[ARC5] ARC-5 Transmitter

Sun Dec 19 00:33:52 EST 2004

I am looking for a relatively simple approach to getting an ARC-5 
type transmitter up an running, but running effectively. The keyword 
here, is simple. Before jumping into this list, most of what I knew 
came from the well known article linked at the ARRL website. Look at 
the last download in the list and check it out.

http://www.arrl.org/tis/info/tranrcvr.html

My impression of this old article is that it was not so much a major 
"conversion" as a  way of developing a practical power supply. The 
conversion of that actual transmitter (aside from the Novice Xtal 
outboard unit addition - which I was going to skip anyway) seems 
mostly to be about powering the tube filaments. The only other thing 
suggested was the addition of a place to plug in the cw key.

So here are some of my naive questions:

1. How was it keyed originally, and aside from the "museum" 
consciousness, is adding the key jack as suggested in this article 
considered an actual circuit "modification". I cannot tell since I am 
not clear how it was keyed in an original set up. How are you purists 
keying this? Where are you plugging in your key?

2. I do not recall in this article, that they suggest adding a coax 
connector for the antenna. Were some hams in the 50s simply 
connecting their antenna wire to the original terminal, and if so, 
what would be the most effective and simple antenna that one could 
use now, to avoid what seems like complexity as indicated in the 
discussion we are currently having. Was it an end-fed wire, how long? 
(Not such a good antenna right?) What other antenna systems would be 
simple and effective for getting these units going? How difficult is 
it, when you boil it down to simplicity, to add a way of feeding a 50 
ohm line. I am somewhat confused by all the discussion. Who is doing 
well? and what are you doing?

3. I assume that one would need to add a good grounding terminal to 
the unit. Cannot say I understand how that was done in the airplane, 
but for ham use, I assume those who are using these units are 
grounding the chassis to a good earth ground. Does this not mean 
adding some kind of a terminal lug to this chassis?

4.  Aside from the issue of preserving these units absolutely intact 
as museum pieces, what really are the operating problems that would 
ensue from pursuing the informtion in this article. The only thing I 
now understand is that the voltage to the oscillator needs to be 
better regulated than the power supply that was suggested in the 
article. The other down-sides seem mostly to relate to
physical alterations rather than circuit alterations. Those being: 
changing power plug, addition of key jack, changing how the filaments 
are powered. Other than that it seems that the circuit is still close 
to original, unless one tries to utilize antenna systems (50 ohm) 
that were not considered in its design which apparently is 
problematic.

That article, outdated as it may be, at least seemed like a simple 
thing I could do. Now I am so overwhelmed with these other 
considerations that the project is momentarily at a standstill. What 
we need is for some mentor to publish a new manual or pamphlet on the 
best current way(s) for getting the ARC-5 transmitter into ham use 
operation. As you can tell I am not really an engineer, just a ham 
willing and quite able with a soldering iron and a schematic 
instruction book.

Anyone ever hooked up an ARC-5 transmitter using an old Johnson 
Matchbox with balance line?

Who here has been really successful using this transmitter, can you 
describe your approach in terms that another ham can simply adapt?

For example, if one already had one of those power supplies indicated 
in that article mentioned above from the ARRL website, what specific 
additions to that power supply would remedy the problems?

Thanks for any help,
73
Bill
K6TAJ

>>Stinson's method is part-complete and unsupported with an explanation
>>
>>Your problem is to adapt the thing to run into something beside
>>a short random wire that looks like low-R and big Xc
>>
>>Scroll the loading coil back to left to discard the Xc compensation (with Xl)
>>
>>Then connect a broadcast (370pf) variable in series w. 50 ohm resistive
>>load and spring-terminal on front.  This'll be adjustable Xc to tune out
>>Xl of swinging link in ARC-5 tank coil. Same boolah-boolah for a T19 or a
>>T-22.
>>
>>Connect "command set" to dummy load & muck with broadcast variable
>>& link adjustment 'til you get, say, 40W into dummy load when 600V
>>on 1625 plates.
>>
>
>Insertion of a series Xc is an old technique, and is precisely what 
>Dave suggested.  It is also part of the tuneup procedure cited in 
>the command set manuals for certain antenna length and frequency 
>combinations.  So far I don't see a different pony in the pile 
>above.  The idea of using a /variable/ cap is a good one, at least 
>for establishing a reference point - it will give you a ballpark on 
>a particular antenna for a particular transmitter at a particular 
>frequency.  The risk in using it permanently is arcing in the 
>receiver capacitor if the antenna VSWR is high, though - several of 
>us have experienced that.  I suspect that's why Dave was 
>recommending using a 1 kilovolt minimum cap.  The MOPA design is 
>fairly sensitive to changes in antenna impedance, and using an NPO 
>cap simply makes things more stable.
>
>>Don't screw with the Un-Un hooey since it doesen't address the reactance
>>questions at all.  The Un-Un procedure is as effective as putting
>>a fotograph of a parakeet in with the 1626
>>
>
>Well, let's think about this a bit.  I wouldn't call an unun 
>transformer 'hooey' unless I had solid technical reasons to do so, 
>and I don't see anything except denigration without a basis - sorta 
>"part-complete and unsupported with an explanation..."   :-P   That 
>raises my professional hackles a bit.  I'll just chalk it up to 
>having a bad hair day that caused you to be unusually didactic, not 
>to mention discourteous concerning Dave's practical guidance.
>
>I seem to recall that your mentor told you that you needed a 
>conjugate match for maximum power transfer.  I also seem to recall 
>you saying at one time that you weren't overly impressed with that, 
>but the math doesn't care what anyone thinks about it - it's just 
>physics.  Sure, you don't have to have maximum power transfer to get 
>a good signal out - the QRP guys communicate with tiny power levels 
>all the time.  I have absolutely nothing against folks who enjoy 
>playing slop pool, or shooting a few baskets in an offhand manner, 
>or throwing an antenna over a tree branch and using the system at a 
>fraction of its capability.  Lots of fun doing that sort of relaxed 
>activity - all of us have enjoyed the instant gratification element. 
>But sometimes it's just as much fun to chase perfection as it is to 
>throw a wire over a limb and call it good.  That's one reason I have 
>a problem with dismissing a feasible engineering solution for those 
>who are trying to get the last watt out of a system - at least 
>without having solid technical reasons to do so.  For those who 
>pursue the goal of maximum power transfer, I'm afraid I'd have to 
>disagree with your assertion about its effectiveness.  Just so I 
>don't leave this unsupported by any explanation, here's my view:
>
>Ideally, the PA tank coil and rotating link at resonance in this 
>particular case is simply part of a transformer, matching the plate 
>resistance of the parallel 1625 tubes with the 5 to 12 ohm resistive 
>component of the aircraft antenna.  Like many aircraft sets (the 
>ART-13 is a partial exception,) the command transmitters used the 
>capacitance of the aircraft antenna as part of the tank circuit. 
>Its presence is also important in reducing harmonic components - one 
>of the reasons the command sets developed a bad reputation for TVI 
>when fed into a straight 50 ohm resistive antenna.  The extreme in 
>this practice is the Bendix TA-2 series, which had no tank capacitor 
>whatsoever - it depended /completely/ on the antenna (and residual 
>capacitance) to furnish that component.
>If you have a 50 ohm pure resistive ham antenna fed by a 50 ohm 
>transmission line and are lucky enough to have tuned out capacitive 
>or inductive reactance at the driving point where it would connect 
>to the transmitter, you'll still need a capacitive reactance to load 
>the command transmitter properly.  You will also still have as much 
>as a 4:1 pure resistive mismatch with many of the command 
>transmitters that does not satisfy the conjugate match requirement 
>for pure R.  You can solve that in a number of ways - with outboard 
>network components (like a pi network), winding more turns on the 
>rotating link, or putting an outboard wideband transformer (an unun 
>or balun, depending on the transmission line and antenna you're 
>feeding) at the junction of the transmitter output and the 
>transmission line.  Failure to do so is like feeding a 16 ohm 
>speaker with a 4 ohm transformer output - you may hear the music 
>fine, but you won't be able to pump the room with full power until 
>you get the strapping correct.  Frankly, for me the unun is a lot 
>easier solution than the others, but it's not the only solution. 
>The other thing you may have forgotten is that a wideband unun can 
>also transform the reactive component appearing at the driving point 
>- not just the resistive component.  That sometimes brings the 
>oddball antenna system impedance that most of us have from a 
>practical standpoint at our QTH  into the range that can be 
>accommodated by the command transmitter tank and output network.  It 
>also makes the antenna current meter stand up straight and start 
>working again, which is another reason I favor getting the Ro match 
>close to correct.  Not a big thang, but it's neat to watch it swing 
>up to 30-40% of FS with every key stroke.
>
>So, by all means, don't use an unun if you think it's hooey - 
>doesn't make any difference to me.  But don't mention parakeets 
>along with it or you'll get another boring dissertation from me 
>again.  Next time I'll force you to endure the math - complete with 
>derivation from Maxwell's equations...aauugh...
>
>Hmmm...come to think of it, I wonder if putting a parakeet picture 
>next to the M.O. would enhance the transparent highs and effortless 
>lows I've been trying to find?  I'll have to try that...heh...
>
>73,
>Mike
>
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