Icom Accessory Port Power Considerations

Although this article is aimed at owners of Icom transceivers, there may be other transceivers, which exhibit the same shortcomings. Since my experiences are limited to just a couple of others, any determination pro or con is left up to the reader. More specifically, if you power any ancillary devices via the accessory port on your transceiver, then I suggest you read on.

Any of you who have been following my mobile radio articles know my basic story. Those who haven't perhaps need a refresher. So I don't bore everyone, here's the basic setup. I have a 3 1/2 year old Icom IC-706MkIIG, and a recent SG235 autocoupler, and SG500 power amplifier. These drive a homebrew 13.5-foot long antenna resonant on 20 meters, which incidentally is where 80% of my mobile QSOs take place.

The SG235 is more or less automatic in operation and has the benefit of a pass-thru PTT to the SG500 amp. The SC500 amp even supports auto-band selection. To use a much over-trite term, it is plug-and-play. -- At least as P&P as you can get. The drawback is, all of this PTT and band selection takes a few moments to occur. I like VOX, especially mobile as it's less distracting, and in the case of Santa Fe, NM, the only legal means of in-motion operation. So I set out to provide a solution to the inherent delay.

Some years back, I remember seeing an article by Nigel Thompson, KG7SG, which described a remotely controlled antenna switch (QST, April 1993, page 32). This comparator-based device used an open collector design which was just the ticket needed to convert the band selection output of an Icom transceiver to the appropriate signaling to the SG500. I purchased the necessary board from FAR Circuits (www.farcircuits.com), and the components from Digi-Key (www.digi-key.com). I stuffed the board, put it into a good-quality aluminum box, made the cabling, used the accessory plug as supplied by Icom, and presto the whole mess worked like a charm. Almost! Here's the rest of the story.

Everything worked the first time just as I was sure it would; when I changed bands on the Icom, the SG500 amp followed perfectly. The PTT delay from the SG235 autocoupler to the SG500 was still there obviously (<.4 seconds), but the band selection delay (about 1 second) was gone. Eureka! I was so delighted! -- Until I tried transmitting with the amp on.

The comparator circuit was trying to select more than one band causing the band selection relays in the SG500 to chatter. My gosh, what could be causing that (?) I proposed to myself. Gee, it worked on my IC-746 Pro, it worked on the bench, so what was up now? After calming down for a few moments, I realized something was causing the band selection voltage or the supply voltage from the Icom to change.

Digressing for a moment, the SG500 does draw a lot of current. -- Forty to 60 amps average, and as much as 100 or more on peaks. But what the hey, I had two batteries, a 140 amp alternator, and I never saw the voltage at the amp drop to less than 12 volts unless I had the air conditioning, all of the lights, and the rear defroster on at the same time! Something was amiss.

Since I could operate the PTT from the trunk (the IC-706 is mounted underneath the package shelf in my 1998 Mercury Mystique and has a mic input on the rear apron), I started checking the various voltages from the Icom's accessory socket while transmitting an RTTY carrier into a dummy load. Just for the record, I was powering the SG235 autocoupler from this socket. Icom states you can draw up to one amp, and the SC235 is rated at 900 mils maximum, so I thought I would be okay. It wasn't!

The input voltage at idle with out any automobile accessories operating is about 14 volts. The measured voltage at the accessory jack was 13.4 with out any load connected. When the SG235 was connected this dropped to 12.8 volts, and if it was in operation, 11.9 volts, or just a few tenths above its minimum. Add in the draw of the Icom's output stage running 40 watts, the SG500 amp running 500 PEP out, the voltage at the accessory port dropped to 10.4 volts (!), albeit the input voltage was 12.4 or so.

And the band select voltage was even worse. (The band select voltage varies between 0 and 8 volts depending on the band selected and is referenced to a regulated 8 volts, which the aforementioned comparator uses to make the correct band selection.) On 20 meters for example, the band select voltage should be about 4 to 4.5 volts. At load, this dropped to less than 3.5, which caused the 18/21 band output to activate on voice peaks. The effect was worse on the 24/28 output which dropped to less than 2 volts which activated the 10 meter band select and caused the SG500 to drop off line to protect itself. Even if I powered the comparator from a stiff 13.6 volt bench supply, the band select voltage still caused a selection error. Obviously, this was not, and is not, acceptable.

Incidentally, the IC-706 is shipped with an accessory plug which is pre-wired as the individual pins are crimped and can not be easily replaced (I just spliced into this existing wiring). The wiring appears to be SWG 30, which relates to AWG 28. In any case, the maximum draw according to Icom is 1 amp, which seems high for size 28 awg wire.

I did try to use the AH-4 accessory Molex connector to provide the nominal 13.6 volts supply, but alas it too suffers from the same fate. The reason isn't as simple as it sounds, but relates to the fact the Icom uses a transistor to switch these ports to the incoming supply, what ever that may be. For the record, I did measure the voltage for the nearly identical port(s) of my IC-746 and the drop is almost the same.

In my case, this negates the use of the accessory band switching scheme I was hoping to use. In its place, I decided to use manual switching, and because most of my operating is 20 with an occasional foray to 17 and 15 (which uses the same band pass filter on the SG500), the personal impact is negligible. But this leaves an open question to any one using the accessory sockets for any purpose.

For example, I use one of the ports on my IC-746 to power a remote-controlled antenna relay, which draws just 90 mils. Without activation the 13.6 nominal voltage at the Accessory Port 2, pin 7 (or Accessory Port 1 pin 1) is 13.4 volts and with the relay activated it is 13.1 volts, and this with the base-station power supply! Those folks using sound-card applications, or other ancillary equipment, which uses power supplied by any of these ports, may have similar problems.

In short, if you power any external device via an Icom accessory port (perhaps many other transceiver accessory ports), and you experience problems, it just may be the voltage drop caused by the internal switching architecture. In a word or two, Caveat Emptor!

Alan Applegate, KØBG

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