50 Years On the Air - WN2ZPS First Station Replicated With Original SLRM

[Chris Bowne]

Been a long time since I checked into this forum. Just wanted to share a photo of my shack featuring my original SLRM and a replacement DX-40, a replication of my orignal novice station that put me on the air 50 years ago on January 4th, 1967 as WN2ZPS. The SLRM has been OOC for a few years, but over Christmas, with the goal of getting it back on for the upcoming 50th anniversary of me getting on the air, I put it on the bench and cleared up some problems with open resistors in the power supply, specifically with the voltage divider that includes the pilot light. It's working great now. One "upgrade" I did to improve its performance on CW and SSB was to loosely couple an external BFO signal into the IF chain with a gimmick cap (two wires twisted together for a few turns). I am using a Rockland 5100 synthesizer that can be seen in the rack to the left and below the SLRM. The adjustable output of the 5100 gives more than enough injection for even the strongest signals. This allows operating with the internal BFO off in the wider selectivity positions, with AVC. I back off the injection level of the 5100 when receiving weaker signals so not as to let the BFO signal drive down the AVC controlled gain. This technique works very well. The set as built always had less than spectacular performance on CW due to the weak BFO injection level. If you look at the SLRM schematic, you will see that there is no direct coupling of the BFO to the IF stages or first detector, it just has stray coupling. Whether this was by accident or by design to limit spurious radiation at the IF frequency is not clear. The SLRF/RCH receiver, which was used in the McKay HF radio installations on merchant ships does have ample BFO injection. In the photo you can also see my RBO which I also brought back to operation this year. I bought it more years ago than I can remember at a Hosstraders hamfest. It came right up to life on all of the original oil filled power supply caps, and only need to have an open coupling cap from the first detector to the first audio stage replaced. The dial indicator needs to be re-strung and I need to replace the dial plexiglass, but for the time being I like how it looks with the dial cover removed. My RCH is out of the photo on the bench. It also was put back in operation after the failure of a couple of if stage screen bypass caps which also took out the associated screen dropping resistors.

Next step for the three receivers is cosmetic restoration. I am looking for cabinets for the RBO and RCH, as well as the front panel pull handles for the RBO. For the SLRM I am looking for about six of the thumbscrews that go into the L brackets that secure the front panel into the cabinet. Anyone have a source for these? I have the RCA RBB and RBC receivers that use similar screws but the threads are not the exactly the same, and are not interchangeable.

Also wondering if anyone out there with an SLRM also has the problem of the pilot light frequently burning out and causing the set to go dead due to the increased resistance seen by the bias divider circuit in the weird power supply design of this set. This probably is due to the surge in current through the bulb as the filter caps charge up on set power up, you can see the "flare" in the bulb brightness as this happens. I'm thinking of maybe adding a zener diode across the bulb to limit the voltage applied to the bulb to nominally its design 6.3 V.

One thing I noted when I set up the Rockland 5100 for use as a BFO, is that the IF stages appear to be aligned to 465 kc, contrary to the SLRM tech manual which states that the IF frequency is 455 kc. I am speculating that it was set to 465 kc either at the factory or on a subsequent servicing, as it looks like many of the Scott broadcast sets did in fact have a specified 465 kc IF frequency.

73 de Chris, AJ1G Stonington, CT