[ARC5] SCR274 AM modulation question (CHRISTOPHER BOWNE)

[Charles Morris]

-----Original Message----- 
Message: 1
Date: Wed, 15 Mar 2017 22:44:55 -0400 (EDT)
From: CHRISTOPHER BOWNE <aj1g at comcast.net>
To: J Mcvey <ac2eu at yahoo.com>, J Mcvey via ARC5 <arc5 at mailman.qth.net>
Subject: Re: [ARC5] SCR274 AM modulation question
Message-ID: <1339659185.50106.1489632295601 at connect.xfinity.com>
Content-Type: text/plain; charset="utf-8"

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
------------------------------------------------------------------------------------

I can't speak to the original microphone and modulator, but I built my own
screen modulator (pretty much cook-book design I pieced together from
various '60's ARRL Handbooks and the old Heathkit DX-60 screen mod
schematic). There was an extensive discussion of this on AMfone.net about
two years ago, check it out.

http://i1181.photobucket.com/albums/x426/DrCharlesMorris/ARC-5/P01-31-15_09.081_zpsru4lh22l.jpg

I can attest to #2 above that loading is VERY critical and it is likely 
not
where you are used to loading up for max smoke. Don't put one of these on
the air without a scope!
Fortunately at 3.5 or 7 Mc. it is easy to sample the RF output directly
without a diode detector, on almost anybody's bench scope :)

It won't quite go to 100% negative modulation without distortion ("tetrode
kink" as the instantaneous plate voltage dips below the screen voltage).
Here is a 1 KHz sine input:
http://i1181.photobucket.com/albums/x426/DrCharlesMorris/ARC-5/1%20KHz%2040m%20ARC-5%202_zpsr4sjwpdy.jpg

-Charles
WB3JOK/0

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