Rig/Amp Selector

Ever key a transmitter into an open circuit?

Once upon a time, not too long ago, I decided that I needed to make my station more idiot proof. After all, the Chief Operator here at W7KF can sometimes display certain, shall we say, less than desireable traits..

It occurred to me that it would be A Cool Thing to have some sort of Rig and Amp selector switch. It would be really nice if I had some sort of accessory that would:


OK, repeat after me: "Oooooooh, that would be A Cool Thing"

Yet another cool thing would be if the device were totally passive. No active components, no CPU required. It should be functional yet simple. It should be (dare I say it?) Idiot Proof.

So, I dragged out the proverbial blank sheet of paper and proceeded to draw up the following:

Rig/Amp Selector Schematic

Rig / Amp Selector Schematic (click for full size view)

As you can see, the heart of this thing is a Six Pole, Three Throw (6P3T) switch.

The first three poles are used to switch the RF and the logic is quite simple.

When the switch is in position #1, transmitter #1 is connected to the output. Transmitter #2 is connected to the first dummy load and transmitter #3 is connected to the second dummy load.

In position #2, (shown in the schematic) the second transmitter is connected to the output while transmitter #1 is connected to the first dummy load and the third transmitter is connected to the second dummy load.

In position #3, the third transmitter is connected to the output while the first transmitter is connected to the first dummy load and the second transmitter is connected to the second dummy load.

It doesn't get any simpler than this!

The second three poles of the switch are used to either route a single input (say, the CW keyer) to one of three outputs (the rig key jacks). Or, to route three inputs (say the rig speaker outputs) to a common destination (a speaker).

The second switch is for selecting which amplifier is in-line. Basically, it's a 3P2T and simply routes input control lines to one of two possible output lines. I'll use one set to route the Key-Line to the proper amp and one set to route 24 VDC to a pair of RF relays that will switch the RF In and RF Out lines of the two amps. The third set is held in reserve in case something else comes up later. Again, it just doesn't get much simpler than this!

Each of the connectors for this signal routing are bypassed to ground since there is a little RF inside the box.

Now, about the dummy loads...

No way I was going to pay big bucks for commercial dummy loads. Hey, all I need is something near 50 Ohms, non-inductive. So, I went to my favorite Junque Box -- eBay. There I found an outfit in Ohio selling 50 Ohm, 10% tolerance, 90 watt, non-inductive resistors complete with mounting clips. Exactly what the project needed. A pair of these beauties ended up costing me about $30 including shipping to Montana!

Well, it all looked good on paper. The box, the switches, the dummy loads, the RF relays and the knobs were all in hand so I only had to order a kazillion RCA Phono connectors and scounge up some SO-239 connectors. And, then find the courage to begin!

Rig/Amp Selector, Rear Panel

OUCH! That's a LOT of RCA connectors! There are 21 of them! Plus the four SO-239 connectors. It was a tight fit but I really wanted to use this particular box. It is the exact same size and configuration of the box I used for the control head for my Switched L-Network Antenna Coupler and I wanted them to match. Fashion is everything, you know..

Rig/Amp Selector, Rear Panal Finished

OK, the rear panel is done. I had to make sure all the connectors were labelled so I had at least half a chance at getting everything hooked up properly. Guess I better start building up RCA interconnect cables too!

Rig/Amp Selector, Interior

With the rear panel in place it was time to start wiring up the whole affair. The first step was to mount the front panel switches and the two dummy loads. And, then to remove the rear panel and install a kazillion RF bypass capacitors, one on each RCA connector. Then, it was a simple matter to start running wires and soldering them in place. Once each connector had a wire attached I knew I was done.

Rig/Amp Selector, Interior - All wired up

And, that's about it! Except for the final step, installing the beast.

Rig/Amp Selector, Installed

In the current configuration of W7KF, two of the banks of Rig Selector RCA Phono connectors are in use. One routes the output of the CW keyer to each of the three possible rigs. The other routes the 8 Ohm speaker output from each of those three rigs to a common speaker.

Two banks of Amp Selector RCA Phono connectors are in use. One switches the Amp Key line to the appropriate amp and the other routes +24 VDC from a wall wart to the two RF relays (not shown) that route the RF into and out of the appropriate amp.

It's done! Now I can select which rig I want and which amp I want with the mere flip of a switch. And, the two un-selected rigs are routed into 50 Ω safety loads so that even if I try to tune one of the off-line rigs nothing bad happens.

So, there ya have it. The Rig/Amp selector -- it switches one of three rigs into one of two amps safely. It's pretty convenient to be able to fire up any transmitter and not have to think too hard about whether or not it's connected to anything. It's also pretty nice to be able to A/B compare signals with various rigs.

And, hey! It's idiot proof. Unless I get better at being an idiot...