (Replacement of the main board would have cost around £500 plus labour so I was very happy with their bill of £160!)

Having got the rig back I was determined that the problem should not happen again. I first tested the switching current on both Linears. The Expert SPE 1K was no problem – voltage about 4V and current minimal – about 2mA!! However, the home-brew 4M linear made by Geoff – a G3 [- was a different kettle of fish with a current exceeding 250mA (FSD on my meter!) so clearly the relay box was essential. I then tested the lead to the relay box and found to my dismay that the pre-made lead to the RCA phone plug had an internal short across the two terminals, probably within the sealed plug. I suspect that by overloading the 13.8V feed from pin 8 (1 amp max!) I had damaged the switching circuit too. Anyway another lead is being put into service and in due course it should be all working OK. I also substituted the relay in my box for a smaller one that takes only 70mA to trip. I also plan to add a low current fuse in line with the rig output if I can find a fuse which takes 100mA – 250mA in my junk box.

Perhaps the morals are:

1. Check your linear switching current by putting a meter across the send to ground (the Expert SPE is great) ;

2. Check pre-made leads for continuity and shorts.

3. Very pleasantly surprised with the efficiency of MLS repairs once you can get through to them (my first two telephone messages never reached their service department).

4. One advantage the IC7300 has over the FTDX10 is that you can adjust the power output to about 1 watt which suits my 4M linear, whereas the FTDX10 minimum is 5 watts which overdrives it.

73 de Charlie G0SKA

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