[ARC5] My ARC-5 40 meter station

[J Mcvey via ARC5]

You get alot more than 8 watts out. The output was designed for low impedance (5-12 ohms ) a few hundred ohm capacitive 5-J200 antenna.Specs say you should exceed 40 watts in CW  and have a 15 watt voice carrier.Perhaps try using a 9:1 "unun" to match 50 ohms.
 

     On Saturday, February 21, 2015 11:42 AM, Charles <charlesmorris800 at centurytel.net> wrote:
   

 Hello everyone... having recently become re-interested in the Command Sets (my first receiver in ‘78 was a NOS R-26 that cost me $5.00), I now have a working station on 40 meters.
(About ten years ago I acquired a BC-696A 3-6 Mc transmitter, built a screen modulator which didn’t work well, also built a linear amplifier with four 4-125A in parallel, and then life intervened and I lost interest.)

A month ago I happened to see at a local flea market a 7-9.1 Mc ATA transmitter without covers or tubes and a bent front panel, and also a complete clean 7-9.1 Mc ARA receiver with dynamotor and tuning knob, missing only the bottom plate. Got them both for $17! Straightened the transmitter front panel, replaced the tubes from my junkbox,  lubed the dynamotor, did the usual front panel mod to the receiver, replaced a couple of suspect bathtub caps, added IF regeneration and a single crystal filter inside the 1st IF can, based on Mike WU2D’s information.

I have now learned that heavy loading of a screen modulated transmitter is critical, and that the stock units do not have enough turns on the variometer link. I added one turn to the coil of the 40m transmitter and two turns to the 80m unit. After reading through Dave Stinson’s helpful notes,  also a 47 pf Russian doorknob in series with the roller inductor (68 pf for the 80m). Now they load into 50 ohms and modulate properly with the Antenna Loading set to about 60% coupling, and 60-70% on the roller inductor.

I quickly found that I had significant distortion at low frequencies (below 400 Hz). So I partially redesigned the screen modulator and power supply with some helpful recommendations from the folks at AMfone.net and fixed that problem. However, as built with 560V on the 1625 plates I only had about 8 watts of carrier (with the transmitter adjusted for 4 times that on 100% modulation peaks). That is not enough to make contacts much of the time on 40m with an off-center-fed dipole up about 25 feet. With higher B+ (700V) I have a clean 15W carrier, 60W PEP (and more if higher upward modulation is desirable). The complete thread including .pdf’s of the schematics can be found on the QSO forum at AMfone.net if you want the details.

I then turned my attention to the linear. It runs in Class AB1 with 3.2 kv on the plates, and requires very little driving power, mostly for the grid swamping resistor. 200W carrier, 800W PEP although I usually run it at 150W carrier in the interest of long tube life. 4-125A’s are cheap but they’re not that cheap. The ARC-5 can easily drive it to full output with nice orange plates.

Now I can join the daily AM activity on 7295 and 7160 with my “ARC-5” station! Currently looking for a decent 3-6 Mc receiver to go with the 3-4 Mc transmitter.
I have pictures but apparently this list didn’t like the Photobucket link?

-Charles
WB3JOK/0

ps I am trying to find a bottom plate for my receiver – anyone got a spare? thanks.
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