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Not sure I understand your question so feel free to disregard this
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if its nonsense. T-10, 11, 12, 13 are the plate tank of the RF amplifier stage. The common connections go to B+ the other side go to one side of the the band switch. The secondary coils go to the other side of the bandswitch which then goes to the signal grid of the mixer tube. The LO is sort of a separate tube within the mixer. It has a common cathode with the mixer part and a common grid. The plate is separate. The LO coils are T-14, 15, 16 and L-1. These coils/transformers go between two sections of the band switch which connects one side to the oscillator grid and the other to the oscillator plate. I have lost track of whether the set does not work on any band or if its the low band only that isn't working. About telling if the LO is working, I agree that the simplest method is to use another radio receiver to pick up the LO. It should be audible without direct connection between the receivers, or just a wire between the antenna terminals. I did try it with a Millen GDO without success, I used the GDO as a hetrodyne detector (most sensitive arrangement) but could not hear the LO in the headphones. However, I coupled to a stub antenna connected to the receiver rather than trying to couple to the oscillator coils. I suspect that coupling to the oscillator coils might load the oscillator enough to kill it. In any case another receiver will work. Keep in mind that the LO will be 455Khz ABOVE the signal frequency on all but the highest frequency band, where it will be 455 Khz BELOW the signal. Also, I did my GDO test on an S-40A, which is very similar but uses a different converter tube so it might not be valid. There is a data sheet on the 6K8 at Franks Tubes on the web and a fair amount of design information in the Radiotron Handbook 4th edition. At the time the S-20R was designed it was a good choice for an all band receiver. The 6SA7 used in the S-40A is a superior tube but was not available when the S-20R was designed. On 3/6/2025 3:36 PM, don Root wrote: An aside from ongoing topic ??S-20R mixer cct. ?… -- Richard Knoppow Los Angeles WB6KBL SKCC 19998 |
