Receivers with Tunable IF's?

[John Ely]

Well Doc...

My favorite RX of all I've used or owned (not all that many) is the Collins 
75A-3.  It has a tunable 2ond IF on the higher bands.  I don't know of any 
lists of radios with variable IFs and I don't know how many RXes used this 
scheme.  The 75A-4 also used this method, but it has no XTAL filter (but 
does have passband tuning).  The S/Line (and KWM-2) do, too, but you must 
peak the pre selector tuning (which is not a big deal, but they don't have 
the XTAL filter).  The 75A-3/4 have 160 - 10m (but not WARC).

Of course I grew up in Cedar Rapids and discovered Collins after I got my 
ham license (and later worked there 30 years).  I've heard all sorts of 
stories and I still love that A-3.  I also have a 75A-4.  Both have been in 
storage over 26 years now.  My A-3 has been modified a lot.  New front end 
(6DC6), new mixers, product detector, and awkward planetary drive from  a 
Swan connected to the tuning knob (who can afford the fancy Collins 
knob).  I dream of that radio.  Maybe someday I will use DDS to vary the 
BFO and HFO in a way that adds passband tuning to the A-3 (using a 
micro-controller).  I have been dreaming of that for years.  If somebody 
else does it first, I'll just plan to add their design to my A-3.

It didn't begin that way, but for the last 45+ years, I have liked CW.  The 
A-3 seemed to work for me.  On 'phone it works, but I removed to "over 
coupling" caps in the IF to narrow the bandwidth.  And it is a very simple 
design with no weird circuits.

Having said all that, I played around with a Collins 651S-1 when it 
first  came out.  I don't know much about its mixing scheme, but as I 
recall it used a really high IF (109 Mhz??).  I tested some of the first 
mixer thin films for it (six FETS) and built the test fixture that was used 
for checking the first circuits..  Later I got to play with the radio for a 
little while and it was really neat (in 1969-early 70).  I really would 
like that 40 year old radio to take home and play with!   Others like them 
enough to raise the price beyond what I will pay.

The question you must ask of yourself is:  Do I build my best effort RX or 
do I buy one already made?    I always seemed to me the answer was take 
something already made and make it better.  Of course you always wonder if 
the specs are really worse and you just like it better, but it you like you 
can always figure out a  way to run some "lab quality" tests to see how 
they come out.

Until after 1972, I think Collins built he best ham radios.  There was a 
lot of inertia, but as you know, they don't build ham gear now.  About all 
the advice I can give is find some guys with some of these radios and get 
them to let you play with them.  See what you like.  Radios should be 
getting better as time goes on.  :)

At minimum,  a used razor blade, safety pin, some wire and a headphone make 
a receiver,  I worked a guy a block away receiving on that "radio".   Once 
I asked a guy I worked for how he wanted to bias a switch.  His reply: 
"What ever turns you on."  Sage advice when choosing a receiver too, I think.

John, W0GN












At 10:23 PM 4/10/10 -0500, kd4e wrote:

>Is there a list somewhere of boatanchor HF
>receivers with tunable IF's?
>
>I am curious for the whole list but am mostly
>interested in more compact, perhaps mobile,
>receivers.
>
>All tubes, preferably 160-6m, and not currently
>so rare that the cost to acquire rivals the
>skyrocketing USA national debt.
>
>Anything come to mind?
>



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