Getting an "ARC-5" Transmitter On the Air.

David Stinson arc5 at IX.NETCOM.COM
Sun Mar 28 13:18:33 EST 1999

I've had about a dozen recent requests for this,
so it's time to post it for general information again.
Copied from a letter I sent to a fellow mil-radio sufferer ;-)

If I get a few requests for power supply information,
I'll post it along with diagrams on my web site.

I've cleaned out quite a few soldered-in rear connectors.  They're
just hollow tubes so if you have some solder wick or a solder
sucker, you should be able to get them cleaned out....

My first "dit" on the air (1972) was a BC-459 and I've been an avid
collector ever since.  At the time, I followed all the "conversion"
articles, not knowing any better, and hacked-up quite a few.
Now I appreciate the history and romance of these old war heroes.
I've restored many and put them on the air.  My first contact with
a complete-everything-dynamotors-too 274N rig was on 75 meters.
I fed it to a Swan Mark II amp so it was kinda loud for an "ARC5" ;-).

Now I have several complete sets working.
The only un-original part is the T-17 microphone element,
which I replaced with a telephone element.
The T-17 elements are very insensitive, which was fine
in the cockpit of a P-47 but not good in a ham shack.

Here's a few tips they didn't tell you in the conversion books:
All the talk about TVI in these rigs is pure, unadulterated B.S.
I've done the work with an IFR COM-120 and other instruments
and it just ain't so.
I think some person just repeats what he's heard said, etc.
The output tank was never designed to tune a 50-ohm antenna.
Hams typically ended-up with zero loading coil, full coupling
at the link, low power and too-high harmonic output.

The correct way to tune for a 50-ohm antenna is to put
a capacitor in series with the antenna lead before the coax.
If you don't have the external antenna switch with its
vacuum cap, you can use a high quality 1500V or better
*NP0* cap.  It must be NP0 because the cap will dissipate
some heat and if it drifts it will detune your tank and
cause chirp.  Just solder it in place of the lead between the
tank link coil and the loading coil.  About 50 pf for 40 meters
and around 75 pf for 80 meters.  This will give you full output
at about 2 o'clock on the link and around 1/3 rd of the loading
coil, if memory serves.  I've made extensive measurements with
hi-tech comm equipment and the transmitters are cleaner then modern
synth rigs when tuned properly.  You can also use a 4:1 "UNUN"
toroid.  This will eat a little power but works fine and
you get to watch the RF ammeter in the BC-442 swing ;-).

The only "modifications"  needed to a Command transmitter if you're
not going to use the whole set-up are simple and 100% restorable.
As far as I know, they were first suggested by "Boatanchor Bob."

1.  (Except this one.  He and I disagree a little here.
He's never found a bad one and I've never found a good one!
Probably because he's a nicer person then I am ;-).
Replace the three capacitors in the bathtub on the back.
Even if you think they're good, they're bad.  Trust me on this.
Leakage in this bathtub cap causes big headaches.
If you're not crazy like me (I open the bathtub, replace
the caps then seal it back), here's an easy way-
Snip-off the leads of the bathtub cap, leaving the wires intact.
Coat them with insulation.  "Liquid Tape" by GC is good and available
at most electronics parts places, or just use tubing.
Leave the wires where they are as their distributed capacitance
is part of the design.
Get some of those little mylar .047s from the parts house.
They're about the size of a pencil eraser.  Trace the wires from
the bathtub to where they connect, then solder the cap between
there and the nearest ground.

2.  Unsolder one lead from the antenna relay.  Roll the spring
contact from the loading coil around so it's always in contact.
Either put some Cramolyn on this contact or polish it as
detailed below.

3.  Check the selector relay and make sure that the osc. B+
contact makes before the 1625 cathode contact.  Carefully bend
them to make sure.  You key the rig with this relay.
Put a "spike killer" diode across the relay coil winding
(anode to positive voltage lead) to keep the inductive
kick off the filaments and keying leads.

That's it.  All the hole drilling and output hacking and
"de-TVIing" of the 50s and 60s was completely unwarranted.

If you really want to make it easy to tune and remove a
source of possible "chirp," here's a good exercise.
This is some work but well worth the effort-

All the roller coil assembly metal parts (less screws)
are plated in silver. This gets oxidized (that black stuff).
When you're loading the rig, you'll notice "intermittent" power
out as you roll over the gunk.  If your loading point is
cruddy with this black stuff, it will heat when you key
and change the loading enough to introduce some chirp.

Go to Wally World and get some cream silver polish.
Don't waste your money on the "dip" stuff.
Put a towel or other "parts catcher" on your bench.
Completely disassemble the roller coil unit.
The leaf-spring contact on the coil axle (ceramic support end)
needs to go back right-side-up, so you might want to mark it
or just notice how it goes back in.
Use a linen or other lint-free cloth to polish the coil,
roller bar, roller, axle leaf spring contact etc.
Use a "Q" tip to polish the inside of the roller wheel as well.
Once all the crud is polished away, rinse off all the polish
with clean water and allow the parts to completely dry-
especially the coil.  Don't want water under the turns.

As you reassemble, lubricate the thumb-wheel end and thumb-wheel
axles with petroleum jelly or graphite.  Do not lube the
ceramic-to-coil joint, but do lubricate the roller coil axle and
leaf-spring contact with paste Cramolyn or other
good contact lube.  Be careful not to overtighten the screws
that are threaded into the ceramic!
Just finger-tight them and lock them in place with some
paint or fingernail polish.  Seal the roller-bar screws
with paint as well.  Now enjoy easy tuning.

On "chirp:"  This is a classic MOPA rig.  While you can expect
some small amount of chirp,  I've got many transmitters in which
it is difficult to hear any at all.  I've never had one that,
after I finished restoring it, had what I call "bad" chirp.
Noticeable chirp in and "ARC-5" transmitter can usually be
traced to a few sources.
Here they are in order of their usual appearance:

1.  Poor power supply regulation.
Your supply needs guts enough to maintain 500-600 VDC at 180 mils
and 24 VDC at about 2.5 amps.
(if you don't want to use DC on the fils, put a diode and
filter cap at the relay)

2.  Leaky bathtub caps.  Reduces osc. drive, messes with
B+, reduces grid bias to the 1625s.  See above.

3.  Cruddy output tuning network or antenna connections.
Anything that changes the loading- like heating the crud
between the roller and coil- is going to pull an MOPA rig.

3.  Low under-load emission in the 1626 Osc.
Swap it out with a known-good one.

4.  Grid emission in one or both 1625s.  Especially
noticeable in one that's been run at 800+ volts.
Swap with known good ones.

I've seen changed resistor values in the osc. stage
and misc. other stuff, but these four are the usual bandits.
While I've heard of the other mica caps going bad, I've
worked on many dozens and never seen it happen.  I've
also never seen that custom "button" cap go bad, though
I've heard of that, too.

I hope these tips will be of value to you.

Good Luck with it!

73 DE Dave Stinson AB5S
arc5 at

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