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.
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