HQ-170 AVC Problems

[Roger W. Stevens WA3FLE]

Fellow and Prospective HQ-170 Owners,

Something I discovered might shed some light on the situation here.
Unlike the HQ-180, whose design was, as far as I can tell, frozen
during its entire production run (with the obvious exceptions of the
A-Series solid-state rectifier and always-on hf-oscillator filament,
which holds true for the HQ-170 model as well), there were slipstream
changes made to the HQ-170 design during early runs in the--guess
what?--AVC circuit.

If you look at the schematic for the latest run of the 170, it and
the 180 are virtually identical in the AVC and audio area.

I have an HQ-180 and an HQ-170C.  I have a "universal HQ-180/180A
manual copy, and a copy of a manual covering one of the early runs of
the HQ-170.  My 180 agrees perfectly with the schematic.  My 170, on
the other hand, agreed with neither the 180 schematic nor the 170
schematic supplied with it.  It had remnants of 180 AVC design elements
(the resistor values switched by the AVC switch), but had an extra coupling
capacitor and a funky bypass cap, if I recall.  As I liked the performance
of the 180 so well, and it looked like Hammarlund finally settled down
and froze both designs as the same, I just made those changes necessary
to bring the 170 into "modern day" compliance.  Now I like it better than
the 180--go figure.

I read recently about this circuit creep--it may have been in Fred
Osterman's book.  Anyway, now I have to see if the 170 exhibits the
pulling that is at the core of this thread.  But for what it's worth,
it took Hammarlund a few tries, but they eventually got it right.

73, Roger WA3FLE
Norristown, PA
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Roger W. Stevens (rstevens at voicenet.com)
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