Curt -
The AGC in the 4 Series is not particularly problematic. You may be
thinking of the 2-B AVC, which was extremely high impedance and subject
to leakage of components and even tube sockets.
Most AVC problems in the 4 Series receivers are caused by gassy 12BA6
tubes in the IF. There is a ZERO adjust pot under the chassis along
with a full-scale CAL adjustment. You might try just adjusting those
according to the manual. IF the meter then works OK, you're done! If
the S-Meter tends to "wander" over time as the receiver warms up, you
probably need a new 12BA6. Unfortunately, not even all NOS tubes are
good enough so you may need to try several to find one that has low
enough grid emission for the circuit.
There are two more tubes that affect the AVC, but V4 and V5 are the
primary culprits in my experience.
I have attached a more detailed adjustment procedure from my CD.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line & TR-4/C Service Supplement CDs
<www.k4oah.com>
Hulett, Russell wrote:
I recently acquired an R-4 that performs well enough on all bands for routine
use. All the controls and switches work well. But, it seems to me that the
AGC needs some attention. I guess this isn't unusual for this receiver.
Evidently, proper AGC performance is problematic. I've changed tubes in that
section without improving performance, with AGC off, sensitivity is fine, with
it in fast or slow, the receiver is attenuated and the S-meter reads upscale,
rather like the RF gain were backed off. I've also changed out RF amplifier
tubes with no improvement noticed.
Cosmetically it is a nice looking receiver, so I am considering taking it to a
professional to diagnose, repair, align, generally bring it up to par. I'm
wondering if there is a competent technician within about 200 miles of Houston
who would be capable of repairing an R4? And, is it reasonable to expect that
a few hours of effort would suffice to accomplish this task? Or, are problems
with the AGC just something one needs to accept with this receiver?
73, Curt KB5JO
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