Back when I was a freshman in high school (a very long time ago), a family friend gave me a BC-454 ARC-5 receiver with an AC power supply he had built for it.
It provided much listening pleasure for me, but in the fog of the intervening years it eventually disappeared to who knows where.
Over the past decade or so I've picked up a few ARC-5 receivers at various swap-meets, and now that I'm retired I thought I'd try to get them (or some of them) back on the air. With that goal in mind, I thought I'd try running them from dynamotors (as they were designed to be operated), rather than spending time to build an AC power supply.
The images below show one of my dynamotors (a little worse for wear!).
Top view:
Bottom view. Note that the dynamotor is labeled "DY-2/ARR-2", identifying it as a dynamotor for the AN/ARC-5 series of receivers (e.g. R-25, R-26, etc.).
(Note: The SCR-274 series of receivers (e.g. BC-454, BC-455, etc.) used the DM-32-A dynamotor (see table here: Wikipedia) -- the two dynamotors are essentially identical.)
The figure, below, shows how the dynamotor's jacks are defined for High Voltage (H.V.), Low Voltage (L.V.) and Ground:
The two dynamotors I had would not rotate when 28 volts was applied, implying that their bearing grease had solidified over the years. I would need to disassemble them and relubricate the dynamotor's bearings.
Disassembling the Dynamotor and Cleaning its Bearings:
To access the bearings the dynamotor's two end covers must first be removed.
Note that the screws holding these two covers to the dynamotor body might be held in place with a "safety wire" (to, I assume, keep the screws from vibrating loose). Cut this wire and remove it, and then unscrew the two screws holding a cover to the body. Remove the two screws and their associated washers and lift off the cover off the dynamotor. Repeat for the second end cover.
Below is a picture of the dynamotor with its end covers removed:
Because I will be cleaning and relubricating the bearings without removing the bearings from the dynamotor assembly, I first removed the four brushes (two on the L.V. side and two on the H.V. side) so that I would not get degreaser or grease/oil on them. When you remove your dynamotor's brushes, keep track of which brush goes into which brush holder, and note the orientation of each. Upon reassembly of the dynamotor you will need to return the brushes to their original brush-holders and in the proper orientation!
As an aid, note that the end brackets have embossed upon them "HV" or "LV" to identify which end of the dynamotor it is, along with "+" or "-" near each of the brush caps, as shown in the four images, below:
Remove the brushes. Note which one goes into which brush holder, and the orientation of each brush. (In my dynamotors the brushes each have either a "+" or a "-" engraved on one side of each brush, and they have always been oriented in their respective holders with the engraving facing up, not down. So all I really need to remember is which two brushes go into the L.V. side and which two go into the H.V. side.).
Next, remove the two screws of the end shield that covers the bearing, as shown in the image, below. Then remove the end shield itself and any washers (also known as shims) that were between it and the top of the bearing. (Note that on the L.V. side of my dynamotor there were four washers between the end-cap and the top of the bearing).
While work continues, let the end-cap and washers soak in a bit of the degreaser fluid.
Next, take a look at the bearing grease covering the bearing. The bearing-grease in my dynamotor was, hard, dried-out. I used a toothpick to remove the grease from the top of the bearing. (Note that this will NOT remove all of the dried grease!)
After I had removed what grease I could with a toothpick, I used a toothbrush and degreaser fluid (in this case paint thinner) to scrub away the remaining hardened grease. This process will take some time. Continually turn the the armature while you scrub -- much of the bearing (and its grease) is not easily accessible.
Note that while scrubbing I keep the bearing always pointed down to prevent the degreasing fluid from flowing onto the armature and possibly affecting the armature's lacquer, etc.
Also, I recommend doing this degreasing outside (to, hopefully, minimize inhaling of fumes), and wear protective gear (e.g. gloves).
Periodically use a clean cloth to examine at the degreasing fluid dripping from the bearing (I will set the bearing on top of a cloth to capture the dripping fluid, as shown below). The goal is fluid that is clear and not be dirty.
Below, after I finished the L.V. side, I started scrubbing away the grease on the H.V. side bearing. The piece of cloth helps prevent the degreaser from splattering on me, and the cloth is on top of a clay flower-pot base (to keep the degreaser-soaked cloth off of the table-top).
By the way, the image below shows the end-shield and single spring washer that covered the bearing on the high-voltage side of the dynamotor.
An Oops Moment!!!
During the cleaning process I accidentally dropped the dynamotor onto the ground, denting one of the pot metal end brackets and bending one of the clamp screws (that is, one of the two long screws that hold the two end-brackets together). To repair the damage, I further disassembled the dynamotor in an attempt to un-dent and unbend the parts I had mucked up...
With the dynamotor disassembled, here's a look at the dynamotor's armature:
Relubricating the Bearings:
To relubricate the bearings, the
Maintenance Handbook recommends, "Apply three or four drops of a light machine oil to the balls and repack the outer side of the bearing with a small amount of AN-G-15 grease. Add only enough grease to cover the bearing. Do
not pack the bearing full."
I used 3-in-One oil (that I had on hand) for the recommended "light machine oil. I did not have any AN-G-15 grease, so instead I searched Amazon and found a polyurea grease, manufactured by Gennel (China?), which claims to be applicable to high-speed fan bearings (which turn at high speeds like dynamotors, right?). Unfortunately, there is no technical info on this grease, so I'm not sure how well it will work. Time will tell if my choices for oil and grease were good!
After the bearings are relubricated with new oil and grease, the washers and end-caps are re-installed on both the H.V. and L.V. ends of the dynamotor
Finally, the end-cap screws are locked down with some nail polish.
(Note: If you look closely at the picture, above, you will see that the two end-cap screws are Philips-head screws, not slotted screws. I somehow lost the original two screws (probably when I was banging on the pot-metal end bracket trying to straighten it), and I had to dig through my junk box to find two other screws that would fit).
Other Notes:
1. SCR-274 vs. AN/ARC-5: Per
Wikipedia, there were three separate series of Command-Set gear: 1) the ARA-ATA series, 2) the SCR-274-N series, and 3) the AN/ARC-5 series. These three series have now become known, informally, under the umbrella term: ARC-5.
Note that equipment designs can differ between series. For example, AN/ARC-5 receivers seem to use a 12SF7 tube for the second I.F. stage (this tube has an internal diode that is used to derive an AVC voltage), while the SCR-274-N series receivers seem to use a 12SK7 for the second I.F. (i.e. no diode, and thus no AVC).
2. 14V vs 28V dynamotor: At least one version of the AN/ARC-5 receivers was set up to run from 14 volts in lieu of 28 volts. If your receiver came with a dynamotor, don't assume that it matches your receiver's voltage. Someone along the way might have rewired your receivers filaments, or replaced 12V tubes with 6V tubes, or added an unknown dynamotor.
To check filament wiring, remove the receiver's bottom plate and verify if the the 12 Volt tube filaments are all wired in parallel (e.g. to be used with a 14V dynamotor), or if pairs of tube filaments are wired in series (i.e. to be used with a 28 V dynamotor).
Similarly, if your dynamotor does not have any identifying information on it (as was the case for one of my three dynamotors), remove the L.V. end-cap and take a look at the voltage specification printed on the end of the armature:
Resources:
A Google search for various ARC-5 terms will return a plethora of hits. Below are some of the sites I found useful.
T.O. 12R2-3SCR274-2 (formerly 16-40SCR274-5). Handbook, Maintenance Instructions, Radio Set SCR-274-N (revised 25 July 1956). Note that this handbook only applies to the SCR-274-N equipment (e.g. receivers BC-454, BC-455, etc.). As such, for example, all receiver schematics show the second I.F. tube as being a 12SK7, not a 12SF7, as seems to have been used in the ARC-5 (not SCR-274) series of receivers. 228 pages.
AN16-30ARC5-2. Handbook, Maintenance Instructions, AN/ARC-5 Aircraft Radio Equipment, LF, MF, HF Components. Note that this handbook only applies to the AN/ARC-5 equipment (e.g. receivers R-25, R-26, etc.). As such, for example, all receiver schematics show the second I.F. tube as being a 12SF7, not a 12SK7, as seems to have been used in the BC-series of receivers. 354 pages.
NAVWEPS 16-30ARC5-501. Handbook, Bench Test and Alignment Procedure, Radio Equipment AN/ARC-5. Specifically for the AN/ARC-5 series, but can probably be used for the SCR-274 series (e.g. receiver I.F. schematic shows 12SF7 as the second I.F. tube). 56 pages.
Video, ARC-5 Receiver Dynamotor Repair and Demonstration. Companion blog-post is here: https://hackaday.com/2019/12/12/wwii-aircraft-radio-roars-to-life-what-it-takes-to-restore-a-piece-of-history/
Video, Basic Dynamotor Maintenance. (He describes the dynamotor for a BC-603, which looks similar to an ARC-5 Dynamotor).
Dynamotor Refurbishment. Lots of good information on refurbishing old dynamotors.
Adapting an ARC-5 Receiver for Ham use. AD5X.
Stevens (G2BVN), The Command Set Receivers, R.S.G.B. Bulletin, February, 1957. Discusses the command set receivers (does not mention the variant with the 12SF7 as the second I.F.) and modifications thereof.
Stanley, J., K4ERO, "Using Unmodified Command Set ARC-5 Radios on the Ham Bands," QST, January 2016
Standard Caveat:
As always, I might have made a mistake in my equations, assumptions, drawings, or interpretations. If you see anything you believe to be in error or if anything is confusing, please feel free to contact me or comment below.
And so I should add -- this information is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
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