HQ-170 AVC "pulls" freq - update

Bill Turner wrt at ESKIMO.COM
Wed Apr 30 21:49:52 EDT 1997

On Wed, 30 Apr 1997 06:13:16 -0700, Doug Hall <dhall at JPS.COM> wrote:

>Hi all,
>   A few days ago I posted an inquiry about problems with the HQ-170 pulling
>the frequency when the AVC was on. With the AVC off, signals are stable.
>I've received over a dozen replies, and out of those replies, 8 people have
>reported the same problem, either with a 170 or 180 receiver. So I'm led to
>believe that this is a common problem with Hammarlund receivers. After
>checking the tubes and replacing the questionable ones and verifying proper
>DC voltages, the problem persists.
>   Basically it comes down to the question of what is being "pulled." It
>seems that it has to be either the HF oscillator (6C4) or the BFO. I'm going
>to pull the 6C4 and feed a signal in directly from a signal generator and
>see if the problem persists - if it does, the BFO is suspect. My gut feel is
>that the problem lies in the HF oscillator, although I can't see why AVC
>voltage would have an effect there.
>   So come on, guys and gals, dig out your VTVMs and sig gens and let's
>figure out what's going on here. There are a lot of HQ-1XX owners that could
>benefit from a solution to this problem.
>Doug Hall, KF4KL
The problem is almost certainly the B+ voltage varying due to the AGC
circuit causing the plate current of the IF/RF tubes to vary.  When
the AGC is off, there is no variance and hence no drift.  

The solution is to regulate the B+ to whichever oscillator is being
pulled - the first converter or the BFO (or both). In keeping with the
Boatanchors spirit, a simple VR-105 or VR-150 should do the trick.
Consult any ARRL handbook for details.  

73, Bill W7TI

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