in need of switching information

[unit513]

Im trying to help a friend recently back from the war on bs,
he has lost the use of both of his hands and he is a radio guy.

here is a little view of his setup

he is running a 2995dx with an astatic mobile max for the "goodies"

i am currently making a complete foot switching setup for him so he can use his feet

i have already opened the case on the 2995 and connected to the dispay board switching so that he can change the channels and move the cursor for tuning via momentary switches.

With him using the mobile max it has plenty of extra gain for an UN AMPED mic.

What i have have done is wired up an d104 on a TUG 8 stand for him with the audio wired hot so that all he needs to do is switch the radio to transmit to talk

The moblemax uses a standard 4pin cobra plug, what pins to i need to use for the foot switch to key the transmitter?

I know this seems simple and im sure that it is; but i dont know for sure and i dont want to burn something up by guessing.

Any help would be great and know that you are helping guy that would give you the shirt off his back just to talk on the radio.

Thanks Again. ~513

 

[unit248]

The usual '4-pin cobra' scheme is that one pin is audio, one is ground, one is TX and the other is RX. The ground is used for both the audio shield and the RX/TX switching.

The RX pin is connected to ground to engage the receiver. The TX pin is connected to ground to engage the transmitter. With the D-104s that I've seen, you use the blue, black and red wires for switching: blue is common, and I think black is RX and red is TX. You can confirm this with an ohmmeter. When the mic is unkeyed, blue and black are connected. When the mic is keyed, blue and red are shorted.

Note that in this particular configuration, a simple single pole foot switch might not won't work: you can use the switch to connect the ground and TX pins together to engage the transmitter, but then letting go of the switch doesn't engage the receiver. I'm not sure of the 2995 works like that though: some radios just one mic pin that has to be connected to ground to key up, and the radio's internal switching circuity handles the rest. I think most ham rigs are like that. CB rigs tend to use both RX and TX control. A quick way to tell is to just unplug the mic: if the receiver shuts off, then you have an RX control pin in the mic plug, and if it stays on then there's just one PTT pin.

If you do need RX control and the foot switch is only single pole, then you may need to cook up something with a relay.

I have one question though: you said you set up a TUG-8 "with the audio wired hot." What does that mean? Did you fix it so that the pre-amp circuit in the base is always turned on? If so, the one problem with that is that you'll be constantly drawing power from the 9v battery. It'll go dead pretty quick that way.

This is also an issue if you want to use a D-104 TUG-8/TUG-9 mic for VOX. With VOX operation, the mic has to always be providing audio to the rig, but with a D-104 you don't get any audio unless you key manually first. Some people work around this by adding a switch to the TX control line so they can disconnect it and then slide the locking ring on the grip to talk bar up when they want to operate VOX. This engages the audio pre-amp forcing the radio to transmit. You need to eventually unlock the mic to keep the battery from going dead though. Another approach is to try to supply power to the mic from the rig through one of the unused wires, but this assumes you also have a spare pin in the rig to supply power through. It's really easy to create a ground loop this way. (Which is why I stopped doing it with my mic.)

-Bill

 

[unit513]

unit248 - the tug8 stand has no amp circuit in it.
i have takent the audio line straight from the mic head straight to the audio pin on the mic plug as is the ground/shield
there is no switching used at all on the stand - it is "wired hot"

there is no VOX mode on that transmitter

the way that the additional stand-alone preamp/nose box box is wired, it does not need RX control.

If i have read what you said correctly; all i need to do is wire the remaining foot switch between the TX PIN and the SHIELD/GROUND PIN.
So that would be 3 and 1 on the 4 pin plug mic plug.
When the switch is pressed the transmitter is keyed and the audio line is already hot so this should work!

I want to thank you for your reply. Any other comments or suggestions would be great!

 

[unit248]

Hm. I'm not sure how well that will work. The D-104 crystal element has a very high impedance (over half a million ohms). This was fine for use with older tube gear that had a high input impedance in the first place, but modern solid state rigs usually have a much lower mic input impedance. Depending on the radio, it could be as low as 500 ohm, and maybe as high as 50K ohms, but probably not much higher.

The pre-amp circuit in TUG-8/TUG-9/TUP-9 stands is intended to provide impedance matching as well as extra gain so that D-104 elements can be used with solid state equipment. If you remove/bypass it, you may end up with very low audio.

You didn't say if you already tested it -- it sounds as if you may have, in which case the result is probably satisfactory or else you would have mentioned otherwise. It could be the external pre-amp/noise-box is compensating for the missing pre-amp in the mic. If so, then disregard my prattling. (I wasn't sure if this was the case since I figured most noise machines would be designed to have low input impedance too, but if it's working the way you want it, then that's all that matters.)

It sounds like you have the right pins. Like I said, if the receiver stays on with the mic unplugged, then that means you just need to ground the PTT (push to talk) line to switch the rig into TX mode. From there it's just a matter of wiring everything up neatly.

-Bill