Re: S-85: how are the RF amplifier tube (v1) and the MIX/OSC tube (v2) coupled in band1?
开云体育Emanuele ? Yes, I prefer to look at R26 and C29 as the plate load Impedance, so the V1 output voltage is developed thru it, and as you reiterated there is no V1-V2 obvious intended coupling in band 1, but the switch has inherent capacitance across the wafer and perhaps that is enough, as your tests seem to indicate. Any yes R65 is probably to reduce overload tendencies on the BCB. ? Recall that C62 began with the S-40A [not the same spot] , and perhaps the S-40? ? From: [email protected] [mailto:[email protected]] On Behalf Of Emanuele Girlando via groups.io
Sent: Wednesday, February 26, 2025 12:46 PM To: [email protected] Subject: [HallicraftersRadios] S-85: how are the RF amplifier tube (v1) and the MIX/OSC tube (v2) coupled in band1? ? This topic raised in the "TRIMCAPs don't peak" thread. I think it's worth its own thread. Here it is. ? In the S-85 there is no obvious coupling between the RF amplifier tube (v1) and the MIX/OSC tube (v2) in band 1. The output of V1 seems to get lost in the R26/C29 couple. ? Someone argued that the signal path could go through the S1B wafers. Other argued that there is some other stray coupling in the circuit wiring. Someone noted that the S-40B has an identical circuit but it is equipped with an additional capacitor (C62), positioned just between the V1 output and the v2 input. The following picture sketches the S-85 circuit with the possible signal path and the phantom C62 highlighted. ? ? ? So I've made some measurements on my S-85. Using my 9640B sig-gen connected to the ANT A2 input via a RTMA dummy antenna (A1 shorted to GND), AM modulated at 30%, I got the following: Below the indicated value the signal gets lost in the noise. As you see I also made a test shorting R65 in the input circuit as someone argued that it could impact (as it does) the receiver sensitivity in the higher part of band 1. ? Thoughts? -- Emanuele (IU1KNR). -- don??? va3drl |
don Root