The minor changes to S-meter reading (transient or otherwise) as you introduce or remove AVC path components (Fast vs Slow and BFO) don't seem very revealing to me. The slow drift in the S-meter over several minutes is the revealing symptom. This latter behavior could be caused by a gassy tube whose grid is being fed from that very high impedance AVC line as Garey has suggested or by the grid having become contaminated and thus acting as a virtual cathode. So swapping out all those gain-control tubes with a fresh tube one at a time would be a good experiment.
Dennis AE6C On Fri, Dec 26, 2008 at 3:26 AM, <py...@integral.com.br> wrote: > Garey and Dennis, thanks agian for the inputs. > > Regarding tubes, I have replaced those afected by AVC/1 and AVC/2. Nothing > changed. Maybe I will need to try to redo this based on what Garey wrote. I > will get special attention to the 6BA6 which is driven by AVC/1. > > These are some effects that I would like to line up and make sure you have > the details > > - After 10-15 minutes Smeter goes to S1. If I turn the radio off and on it > will do again, but journey is > shorter. The S-meter will come from S4 to S1 in 5-6 minutes. > - During the journey to S1, after start up, if I am in Fast AVC and switch > to Slow (introducing the 1uf capacitor), the S meter's needle deeps to the > left and gets up again. I hear the RX background noise to pulse too. It > does that once. > - In any circusntance there is a slight S meter difference when I switch > from SAVC to FastAVC and vice-versa. In SAVC it reads about half S unit > more (to the right). > - I have realized that when th BFO is on, the S-Meter moves a bit do the > left. > > Normally I would answer to someone that would have posted something similar > to it that the AVC discharge path is malfunctioning. I have checked the > resistors in this path. I have also checked the time constant capacitor in > the AV/2 and AV/1 circuits. > > I will have time to check spurios oscillation and/or grid leak whan I get > back home. I will be away until year's eve or so. > > Please send any ideas that are very much welcome. > > Best regards Fred > > > > On Fri, 26 Dec 2008 04:19:24 -0200, Garey Barrell <k4...@mindspring.com> > wrote: >> Fred - >> >> I have been traveling and missed some of this thread. Try subbing tubes >> in the AVC stages. Often an IF tube will be gassy, and the grid will >> "lose control" after 5-20 minutes of warmup. The 12BA6 is particularly >> susceptible to this problem, even in "New Old Stock" tubes. You >> sometimes have to try three or four before finding a "good" one. The >> AVC circuit is so hi-z that it takes very little grid current to upset > it. >> >> 73, Garey - K4OAH >> Glen Allen, VA >> >> Drake 2-B, 4-B, C-Line & TR-4/C Service Supplement CDs >> <www.k4oah.com> >> >> >> >> py...@integral.com.br wrote: >>> >>> Dennis, merry xmas to you and yours, firts of all. >>> >>> >>> >>> Thanks for the input. I don not know if I my explanation was clear >>> enough. The long S-meter journey to S1 just occurs after power the >>> unit up and for 10 or so minutes. After that the behaivor is normal. >>> Yes my first shot was a opened path to discharge the AVC circuitry. >>> Unfortunately I have not found anything to blame. Did you consider >>> that after the initial pb the behaivor seems to be normal ? Regards Fred >>> >> >> _______________________________________________ >> Drakelist mailing list >> Drakelist@zerobeat.net >> http://mailman.zerobeat.net/mailman/listinfo/drakelist > > _______________________________________________ > Drakelist mailing list > Drakelist@zerobeat.net > http://mailman.zerobeat.net/mailman/listinfo/drakelist > _______________________________________________ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
