K3HVG wrote:
Purpose built VFO's, as you call them, generally allow for presetting an
offset or offsets such that a "dialed-in" frequency reads out directly
in the use frequency rather than the actual mixing or insertion
frequency. Additionally, as with S-Line equipment, etc. that use
band-dependent, multiple mixing formulas, multiple offsets may be set
into VFOs such as the S&S unit I mentioned. Cost wise, they probably
cost the same initially but I'd hate to have to pay to repair an HP. I
personally have not experienced the front-end problems as you describe,
but then I generally use my R-390A's. The 51S1AF/551G sort of sit on
the shelf, looking pretty. I did an article on general-coverage for the
Drake "C" line via a purpose-built VFO in an ER article last year. You
might, perhaps, be interested in having a look at it.
Regards Jeep/K3HVG

I think ER is a noble effort but it's a little expensive for someone
who is not actively hamming.

The S&S I'm not familiar with but I remember AOR made the dual section
vfo that replaced both the xtal section and the pto on S-Line. Looked
vaguely like a tiny S-Line. I guarantee it would be no cheaper to fix
than a synthesized HP if you were not too concerned with the precision
of the attenuator of the HP. In fact if you can get a 8656A that was
not field upgraded to the PIN diode attenuator, and the mech attenuator
went bad, (they do) I doubt it would bring more than a hundred bucks.
Use it straight through at +13 or +17 or get the little rotary stepped
job IFR uses in the 500/1200. I knew a guy who worked there who
summarily changed every one he worked on for over a year : he gave me
half a dozen and said they were all good under 400 MHz. IFR charged a
couple hundred apiece but they were like $60 from the source, maybe
Berkleley Varitronics.

IIRC the 8656/57 have 100 preset frequencies you can program in, so
you just use it channelized, since you only need to select for the
"band".

The limits on fixing 8640s is the presence of some HP custom IC's. I
have no idea if there are proprietary ICs in the 8656/57. In general
fixing HP equipment is easy. IFR is a mechanical nightmare but
electrically simple. Systron Donner-throw it off a cliff!

Someone suggested that the newer generation of arbs had the frequency
range and level to do the job and as well most are USB controllable. I
don't know if their spectral purity is good enough though.

The R-390 is the Marilyn Monroe of HF radios, though. Isn't it?