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Hotfoot keying adapter for ham linears. No joke.

nomadradio

Analog Retentive
Apr 3, 2005
7,589
12,465
698
Louisville, KY
www.nomadradio.com
Here's a product I never thought I would announce. For years my advice to keying a ham linear from a CB or 10-meter radio was to install a relay inside the radio. A patch cord keys the ham linear as soon as you key the mike, and drops back to receive when the mike does. No chatter, no sideband switch.

Smooth.

But some folks just don't fancy opening up the radio to install a relay and socket. They want external gadgets for their noise toy, echo and whatever.

So here is the prototype for a "Hotfoot", a keying circuit in an external box to use with a ham linear and a radio that has no amp-keying feature on the rear panel.

PcZ3En.jpg


Dirt simple. This end just fits the linear's 'radio' socket.

1HXZcR.jpg


The other end takes the coax jumper to the radio.

h6CT2W.jpg


Not much on the inside. The same keying circuit we sell on fleabay to put inside your linear. A short coax and the wire with the RCA plug for the linear's "relay" socket.

fEYwRO.jpg


Biggest expense building it is the labor. Punching/drilling holes and bolting things down takes longer than stuffing nine parts into a tiny pc board.

No, there's no sideband switch. If you're serious about sideband you'll put the relay inside the radio. Should be okay on AM.

Oh, and if the amplifier says "National" on the front, this won't work with it. That one uses AC to power the relays. Likewise if it says "Heathkit" on the front. SB220/221 or SB200/201 use a high-voltage relay coil. It's not built for that. The only one with the Heathkit brand it's compatible with is the SB1000 model. That one isn't really a Heathkit, it's an Ohio Ameritron AL80A with Heathkit's name on the front. Works with Ameritron, Drake, AEA or any amp with a 12-Volt or 24-Volt DC relay.

No idea of an asking price yet. This is the only one so far. I'll build the next half-dozen and average out the time required. Only then will I have any way to know what each one really costs me to build.

Odds are the selling price will be stupidly extravagant. Got no source of cheap labor who can do that kind of work.

And if the price is so high they don't sell, at least I'll know not to build any more of them.

Film at 11.

73
 

Had a very-minor brainstorm. Couldn't figure out a way to make our existing keying circuit work with Heathkit linears. Didn't want to have to put a loud attention-getting sticker on the box that says "DON'T USE ON A HEATHKIT". Should work on Drake, Ameritron, AEA, Dentron, Amp Supply and any other with a 12 or 24-Volt DC relay.

But the SB200/201 and SB220/221 both use a relay-circuit voltage around 100 Volts DC. And the polarity is positive for the 220 and negative for the 200.

Decided the best solution may be to just start over and 541t=can the bipolar-transistor circuit. A IRF630 MOSFET rated at 200 Volts will be sufficient. A rectifier bridge will serve to "turn around" the DC polarity for either amplifier.

The one drawback is the voltage drop through the bridge rectifier. Probably won't cause trouble in a 24-Volt circuit. Gonna have to hook it up and try it with a 12-Volt relay.

This is just the brain-storm level of the idea, not complete. The ground for the amplifier's "relay" jack has to be kept separate from the actual keying transistor circuit so it can "float" and accommodate positive or negative ground.

PekMeY.jpg


Haven't built it to try yet. That's next. A universal hotfoot that works on any amplifier just seems more appealing than the first-version design above.

73
 

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