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 Post subject: RAX-1 CG-46115
PostPosted: Jul Tue 11, 2017 5:22 pm 
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Location: Trenton, Michigan
Being an old Navy Airdale (ATN) I was excited to find this in a local antique mall. It's one of three receivers that make up the RAX system which receives up to 27mhz. This unit tunes .2 - 1.5mhz in 4 stages. Basically the AM band. It is in great condition like it has never been used? It is complete with a shock mounted rack also in pristine condition. It is missing the dynamotor.
I have found a schematic and manual.
Schematic: http://aafradio.org/docs/RAX-1_Unit_1.png
I plan on building a power supply to replace the dynamotor. The receiver uses six 12 volt (.15amp) tubes, mounted in pairs, in three parallel groups, powered by the aircraft 28volt DC system. The dynamotor supplies 160 volts DC up to 90ma for the plate supplies.
It looks like the 28 volts is only used for the tube filaments. I'm thinking on running them from a transformer, 25.2 V.A.C. Can you see any problems with that?
I will use a 120 volt secondary to make the B+.
Also, is the audio output transformer 600 ohms or a lower impedance?


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Joe

It's probably just a bad tube or something?
If it has a short, can you make it longer?
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 Post subject: Re: RAX-1 CG-46115
PostPosted: Jul Tue 11, 2017 8:08 pm 
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Looks in great shape and probably was indeed never issued in the military. Someone has been using it though, as the antenna binding post has been replaced with an SO-239 connector. That probably explains why the dynamotor is missing, someone was using it from an AC power supply.

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 Post subject: Re: RAX-1 CG-46115
PostPosted: Jul Tue 11, 2017 9:12 pm 
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Joined: Nov Mon 03, 2008 8:13 pm
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Location: Trenton, Michigan
Thanks Geoff. I didn't notice the antenna connector change. It looks a bit crooked!

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Joe

It's probably just a bad tube or something?
If it has a short, can you make it longer?


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 Post subject: Re: RAX-1 CG-46115
PostPosted: Jul Wed 12, 2017 11:37 pm 
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Joined: May Tue 23, 2017 8:15 am
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At one time I had the middle frequency range Rax-1 and thought it was a pretty good receiver. In moving around in my younger years I foolishly sold it. It was in better shape than your receiver, almost new and unused. I powered it off an external supply. I wish I had kept it. Oh well, I was foolish in my younger days.


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 Post subject: Re: RAX-1 CG-46115
PostPosted: Jul Thu 13, 2017 3:30 pm 
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Location: Trenton, Michigan
My picture is washed out. It too looks like it saw very little use.
It is good to know that you liked your receiver. Sorry you let it go. I have read a couple other threads of owners praising theirs.

I have parts on order to build a power supply that will plug into the dynamotor contacts. I'm looking forwrd to getting it working.

Besides phones, it looks like this receiver will also drive a speaker. Does anyone know the impedance?

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Joe

It's probably just a bad tube or something?
If it has a short, can you make it longer?


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 Post subject: Re: RAX-1 CG-46115
PostPosted: Jul Sun 16, 2017 11:41 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Las Vegas, NV, USA 89052
There's a least one related old thread here on ARF.... maybe 10 yrs ago? At that time I downloaded a RAX-1 "preliminary" manual from Royal Signals site... probably still available.

A quick look through same did not directly answer the question of phone/speaker impedance. Looks like the triplets were suppose to be cabled through a custom junction box and into the aircraft (ie PBY) intercom system. Manual does show AF output tube as !2A6, AF output transformer as having a -turns ratio- of 5.1, and it speaks of "loading" the unit with 300 ohms during alignment.... Squaring the turns ratio and applying it to the data in tube manual suggest ~288 ohms, but I'm probably just stumbling around.

I had one up and running some years ago; used a 600 ohm headphone, or 8 ohm speaker for all I remember :) Either would probably work for testing.

If you're lucky there will be a spline key (for the knobs) inside the little fuse compartment on the rear and a metal tube puller under the top cover.

You've caught my interest, so I'm digging them out for some play time ! Enjoy.


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 Post subject: Re: RAX-1 CG-46115
PostPosted: Jul Mon 17, 2017 7:44 pm 
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Location: Trenton, Michigan
There isn't much info on the web or even eBay about these receivers. I found several old threads on ARF. I used the "Google Site Search". There are even a couple from our late Curt Reed praising his RAX receiver.
I did download the manual from Royal. I also found an updated manual from the RKK Museum. It has several more pages.
Thanks for your insight on the audio out. Parts ore on order to build a power supply and I will check that out when I test the unit. I may have to add an amplifier?
I guess you can say I'm half lucky. I didn't see a place to store a key wrench in the fuse box, but I do have the long handled tube puller under the lid.
I'm glad I could re-kindle your interest. I'm looking forward to getting mine running.

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Joe

It's probably just a bad tube or something?
If it has a short, can you make it longer?


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 Post subject: Re: RAX-1 CG-46115
PostPosted: Jul Mon 17, 2017 8:00 pm 
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Location: Trenton, Michigan
I just found this in an old thread from 2009 that answers my question.

signalcorpsoperator wrote:I noticed that there was a missing sealed transformer, looking at the set from the front in, it would be in the right-hand corner of the radio. I think that this is, or was, a sealed audio transformer, how would one go about replacing one of those ???

AAFRadio wrote:I'm afraid that the output transformer, like those in most WWII aircraft sets, is unique to the overall type. However, it is a simple 5.1 turns ratio transformer with a 300 ohm output, so just about anything you can find that will fit and reasonably accommodate the 12A6 pentode characteristics should work fine until you find a junker with an original somewhere.

I never did fully understand why folks replaced the output transformer back in the day - it would have been easier to just add on an outboard 600 to 4/8 ohm transformer, but of course transformers from trash AA5 sets were plentiful and cost nothing. My mentor (who was a preservationist like I eventually became) had me using them with turns removed from the primary to make such line to voice coil transformers when I was a teenager. Gave me a good lesson in transformer construction.

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Joe

It's probably just a bad tube or something?
If it has a short, can you make it longer?


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 Post subject: Re: RAX-1 CG-46115
PostPosted: Jul Tue 18, 2017 7:03 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1105
Location: Las Vegas, NV, USA 89052
The spline wrench / key is held by the two clips just above where the spare fuse is stored.

Found the bearings and lube to be good on the dynamotor, so I powered up the highest freq unit. It worked w/ 8 ohm speaker though it was obvious that 600 ohm headphones were a better match. Motor started about 16 vdc, and after a couple minutes warmup, ran ok as low as 12 volts w/ reduced audio. Not much difference from 16 to 24 volts except increasing rf hash and audio motor whine as the voltage went up. I was inclined to to run it at 16 volts, where it was pulling 1.5 amps.

Antenna used was just 20 feet of hook up wire run around the room near ceiling, so, a proper outside antenna w/ shielded lead-in would probably drop the motor hash level. Grounding unit to wall socket ground brought hash static down to acceptable level. DC power supply would be even better of course.

Freq stability was quite good based on 20 & 40M SSB conversations. Dial readout was very accurate based on WWV and known ham freqs.

Swapping my one dynamotor to middle freq unit produced the same results. Low freq unit is buried somewhere in "garage stock" so was not tested.

Overall I found them to be pretty impressive for relatively small single conversion units from the late 30's, especially given their obvious used condition and unkown history.


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 Post subject: Re: RAX-1 CG-46115
PostPosted: Jul Wed 19, 2017 5:18 pm 
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Location: Trenton, Michigan
So when you were running it at 16 volts with good reception, was your filament supply at 16 volts as well? If so, THAT'S AMAZING!

My power supply parts came yesterday. I did a breadboard test under load and it looks good. I'll spend more time packaging it before trying.

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Joe

It's probably just a bad tube or something?
If it has a short, can you make it longer?


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