Icom IC-703 QRP Transceiver

The IC-703 is completely different from my other QRP rig. It's modern, it's portable, it's synthesized, it's computer controlled, it's complex as fsck. I'm still coming to grips with all the functions.

"Cheat Sheets"

I reduced the size of some parts of the User Manual to four-pages-per-page: Quick and Initial Settings and Operating Manual.

Calibrating the SWR meter

As received, my IC-703's built-in antenna tuner didn't work. Pushing the TUNER button gave a click and then immediately a beep, as if the SWR was too high -- even when using a dummy load. I then found that the SWR 1 LOAD calibration from page 4-11 of the Service Manual passed (transmitting on all the bands, with output visible on the power meter / dummy load) but that SWR 2 LOAD failed. So I asked on various forums and got enlightening answers like "your SWR is too high" and "check your cable" -- proof of the Dunning-Kruger effect.

I was slightly puzzled by the lack of difference in methodology for SWR 1 LOAD and SWR 2 LOAD so I referred to the IC-706 Service Manual. For SWR 2 LOAD the dummy load has to be 25ohm or 100ohm -- i.e. an SWR of 2:1. That makes more sense.

Power Supply

The manual specifies an oddly specific "915.87 V DC" power supply requirement. 9V I can understand but what's this 15.87V?

I've looked at the schematics and I can't find anything that's liable to pop at 15.88V. I have to conclude that some technical writer who has never heard of significant digits was told that the IC-703 can handle 15% over the 13.8V nominal automotive voltage.

Battery Pack

The optional battery pack from Icom is the BP-228. This is a 2800 mAh 9.6V NiCD pack which provides seven hours of... Idunno, presumably listening a lot and talking a little.

I just happen to have some 18650 Li-ion batteries for a netbook battery rebuild I never did. 3.7V per cell, 3000 mAh. Four of these will give 14.8V. However, li-ion cells can be charged at up to 4.2V per cell, 16.8V for four cells, which means that it's not safe to operate the IC-703 on batteries while they are charging.

On the other hand, charging voltage should be reduced to 3.92V in the interest of long life -- which brings the total voltage while charging for four cells to 15.68V, just under the IC-703 maximum specified operating voltage.

Also in the interest of long life, the initial (constant current) part of the charge curve should be limited to 0.7C, or 2.1A in my case.

SHDesign has about the simplest charger schematic I could find (It's pretty much the schematic from page 19 of the LM317 Application Note. The problem is that the LM317 is good for 1.5A maximum and I want 0.7C where C = 3A, i.e. 2.1A. This can be done by wrapping a transistor around the LM317, see pages 16 and 17 of the Application Note. The current through R1 turns the transistor on and that supplies extra current to the load.

Finals

If you don't like (or can't find) the RD07MVS1 output transistors you can probably replace them with RD15HFV1 like Pascal did.

Some tips on desoldering and re-soldering 2SK2975 or RD07MVS1.

Computer Interface

OK2WY CI-V interface and RA3APW's almost free audio + PTT interface. Some glue required.

Antennas and Accessories

The range of the built-in antenna tuner is a bit limited, so one option is a separate antenna for each band, with a balun. Or use switches, or a switched loading coil. More antenna ideas here.

NorCal short doublet and BLT tuner (BLT Plus adds random wire support).

eBay has a "Days Antenna Tuner" which contains everything you need to build the Norcal tuner, you just have to reconfigure the L/C arrangement. Or in South Africa, get it from Giga (more expensive than eBay but also more convenient).

Links


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