I have two of these sets; one is fairly pristine and I will minimal invasive work to make it work with one of my boat anchors with a 455 kc IF. The other one is in rough shape, so I do not mind making some design changes. One thought that came to mind is that either audio or BFO stage could be replaced with a second mixer stage and 455 kc IFT, this stage would use the LO signal to convert the 85 kc IF back to the original 455 kc IF. This could be "dropped" (hopefully by using tube based project boxes) into the 455 kc IF chain in the main receiver, and possibly replace most of the 455 kc gain stages in that RX. A bit of work, but I think it could provide a viable band pass tuning scheme? Anyone ever try this? Pete
My concern is that sharing the same LO injection might create a 455 kHz blow-by path around the 85 kc band pass tuning IF section. Not sure if isolation (buffering is needed or not..)
I know that it has been done, but I can't recall if it was done with a Command set. The trick would be to "sample" the 12K8Y oscillator stage without loading it down, and send the signal to the second mixer. I don't think blowby would be much of a problem. btw, if this is done, you might want to change out the last 85 kc. IFT for an interstage IFT. That last IFT is designed to feed a diode detector, and using an interstage transformer (or modifying the diode transformer) should give a little more selectivity. 73 es GL de Jim, N2EY
Hi Jim... I figured you'd be the guy who answered. LOL. Not many old geezers left who are into these relics in this century. Yeah, good idea on the last IFT. I do have several cans that I was saving for the HBR project that I never got around to doing. I'll use a beat up R-23A for the Guinea pig. I was going to copy the first mixer stage and simply capacitor couple the LO from control grid to control grid between mixers.
That might work, but might also introduce blow-by and such. Here's an idea: Replace the 12K8 mixer with a dual triode (6SN7?) and move the mixer circuit to the RF stage tube socket. The second 6SN7 triode functions as a cathode follower/buffer to feed the LO to the second mixer. Here's another idea: I think I saw this in the book "Command Sets"..... Disassemble the R-23A and put the circuitry on a new chassis. That way you can add tubes and circuits as desired. The plug-in sections for the IFTs and coil set can be carefully cut out of the R-23A chassis and bolted into the new chassis, with suitable holes. Found it! Page 120: http://w7ekb.com/glowbugs/Military/PDF files/ARC5/command_sets.PDF 73 es GL de Jim, N2EY
Hi Jim I was thinking about adding some series-tuned 455kc traps on the LO lines if blowby is an issue. The control grid on the 6k8 tubes look to be fairly well isolated, however. I think that (traps) might only affect the RX coverage at around 540kc, assuming hi side LO injection. I'll take a peak at the w7ekb' site.
Work has started. It is now going to be a poor ham's version of a Hammarlund HC-10. Unit will convert a 455 kc IF down to a sharp 85 kc, and then back up to an IF of around 465 kc which will be fed to a Central Electronics Signal Slicer for even more apparent selectivity by eliminating one sideband. The Guinea Pig is a R23A, which unlike the BC-453, does have an AGC system. That will allow adding a small S Meter on the front panel. Two visible pot controls are for S Meter calibration. Right now the design is undergoing some tests on the workbench. I will post more photos as it nears completion if there is interest. Pete
Latest under chassis shot for anyone who is interested... all of the "flower pot" caps have been replaced using point to point wiring. The audio stage was converted to a 6SK7 for a third IF at 443 kc to drive a Central Electronics Sideband Slicer. 12AT7 was added and is a Pullen Mixer to convert 85 kc back up to an IF in the 450 kc region. 12AU7 stage added to provide a bridge circuit to drive the S Meter. BFO was modified to use the LO coil from another scrapped BC-453 to serve as the second LO (360-390 kc) to drive the Pullen Mixer. Other unneeded parts were removed to conserve available space. The 2.5 mH RFC is a trap to reduce the 2nd blow by coming out of the IF output jack on the rear apron. Tested on the spectrum analyzer and later on a Slicer tuned to the 443 kc IF. Running the IF in-out offset by about 10 kc to avoid beat or feedback problems.
Before I got my Ham License back in 1964 (WN8LGM), my grandfather took me to Fair Radio Sales in Lima, Ohio and bought me a 6000 to 9000 Kc/s (not KHz) with a store made power supply and audio amplifier. I thought that it was a great radio compared to the Hallicrafters (?) S-120 that I had before that. I'd love to find another one of those radios for nostalgia sake now that I'm an OT.
LGM: The reboxed S-38E (S-120) versus the BC-455: For 40-meters, the BC-455 wins "hands down"! S-120 BC-455 on right hand side. Someone gave me the S-120 so I added it to my "boat anchor" collection. At least it isn't as bad as the National SW-54! The 3 Command Set receivers, BC-453, BC-454, and BC-455 have been acquired over the years. All 3 work fine and, for over 75-year old equipment, still do a pretty good job. Glen, K9STH
