Re: [Drakelist] Drake 2-B S-Meter Calibration/AVC
Dennis, Garey and everyone Happy 2009 to you all. Ihave been absent for few days. Indeed it was a gassy 6BA6 indeed which made all the trouble. For some reason during the quick swap I have not detected it was faulty the firts time. A lot of learning in this 2-B restoration ! Thanks for all the inputs. Fred - PY2XB On Fri, 26 Dec 2008 16:41:07 -0200, Dennis Monticelli dennis.montice...@gmail.com wrote: 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
Re: [Drakelist] Drake 2-B S-Meter Calibration/AVC
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
Re: [Drakelist] Drake 2-B S-Meter Calibration/AVC
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
[Drakelist] Drake 2-B S-Meter Calibration/AVC
Hello. Merry Xmas to all of you. Still need some help on my 2-B that still on the bench. I have calibrated the S-Meter using the manual instructions. The radio was turned on for a long time BTW. The process was straightforward. Too bad that even a 15Mohm voltmeter shunts the AVC line. The manuals procedure takes this in account. Later I realized that when I turned the radio on again, the S-meter did not go to S1 right away. This is what happens: When I turn on the 2-B, the S-meter goes to around S9+10 and then goes down to S8. Then it starts a process that takes around 10 minutes until it goes to S1 (to the calibrated position). I have verified the whole AVC circuit (resistors and capacitors). The AVC is working apparently well. It acts, as it should. I have also replaced the 6BF6 (AVC amplifier). Nothing changes this behavior. I also realized that there is a slight change on the S meter S1 when the AVC is set from fast to slow (or vice versa). On Slow the S meter goes closer to S2. When the resting process is happening, I tried to monitor the AVC2 voltage with the VTVM or with an oscilloscope, but the measurement instruments load difficult the task and even changes the S-meter position. I do not think it has to do with heat because sometimes I turn the receiver off for few minutes and turn it on and all happens once again. I am wondering if someone have experimented that and/or has any tip. Thanks and regards Fred ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
[Drakelist] Drake 2-B S-Meter Calibration/AVC
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
Re: [Drakelist] Drake 2-B S-Meter Calibration/AVC
Thanks Dennis. I will check that. On Thu, 25 Dec 2008 22:18:34 -0200, Dennis Monticelli dennis.montice...@gmail.com wrote: Well, there still may be a thermal effect. It is hard to imagine a time constant of several minutes that is due to a capacitor, while a component or sub-assembly will undergo such time constants. If you have some cold spray, you might try a short blast in specific areas to see if there is a dramatic change. As for what may be the root cause, I would check for evidence of a carrier bleed-through or spurious signal or a possible weak oscillation that is getting through the IF chain and getting rectified by the AVC detector (in other words, masquerading as a real signal). Dennis On Thu, Dec 25, 2008 at 3:01 PM, 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
Re: [Drakelist] Drake 2-B S-Meter Calibration/AVC
I have yet another wild idea. I don't have a schematic in front of me so I don't know if there is a buffer between the high Z node and the AVC line that feeds the grids. If there is no buffer, then perhaps one of the gain controlled tubes has a grid emission problem that is charging up the AVC line and overcoming the discharge resistor. Dennis AE6C On Thu, Dec 25, 2008 at 4:45 PM, py...@integral.com.br wrote: Thanks Dennis. I will check that. On Thu, 25 Dec 2008 22:18:34 -0200, Dennis Monticelli dennis.montice...@gmail.com wrote: Well, there still may be a thermal effect. It is hard to imagine a time constant of several minutes that is due to a capacitor, while a component or sub-assembly will undergo such time constants. If you have some cold spray, you might try a short blast in specific areas to see if there is a dramatic change. As for what may be the root cause, I would check for evidence of a carrier bleed-through or spurious signal or a possible weak oscillation that is getting through the IF chain and getting rectified by the AVC detector (in other words, masquerading as a real signal). Dennis On Thu, Dec 25, 2008 at 3:01 PM, 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
Re: [Drakelist] Drake 2-B S-Meter Calibration/AVC
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
