[ARC5] SCR274 AM modulation question

[CHRISTOPHER BOWNE]

Few things to watch out for:
1. If you are trying to screen modulate an ARC-5 transmitter with a SCR-274N BC-456 screen modulator, you must change the final screen dropping resistor to the value used in the Signal Corps SCR-274N transmitters. The ARC-5 transmitters are set up for plate modulation with the MD-7/ARC-5, and the screen dropping resistor is not the same value as the 274N transmitters. That is the only transmitter circuit change I had to make in my mashed up system with the BC-456 modulator/dyno unit, T-19 and T-22 ARC -5 transmitters, ATA antenna relay, and SCR-274 4 channel transmitter selector/control box, outside of figuring out the correct cable connections across the SCR-274/ARC-5 pin out differences.
2. The AM modulation percentage is very sensitive to how the final amplifier loading is tuned up. You must tune for maximum output in CW mode and then switch to voice mode, do not readjust the loading for max output on voice modulation or you will significantly lose modulation percentage. Some of the transmitters have a warning plate on the front panel regarding how to tune up for voice mode in this manner. Once you get the set loaded up in this manner, you should watch the RF waveform on an o scope and carefully tweak the output coupling control for best mod peaks. Typical RF output in voice mode is about 1/3 that of CW, if you are seeing 50% that is a bit high for full modulation.
3. As a diagnostic, do you get full modulation in MCW mode? It should be at least 100% or possibly even overmodulated with some negative peak clipping. You can use the MCW mode to optimize modulation depth as described in the step above.
3. Modulation depth and quality varies a lot from one 70 year plus old T-17 or RS-32 carbon mic to another, you may in fact have a bad one. Even the best of mine require very close loud talking to get anywhere close to full modulation.
4. Try feeding in outboard low source impedance audio, say a couple of watt mono amp driving an 8 ohm voice coil to 500 ohm matching transformer in reverse, with a series non polarized cap of a couple of microfarads to keep the DC carbon mic excitation voltage off the transformer output winding You should be able to get some great sounding wide bandwith low distortion modulation out of this technique, works well on the TCS transmitters also. Its startling to do an A/B comparison of how these rigs sound driven with a carbon mic vs with high quality external audio chain.
5. If you want to stay on theme with a PTT mil mic, there are several circuits out there to build a carbon mic functional equivalent with a RS electret element and a small preamp/driver stage. The mic element and circuit can be fit into a T-17 case so it looks period correct.

73 and good luck, Chris, AJ1G Stonington, CT


> On March 15, 2017 at 8:37 PM J Mcvey via ARC5 <arc5 at mailman.qth.net> wrote:
> 
>     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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