[ARC5] [Milsurplus] ATD + R4

Sun Nov 5 12:47:53 EST 2017

I don't think a conventional mike amp will work in place of  a carbon.
1) the carbon impedance is very low so it will kill the gain of of a medium-z amp.2) A good working carbon mike circuit develops several volts P-P .None of the Hi-z/medium -Z stuff has enough amplitude.

You can see for yourself with a simple test circuit.
25V>390 ohm> 25 mf cap to ground>100 ohm > carbon mike > ground.Put a scope across the mike and whistle..I think the T-17 was supposed to be in the 50 to 100 ohm area but none of the carts are good that I've seen.Also try an old fashioned telephone carbon mike. those are usually around 300 ohms, so increase the load resistor to 270 ohms.

    On Sunday, November 5, 2017, 11:03:00 AM EST, David Stinson <arc5 at ix.netcom.com> wrote:  

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Thanks for writing, Bruce.

From:  Bruce Gentry

Subject: Re: [Milsurplus] ATD + R4

>… Did you add or modify an existing preamp in the microphone base so you could use the original mic input on the transmitter? 

It’s an amplified-base D-104 mic.  I can’t say what the original owner

did as I’ve never examined another, just checked that 

it had a DC-blocking cap on the output, which it did.

Unlike many WWII Aircraft sets, the ATD has a mic amplifier.

When the ATD audio input is selected for “MAGNETIC,” 

it places 300 Ohms from the audio input buss to ground.

That’s fine for the Lo-Z elements used then, but kills the

output of Hi-Z mics most common today.  Lifting one end

of the 300-Ohm resistor is all that’s needed to use a Hi-Z mic.

I haven’t tried a non-amplified mic yet.  

>Does the transmitter have a built in T/R relay and some sort of muting output for the receiver?

It does have a TR relay, but no muting contacts.  I put a small relay

 across the keying relay coil (glued- no holes) and ran two contacts

 out via a small cable through the side vents- a NC for receiver muting

and a NO for external amplifier keying.  I could do all that with

an RF sensing circuit like they do for Chicken Band amps, but

that takes a lot of fiddling to get the coupling cap right.

Once I get everything mounted the cable won’t be noticeable.

73 Dave AB5S

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