[Milsurplus] [Hammarlund] BC-779

[Miles B. Anderson, K2CBY]

I've found that the most useful tool to troubleshoot an AVC problem is an
old fashioned VTVM used in the DC mode. The meter should have an input
impedance of 10 Meg or so. A VOM -- even at 20k ohms/volt -- loads the high
impedance AVC circuits far too much to give useful readings. (Many of the
contemporary digital meters and the FET input analog meters have as little
as 1 Meg. and even this is marginal.)

Provided AVC voltage is being developed at the detector, what you're faced
with is a very high series resistance (typically 1 Meg) going from the AVC
buss to the grid of each controlled stage with a shunt capacitor (typically
.005 to .01)to ground.

The values of the series resistances are distinctly non-critical. Anything
from 500k to 2 Meg will probably work OK regardless of the original design
value. All they do is decouple the RF, and these resistors almost never
fail.

The shunt capacitors are another story. ANY appreciable leakage to ground is
going to drop the AVC voltage from its proper value (-2 to -10 volts) to
ground. Like all caps of this age, these ARE prone to failure, and the
failure mode is almost always leakage rather than open.

The troubleshooting technique is: (1) Set the receiver to AVC mode; (2)
Apply a strong modulated signal (> 100 uV) at the antenna or tune to a
strong station; (3) Tune for maximum signal strength (maximum audio out);
(4) Verify that AVC voltage is being developed at the detector using the
VTVM; (5) Probe each controlled grid with the VTVM.

The grid where the AVC voltage is appreciably less negative than the AVC
voltage measured at the detector is the trouble spot. Look for a leaky
bypass cap.

Miles Anderson, K2CBY
16 Round Pond Ln.
Sag Harbor, NY 11963
k2cby at optonline.net