Wow, nice detective work Paul! I've had correspondence with John too, but it was mostly about the AGC alignment and the subject of the fast AGC never came up. I would agree with the assessment that it is useless, the poppy-ness on noise alone is just to annoying to put up with, and manual intervention with the RFG control shouldn't be needed. I have another R7 I am working on for someone that exhibits the same behavior, and I have heard similar tales from other users. I just figured that it was the nature of the beast, and haven't taken the time to look deeply at the circuit.
I agree with doing the alignment "by the book" at this point. I tied various alignment methods, and after discussing the how and why of the book method with John, that seems to be the best way to do it. I'll file your notes and try it out the next time I have one or the other of the radios on the bench - a couple of other projects have temporarily sidelined Drake tech-ing for me. Many thanks to John also, for sharing his wisdom! 73 -Jim NU0C On Sat, 11 May 2013 11:39:52 -0500, Paul wrote: >Have had issues with the R7 AGC in fast mode in that the AGC would go >into a low frequency oscillation excited by noise. Looking at the AGC >line showed a triangular waveform. This caused distortion on AM and SSB >signals. This problem went away when the RF gain control was turned >counterclockwise (reduced IF gain). Medium AGC also had this problem >but to a lesser degree. Slow AGC worked fine. > > Contacted John Kriner and he was kind enough to share his thoughts and >suggestions on fixing the R7 AGC problem. I made the modifications to >the IF/AGC/Audio board per John's suggestions and they did help. There >was no more issues with medium AGC and much less problem with fast AGC. >On no noise the fast AGC did not oscillator anymore, but when high noise >or signals present AGC did go into the low frequency triangular wave >oscillation. By turning the RF gain control down (less IF gain) the >problem went away and the fast AGC was usable. Since I don't really use >fast AGC for anything, I'm quitting while ahead. Proper alignment of >the R7 IF/AGC/Audio board is important. > >Has anyone had a similar problem with the R7 AGC in fast and/or medium >modes? Anyone come up with a better solution then John's below? I >think that the R7 AGC circuit has too much gain and becomes unstable >when excited by noise or signals. My TR7 doesn't suffer from the >aforementioned problems. > >Excepts of the emails: > >To: John, >I have a Drake R-7A that works very well except when using fast AGC >and/or the audio distorts when the RF gain is all the way up (in all >speeds of AGC). I have aligned the IF/AGC/audio board per the manual >and your instructions below: > >(John's alignment instructions - AGC pedestal is left all the way CW. >I have argued with some folks who claim it must be re-adjusted. But >this is the way I have always left it. And the S-meter calibration >turns out to be perfect, and S+N/N far exceeds the specifications. 10dB >drop in generator level equals an exact 10dB drop on the 'S' meter. ) > >My S-meter calibration is perfect for microvolt inputs, but audio >distortion at high RF control (IF) gain is horrible. When I turn down >the RF (IF) gain control the receiver works well with no distortion or >AGC pumping in fast AGC. I tried adjusting the AGC pedestal, R1127, >just to see if it made any difference, it didn't. > >I could not find any obvious circuit or component failures. Scoping the >AGC circuit in Fast AGC shows a triangular waveform oscillation excited >by noise. When the RF (IF) gain knob is turned down this triangular >waveform oscillation goes away and the receiver works fine. I think the >audio distortion is being caused by the AGC circuit modulating the IF >signal. The distortion is mainly on AM detection but I can tell that >the SSB demodulated signal also doesn't sound quite right. There also >is popping of the SSB signal at the beginning of voice peaks even with >slow AGC. All of this goes away when I reduce the RF gain control. > >Is this a circuit component(s) malfunction or a design problem? My TR-7 >doesn't act like this. > > From John: >Fast AGC on the R7/A is useless. > >It always has been. No tweaking can correct it. > >I had a customer with a RR3 complain about this as well. He sent me the >radio (basically a ship R7) and I worked with a designer at Drake. This >is the fellow who designed the SW8, Eton E1, and R8A/B. He said he too >knew that fast was worthless.I took a schematic to him and he said that >the time variance between fast and medium was such that fast was 10 >times faster than medium. This is not a good design in principal, >according to him. Instead he prefers a factor of 5 times, that is fast >ought to be 5 times faster than medium. > >He suggested component changes, and I found that changing R1124 to a >470Kohm and C1114 and C1118 to .01uFD did the trick. MUCH better. The >customer was very happy with the change. Try it and I think you will be >too. Let me know what you think afterwards. > >To: John: >I installed the components you suggested and the my R7 works much >better. Thank you. Calibrated the IF/AGC/Audio board exactly per the >manual. Still have low frequency oscillation in AGC modulated by noise >in fast AGC with antenna connected, but can eliminate by turning down RF >gain control. Medium AGC does not suffer from this oscillation any >more. Also with no antenna connected the fast AGC won't oscillate. I'm >going to quit while I'm ahead as the R7 works fine now just the way it is. > > > > > >_______________________________________________ >Drakelist mailing list >[email protected] >http://mailman.zerobeat.net/mailman/listinfo/drakelist -- Ham Radio NU0C Lincoln, Nebraska, U.S.S.A. TR7/RV7/R7A/L7, TR6/RV6, T4XC/R4C/L4B, NCL2000, SB104A, R390A, GT550A/RV550A, HyGain 3750, IBM PS/2 - all vintage, all the time! "Give a man a URL, and he will learn for an hour; teach him to Google, and he will learn for a lifetime." HyGain 3750 User's Group - http://groups.yahoo.com/group/HyGain_3750/ http://incolor.inetnebr.com/jshorney http://www.nebraskaghosts.org _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
