I’d known of the SR-75 for a long time as is it shown in the Chuck Dachis book but I’d never seen one though possibly passed them at swaps and thought, “just another S-38B”.

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So, at the Chelsea, Michigan swap in May of this year, there it was on a table. While I do not need any more radios, this one was worth the fun to see if it could get going.

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The SR-75 as I mentioned is a S-38B but cleverly a 10 watt crystal controlled transmitter is incorporated by adding a 12BA6 crystal oscillator and using the 50L6 audio amplifier as a RF amplifier. It has a keying relay to key the cathode of the 50L6 and unlike many keys of that era only 3 volts is present on the key because two D cells are used to provide the relay voltage. On the back of the radio is Plate tuning, Antenna loading, a switch to change the oscillator to allow 11/10 meter operation and a 5 pin jack for monitoring plate current and attaching the key. There is a single pigtail antenna wire that as shown in the manual that connects to a random wire or a balanced line to a resonant dipole.

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The SR-75 uses 4 different plug-in coils and uses crystals, 3.5 MHz for 80 meters or 7 MHz for the remaining bands. I was fortunate that mine came with the 80 meter coil and a crystal on 3555 KHz

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I brought up the SR-75 on a Variac and to my surprise it received pretty well since it appeared to have never had a part changed except for the volume control since new. The transmitter was another story and it did not work. Oddly, half of the 117Z6 rectifier was not working so there was no negative bias supply. Suspect too, though there was only minor hum, ?were the multiple electrolytics in two waxed paper cases. After getting a new rectifier I had bias voltage but decided to change the electrolytics which actually resulted in more space under the chassis. The outcome was having nearly exact tube voltage readings everywhere. ?

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Next was to wonder where in the world I’d find the phenolic 5 pin plug for the key and plate current. Lo and behold it’s really 4 pins and a 4 pin connector ( the center pin was not used) was found new on eBay to my surprise. It was a sign that this was “meant to be” as was an original manual also on eBay.

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So what was I going to do for additional coils? I’d put out a message here, on QTH, and heard from others who own the SR-75 and almost all had no coils or information except for Chris Farley KC9IEQ who had the 40. 20. and 10 meter coils. Chris graciously provided me with pictures, turn counts and even measured the inductance. With that information I went about finding the correct phenolic coil forms, spreaders and banana plugs to reproduce the coils. I made a coil winder and lucky enough had enough enamel covered wire to make the coils. Later I remade them using the original tinned copper just for appearance sake. ?I sent Chris an 80 meter coil to complete his set otherwise I have had no information on the coils had it not been for him.

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Though the radio received OK enough I went ahead and recapped the whole thing, checked resistors, did the alignment and it now receivers really well for an S-38B.

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I had a hard time getting to” know” the transmitter. What would I do to be able to read power besides measuring RMS volts on my HP-410C. I wanted to see power into a dummy load which Hallicrafters describes for the transmitter. Eventually I was seeing some power as evidenced by burning my finger from the resistor in the dummy load! With a bit of experimentation, I was able to connect a piece of RG-58 though a wattmeter and was able to get 6 to 10 watts output into a 50 ohm dummy load. The next step was to see if my off center fed dipole would be liked by the radio. On 80 meters not so much with only 2 watts output shown but on 40 meters I have a solid 10 watts with about 1 watt SWR.

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Back when I was a novice as KN8AJF, using a S-53A and Globe Scout, crystal controlled on 40 meters with a 75 foot long wire, I’d call CQ and listen all over the band for a reply. I did manage to work quite a few stations including the FCC when my second harmonic ended up on 20 meters.? My sense is that in today’s world few stations would tune off frequency and if so I’d be told I was off frequency! Another drawback of the SR-75 is there is no side tone. I admit to not being very good with a straight key anymore so I used another receiver to provide side tone. The “band” around 7050 KHz is very narrow and the bandspread of the SR-75 is pretty narrow as well so it is easy to miss a caller. I have made one contact so far just to be sure everything worked as I thought. Certainly, back in the day this could have and probably did serve as a complete novice station. Even so I am somewhat intimidated to be using it today.

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The outward condition of the radio is not at all mint though I prefer to say it has some patina with a few touchups here and there. Someone cut a hole perhaps to mount a SO-239 socket in the side but it was done so neatly I thought at first it was original. I don’t know what I will do with this now though I will likely sell it at some point. I hate to see things like this sit on a shelf.

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I still have some coil forms and a few spreaders as well as that scarce 4 pin plug for the back if anyone wants them at a small cost. I can wind coils though while they are close to original they are not as nice looking as Hallicrafters made them

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Thanks to all who have shared their SR-75 stories with me!

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73, Roger, NJ2R

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