Subject: Time:12:43 PM
I'm trying to repair a remote tuned Henry 3K amplifier for a local ham. The
closed-loop remote tuning system went bad; specifically, the load capacitor
in the RF head travelled to its min capacitance, and stopped. The problem
turned out to be an open load capacitor tracking pot (R108) inside the RF
head. I replaced it, and control was restored. A new problem exists: the
load capacitor tracking pot will only rotate 3 turns, whereas it has 7 turns
available from mechanical stop to stop.
Any ideas? My first hunch (and confirmed by DVM check) is that the desk top
controller "load" pot (R1116) is not seeing the full bridge voltage (+10,
and -8 VDC). Instead, it only sees +2 to -2 VDC. There are two fixed
resistors connected to each end of R1116, but these do not appear in the
schematic. These are "dropping" the available voltage to R1116, in an
apparently unacceptable way. Despite being exactly the same circuit
electrically, the desk top controller "tune" pot (R1117) sees nearly the
full bridge supply. Apparently, I have a low impedance shunt in the R1116
circuit.
Personally, I am a QRP operator. I don't know why I'm fixing 3K's !
Any ideas, or has anyone else had this problem? The repair guy at Henry is
not familiar with this circuit.
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