[AMRadio] More weird modulation stuff

John Coleman jc at pctechref.com
Fri Sep 14 10:53:41 EDT 2007

Here's a weird one for you:

	BJ and I use a SB-200 to as an intermediate carrier amplifier to
drive the 250THs class C, because the even though the TS-820 is capable for
short periods it would not hold up over sever hours of AM operation at 40-50
watts of carrier so we run about 10-20 watts out of the TS-820 into the
SB-200 and about 40-50 watts out of the SB200 to the input of the 250THs.
The SB-200 had some weird modulation on the carrier output, even though the
input was a real clean carrier.  At first I though it was just hum from the
power supply so I changed all the electrolytic caps in the HV supply.  NO
HELP:  I then noticed as the input carrier was increased or decreased then
the output's weird modulation would change phase and shape.  Although it was
synchronized with 60HZ it was varying in shape and phase as the carrier
input was increased or decreased.  BTW it only happened on 80 MTRS.  It
ended up being that the RF choke and bypass cap in the SB-200 is not large
enough to reduce the RF at the power supply connection point on 80 mtrs.  So
RF was hitting the rectifiers and they in turn were detecting the RF to a
varying DC which was synced to 60HZ.  Installing a secondary RF choke and
bypass between the existing choke & bypass and the HV supply point corrected
the problem.


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