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5.2 Meter Radio Astronomy Project for 1420 MHz
Disabling the AGC on R-7000 and R-7100 Receivers
R-7000:
I have found that the AGC on an R-7000 receiver can be disabled
by cutting the top lead of R-115 on the IF board. This resistor
is in the base lead to transistor Q18, which is the AGC amplifier.
With this resistor opened, no voltage from the AGC rectifier diode
D28 can reach Q18 so it never attempts to lower the gain of the
receiver.
After you take the top cover off, and facing the front of
the receiver, R-115 is located about 5 inches from the left side
of the receiver, and about 4 inches from the front. It is plainly
marked and is standing on end. The lead coming out of the top
is easy to clip and easy to solder back if you need to. I never
use my R-7000 in AM mode for anything other than radio astronomy
so I did not add a switch for turning it on and off, but it would
be a simple matter to do so. My modification seems to be working
fine for me but I would be interested in hearing from anyone with
problems or a better idea.
R-7100:
The AGC portion of the R-7100 looks just like the R-7000,
except for different part numbers. The resistor to clip is R-252.
It is a 10k resistor and is located adjacent to transistor Q-38.
R-252 on the R-7100 performs the same purpose as the R-115 on
the R-7000. On the R-7000 there is a "Main" board and
an "IF" board. On the R-7100 they have combined the
boards into a single "Main" board.
SPECIAL NOTE:
The above mods allow the receiver amplifier stages to operate
"wide open" as far as gain is conerned. As such, the
receivers are very easy to overload. You may even find that the
noise output from your front end preamp(s) produces enough noise
to "quiet" the receiver. In that case you must add attenuation
in front of the receiver.
Randy Stegemeyer
hamradio@oz.net
Created by Randy, W7HR
E-mail: hamradio@oz.net
E-mail: hamradio@oz.net