[ARC5] SCR274 AM modulation question

[seanbart at fidnet.com]

I had the same problem with low percent modulation when trying to use an original T-17 microphone.  I replaced the carbon element with an electret element and amplifier using the design here:
 
[ http://www.vmarsmanuals.co.uk/newsletter_articles/Electret_Mic_Replacement.pdf ]( http://www.vmarsmanuals.co.uk/newsletter_articles/Electret_Mic_Replacement.pdf )
 
It is pretty tight getting everything inside the T-17 case using through-hole components.  Using SMT components would probably fit pretty well.  I get a nice waveform when doing the trapezoidal pattern and the audio sounds nice on a monitoring receiver.  I have called CQ a few times on 7290 kHz but have not gotten any replies yet.
 
I get 45-50 watts of carrier out on CW and about 20 watts of unmodulated carrier on voice.  This is all with the BC-456, DM-33, and 24V supply.
 
Sean - KB0OVD
 
 
-----Original Message-----
From: "J Mcvey via ARC5" <arc5 at mailman.qth.net>
Sent: Wednesday, March 15, 2017 7:37pm
To: "ARC5 at mailman.qth.net" <ARC5 at mailman.qth.net>
Subject: [ARC5] SCR274 AM modulation question




Been a while since I've played with the Aircraft radios, but I got back to them yesterday.
This system is just about 100% wired as the original design ( except for the ham-made ft120 receiver plugins), BC456, dynamotors and all ..
Everything worked when I fired it up, seemed to put out about 40 watts.  The rf ammeter shows approx 2 amps into  a 10 ohm load in CW mode.
It drops to about half of that in AM dead carrier.
I was rather disappointed with the weak modulation. using a T17. I've used this same mike on other WWII gear with good results, so I don't think it's the mike.
I replaced the cathode bypass cap and the mike supply filter cap, and the 1625 with very little improvement.
I could barely hear myself in another radio . Even with the dummy load , I had to attenuate the receiver. With this kind of signal I would think there would be loud audio feed back, but not even close! 
Not very scientific,i know, but a good rule of thumb.
Any suggestions?
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