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IC-706 Linear Amp Keying Circuit
Bob Varone (W4ETN)
on
September 16, 2009
View comments about this article!
IC-706MKIIG Linear Amp Keying Circuit
The IC706 a very popular HF, VHF and UHF all mode transceiver and has gone under many changes and upgrades to the current IC706MKIIG.
One flaw I have found is the circuit used for keying a linear amplifier. The circuit provides a ground level voltage between -0.5 to 0.8 volts at less than 20 ma. This is not enough to drive a relay directly never mind an old vintage amp with high voltage and huge spiking issues.
The following keying circuits solve that problem and with the relay circuit you will be able to use your IC706 with any amplifier.
I use my IC706MKIIG as a back up to my FT-1000D to drive my Ameritron AL-82. These circuits require no external power supply and I have even listed the pin numbers and color code with the ACC plug that is provided with the radio.
Have fun and see you in the pile-ups.
This article has expired. No more comments may be added.
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IC-706 Linear Amp Keying Circuit
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by K0BG on September 16, 2009
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There's a similar circuit floating around the net which uses a pair of NPNs, and two resistors; http://www.k6xx.com/radio/ic706amp.html
I curious why you need the 10 k between the 8 vdc and the HSEND? It's already pulled to 8 volts internally.
ALan, KØBG
www.k0bg.com
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RE: IC-706 Linear Amp Keying Circuit
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by W4ETN on September 16, 2009
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Hello Alan,
Both circuits will work great. I usually over engineer stuff and having a positive going to ground during XMIT I went with the PNP - NPN approach. In the other circuit the first NPN is conducting until you transmit and then it turns on the other NPN. (grounding collector). Always more than one way to skin a cat.
Best 73
Bob
W4ETN
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IC-706 Linear Amp Keying Circuit
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by K1DA on September 16, 2009
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A two dollar FET from Radio Shack works with fewer components and is not as delicate as those little bipolars.
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RE: IC-706 Linear Amp Keying Circuit
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by WA1RNE on September 16, 2009
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Bob,
Both keying circuits will work with the Al-82 and other similar amplifiers with low voltage keying requirements.
But the second circuit using the 2N3904 NPN output device will fail when keying an amplifier that is designed with a change-over relay in series with high voltage negative cut-off bias like the SB-200, 30L1, etc. which run -120 vdc.
Here's what you can expect to occur:
> The Vce rating of the 2N3904 will be exceeded and will cause the device to fail - probably shorted - and the amplifier will stay in transmit mode.
> If the 2N3904 doesn't fail right away, and assuming the 13.8 volt supply source resistance is pretty low, the -120 volt bias will cause about 1.5 watts of power to dissipate in the 1000 ohm collector resistor. Assuming a 1/2 watt resistor, it will be running at 3X it's rating and will overheat.
> For amplifiers like the SB-220, +120 vdc must be switched for changeover control, causing similar component failure and overloading problems.
For amplifiers that use relays in series with grid bias voltage, what you need is a couple of high voltage MOSFET's or a high voltage bipolar configuration that will handle negative, positive or both polarities.
I would also recommend adding some bypass capacitors at the input and power rail of the keying circuit.
...WA1RNE
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RE: IC-706 Linear Amp Keying Circuit
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by W4ETN on September 17, 2009
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That is why I also show a circuit with a relay for amps that require high voltage switching. I have fould I don't need the bypass caps but if your shack is full of RF it would be good to add them.
Bob
W4ETN
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IC-706 Linear Amp Keying Circuit
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by KA2OVK on September 17, 2009
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W2ENY has had a simpler version out for years:
http://members.fortunecity.com/w2eny/relay_706/
He also has a 7 pin model for other Icom radios:
http://members.fortunecity.com/w2eny/relay/
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RE: IC-706 Linear Amp Keying Circuit
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by K8KAS on September 17, 2009
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A $2.50 small reed relay bought at Rat Shack will also work fine. The reed relay only need's 10 ma to operate it
and most rigs will provide a source for both the voltage needed and a keyed on tx ground. I have used these relay's as buffers for the last 20 years on a number of rig's and amplifiers and never had a problem.
EZ and cheap...Denny
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RE: IC-706 Linear Amp Keying Circuit
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by N4JTE on September 17, 2009
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Speaking of easy and cheap I am using a borrowed MK 706 and I did not want to experiment with any inline
device to key an AL80b. I rigged up a momentary contact switch back to the relay in pin, under my computer roll out shelf, and knee it on when I want to run the amp.
Works great with no chance of breaking a loaner radio.
Tnx for the article.
Bob
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RE: IC-706 Linear Amp Keying Circuit
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by W5HTW on September 17, 2009
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The 706 uses several 12 vdc relays on the filter board. Also there are relays that switch from TX to RX. To me, this meant that somewhere on that board was a switched voltage to operate the antenna change-over relay.
I did some exploring, and found a ribbon connector along the right side of the filter board that had a switched 12 vdc on TX. I simply ran a wire from there to the spare pin on the ACC socket. ( I think that was pin 7, but am not sure now that I no longer have the radio.)
That provided a very low current switched 12 vdc outside the radio. I used that switched 12 vdc to switch a miniature Radio Shack reed relay, which in turn controlled the amplifier's 6.3 VAC relay circuit. Only one component involved - the reed relay. It worked like that for three years before I sold the 706.
Ed
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RE: IC-706 Linear Amp Keying Circuit
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by W4ETN on September 18, 2009
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Hello Ed,
Most guys would not open their radio for fear of voiding the warranty. I am sure there are many ways to key an amp with the IC706. I was just showing an external solution that did not require a power supply and used the existing voltage and control line on the ACC connector. The transisters and resisters are dirt cheap and can be still found at Radio Shack. I built it on a small piece of perf board and inserted it into a small plastic box.
Best 73
Bob
W4ETN
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IC-706 Linear Amp Keying Circuit
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by EA2BSN on September 22, 2009
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Bob,
Thanks a lot for the article.
There is also a a very neat amp keying solution for this rig which, however, requires to open the rig:
http://home.fuse.net/jg/Modifications/ic706.html
73
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IC-706 Linear Amp Keying Circuit
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by K4JRB on October 14, 2009
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Come on gang, Bob is showing you a way to use the 706 with an amp. Present your way without critizing. The circuit Bob presents is exactly the same as Yaesu used in several relay boxes.
Dave K4JRB
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