[ARC5] [Milsurplus] ATD + R4

[Richard Knoppow]

    The resistance of a carbon button will depend a lot on the condition 
of the carbon particles. If you measure one and shake it up the 
resistance will change.
    A carbon mic is a controller rather than a generator. The power 
comes from an external source, frequently a battery but can be a DC 
source in a transmitter or by putting the mic in the cathode of a tube.
    I do not understand why the transmitter under discussion has a 300 
ohm resistor across the mic for a magnetic mic. A magnetic mic is 
electrically identical to a moving coil mic. Its impedance depends on 
the pickup coil and can be virtually any value from very low to several 
Kohms. In general, microphones are not terminated in their source 
impedance (BBC practice not withstanding). Rather even low impedance 
microphones are usually terminated in an impedance several times the 
source impedance. If one desires the best signal to noise ratio this is 
the right way to do it. I looked at the schematic and find the input 
circuit a bit puzzling.

On 11/5/2017 9:47 AM, J Mcvey via ARC5 wrote:
> 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:
> 
> 
> 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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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL