Re: S-20R mixer cct.
¿ªÔÆÌåÓýJim & Tom? The Hexode signal grid has a max? or min? voltage of -3? presumably to the cathode [depends who you read] , but my conclusion from a Sylvania ??blurb? is that it can go as positive as -3 volts, and as negative as -30 where there is little transconductance, and so is often connected to the AVC, but apparently not in the S-20R. but that does not help the situation. As Jim says, on band 1 ?there is no ground, and I say there is no control of THAT grid voltage by ground, AVC or other, so it floats. On the other bands the bottoms of ?3,2,1 can be grounded [all depending] providing a DC ground ?but it goes thru the coil{s} so is strange to me. But band 1 is a real mystery to me too. ? ? Tom, I suggested a test for the bandswitch area? was it not practicle? ? ? From: [email protected] [mailto:[email protected]] On Behalf Of Jim Whartenby via groups.io
Sent: Sunday, March 9, 2025 2:46 PM To: [email protected] Subject: Re: [HallicraftersRadios] S-20R mixer cct. #photo-notice ? Tom That is interesting.? The tube manual indicates that -3 volts should be there.? Have you replaced C5 and C8?? Minor leakage in either could cancel the expected negative bias on G3 of the 6K8.? From the QST article, I expect that there are unintended interactions when the expected voltages are out of spec. ? In looking at the schematic for the umpteenth time, I see how the upper band tank coils are returned to ground through switch 7D rotor that shorts the bottom of the band coils to ground.? Not sure how L13 is grounded when the band switch is in the BCB setting.? If you measure the DC resistance from the grid cap to ground you should measure R12 (33 ohms +/- 20%) plus the coil resistance for each band switch setting.? Is this the case??? ? Measuring from the switch side of R12, you should measure just the tank coil resistance to ground.? That resistance should increase as the band switch connects to the next lower tank coil.? If you don't see the coil resistance then perhaps the oscillator stator sections of either C1 or C2 is grounded? ? Likewise, the switch side of C37 should measure the LO coil resistance to ground for each band switch setting.? Like the above, each lower band coil resistance will be higher.? Is this the case? Remote troubleshooting is a s-l-o-w process, Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy ? ? On Sunday, March 9, 2025 at 11:47:14 AM CDT, Allthumbs via groups.io <btse1@...> wrote: ? ? Thank you, Jim. The 6K8 control grid (grid cap) measures 0.00 vdc, pretty much as expected. ? Tom ? -- don??? va3drl |
don Root