David Stinson’s ARC-5 Posting

December 18, 2004

 Date: Fri, 17 Dec 2004 15:16:41 -0600
 From: David Stinson < HYPERLINK ""
Getting an "ARC-5 " Transmitter Running- Preparing The Transmitter
  (per request)
 The only "modifications" needed to a Command transmitter are simple and 100% restorable.

1. 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, and if it isn't leaking now, it will be. Here's an easy way: Unsolder the leads of the bathtub cap, leaving the wires intact. Make sure multiple wires stay connected to each other, just not the cap.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 (the "short" one, nearest the coil)
 makes before the 1625 cathode contact.
 Carefully bend  them to make sure. You key the rig with this relay

 through it's contact on the back plug-it's called the "selector."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.

4.  Easy tuning.   If you want to make it easy to tune and remove a source of possible "chirp," the next steps are

 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 (yes, believe it or not).

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. Pay close attention so you can get it back together in the right order. 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.

5.  Chirp.  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 an "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 200 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 Osc 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 the freq of an MOPA rig.

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

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

(6.) I've seen changed resistor values in the osc. stage and misc. other stufff.But these five 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 in the MO stage go bad.

 I hope this will help out.
 Next post for the power supply.

 73 DE Dave Stinson AB5S


 Message: 2
 Date: Fri, 17 Dec 2004 15:17:39 -0600
 From: David Stinson < HYPERLINK ""
 Subject: [ARC5] " ARC-5 " Transmitter Antenna Matching.
 On Loading an Antenna:

The SCR-274N and AN/ARC-5 radios were not designed to load a 50 ohm antenna. They were intended for antennas of about 5-12 ohms. In order to load a 50-ohmer you'll need to do one of two things:

1.  Put a Cap in series with output. Cheap and easy- put a good quality NP0 (no thermal drift) capacitor of at least 1000 volts between the antenna terminal and the coax center conductor (in series).For 40 meters, use 50 PF. For 80 meters, use 75 PF. Non-NP0 caps may heat, drift and cause chirp.

2.  Use a 4:1 UNUN Balun. More trouble but better: Wind a 4:1 "UNUN," which is an unbalanced to unbalanced balun, and use it at the antenna connector to match the coax. The design is available in the Handbook. Will cost a couple of watts but worth it.

I hope these tips will be of value to you.

 Good Luck with it!

 73 DE Dave Stinson AB5S


 Message: 3
 Date: Fri, 17 Dec 2004 15:24:12 -0600
 From: David Stinson < HYPERLINK ""
Subject: [ARC5] " ARC-5" Transmitter Power Supply.

On an ARC-5 Power Supply:

1.  3 Flavors of “ARC-5”. First- there are three "flavors" of transmitters commonly lumped under the name "ARC-5"

The first is the Navy ATA, second is the Army SCR-274N and last is the Navy AN/ARC-5. The rear connectors and wiring for the ATA and SCR-274Nrigs are identical.  The AN/ARC-5 connector is physically different and the wiring is also somewhat different. If your transmitter is black wrinkle painted and has a big RF choke next to the tank coil, it's AN/ARC-5.  If it's black and has not such coil,  but has an anchor in yellow paint on the back skirt, it's an ATA.  If it's silver, it's a later SCR-274N. If it's black, has no RF Choke and has a "BC-4xx"  number on the back skirt, it's SCR-274N.


2. Transmitter Plug Pin-out: (Note: If yours has an octal socket or anything other then the 7-pin Mica connector, then it's a modification and all bets are off.) 

For AN/ARC-5-There are seven pins. The pin in the center is pin #7. Locate the pin that is grounded to the chassis. This is pin #4.Count the others *clockwise* around accordingly.

 1. No Connection
 2. +200VDC for the Osc. (needs to be right for stability).
 3. Keying Relay (ground to key).
 4. Ground.
 5. A+ 24-28 VDC.
 6. PA Screen Voltage (use 20K 5W dropping res. from B+).
 7. PA Plate B+ Voltage +550VDC

For ATA/ARA and SCR-274N: There are seven pins. The pin in the center is pin #7. Locate the pin that is grounded to the chassis. This is pin #1. Count the others *clockwise* around accordingly.

 1. Ground
 2. Test point for PA grid current.
 3. +200VDC for the Osc. (needs to be right for stability).
 4. PA Screen Voltage (use 20K 5W dropping res. from B+).
 5. Keying Relay (ground to key).
 6. A+ 24 VDC.
 7. PA Plate B+ Voltage +550VDC.

3.  Power Supply Requirements and Design. The best way to get power to the rig is the way the original engineers intended. I don't mean a dynamotor- I mean a good, solid 550 VDC source and a divider. Both the AN/ARC-5 and SCR-274N units derive their voltages using a voltage divider network. We'll draw a divider that matches the one used in the original power supply. I very much recommend it.

Primary power should be 24-28 VDC at about 2.5 amps for relay and filaments and B+ of 500-600 VDC at about 200 MA. We will assume the designer's specs of 550 VDC B+ and 26 VDC filament. Get a pencil and a piece of paper. Follow along:

Place a dot on your paper and call that point "A".
Draw a line right to point "B".
From point "B", draw a line down and connect a 20 K-ohm, 5 watt resistor here.
Call the other end of this resistor point "C". 

Draw a line right from point "B" to point "D".
From "D", draw a line downward and connect a 15 K-ohm, 10 watt resistor. Call the other end of this resistor point "E".
From point "E", connect a 100 K-ohm 1 watt resistor to ground.
Draw a line right from point "D" to point "F".

Draw a line right from point "E" to point "H". 

+550 VDC in at point "A".
+550 VDC out to PA plates at point "F". 

+270 VDC out to PA screens at point "C". 

+200 VDC out to Oscillator at point "H".
Bypass all outputs with about .01 ufd to ground.
If you wish to regulate the screens for AM, 
connect an 0D3 VR tube with a .5 ufd at 300VDC cap across it between point "C" and ground. This will bring the screen down to 150 volts.

If you use this network, and use correct antenna matching, you will be able to properly load and tune your command set transmitter to rated output.

Voltages lower then 550 VDC will work just fine with the same network. Voltages much higher will cause MO drift and increase chirp.


 Message: 5
 Date: Fri, 17 Dec 2004 16:41:52 -0500 (EST)
 From: "Marty Reynolds" < HYPERLINK ""
Subject: Re: [ARC5] " ARC-5 " Transmitter Antenna Matching.
  On Loading an Antenna:
The SCR-274N and AN/ARC-5 radios were not designed to load a 50 ohm antenna. They were intended for antennas of about 5-12 ohms. In order to load a 50-ohmer you'll need to do one of two things:
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. Don't screw tith 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
 Message: 7
 Date: Fri, 17 Dec 2004 19:59:26 -0600 (GMT-06:00)
 From: Mike Morrow < HYPERLINK ""
 Subject: Re: [ARC5
] Getting an "ARC-5 " Transmitter Running- Preparing
 The Transmitter

 David Stinson wrote:

2.  Unsolder one lead from the antenna relay. Roll the spring contact from the loading coil around so  it's always in contact.

3. Check the selector relay and make sure that the osc. B+ contact (the "short" one, nearest the coil) makes before the 1625 cathode contact. Carefully bend them to make sure. You key the rig with this relay through it's contact on the back plug-it's  called the "selector."

On "chirp-" This is a classic MOPA rig. While you can  expect some small mount of chirp, I've got many transmitters in which it is difficult to hear any at all.

 Hi David,

 It makes one wonder why, when these rigs were in their ham heyday, so much ham "re-design" was thought to be necessary.  The basic design seems to be very sound for the era.  But it would be interesting to see measurement of the spurious output spectrum when the set is operated all original and under design conditions.

With respect to items 2 and 3 above, I wonder if putting the transmitter in as close to it's intended environment would work well, that is:

(1) Maintain pin 3 (for ARC-5) [pin 5 (for ATA or SCR-274)] grounded to energize the two select relays mentioned above (K-53, K54).  This simulates a remote control box select switch having selected this transmitter.  Relay contact closure timing then becomes a non-issue for these relays.

(2) Key the transmitter by using a Morse key to energize an external B+ switching relay.  This would perform the same function as the modulator unit's B+ keying relay K-52.  These sets were keyed by K-52 switching on and off the MO plate, PA screen, and PA plate voltages at pins 2, 6, 7 (for ARC-5) [pins 3, 4, 7 (for ATA or SCR-274)].

These two actions would employ the transmitter in almost exactly the same manner as the original system design did.  I've never tried it, though.  The last command transmitter I put on the air was 35 years ago, back in the hack-em-up days.

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