In article , Bill wrote:

John Byrns wrote:

In article , Bill wrote:

I wish that Jon could actually experience this and see that his
thinking 'outside the box' isn't going to be an automatic revelation
that 80 years of radio has simply overlooked.
Thirty minutes on the bench could save 'light-hours' of ramblings.

I don't follow your reasoning on this, what is Jon going to learn in
"Thirty minutes on the bench"? I would venture next to nothing? Thirty
weeks on the bench might be more like it, and even then there won't be
time to explore all avenues. As far as 80 years of radio go, Jon's
constraints are different than might have existed when tube radios were a
mass application, he may be able to make different tradeoffs than were
practical then.

Don't be silly just for the sake of being a 'devils advocate', John.

I'm not being the "devils advocate", I'm just saying that the problem is
more complex than you are making it out to be and a mere thirty minutes on
the bench is not going to resolve much.

If Jon can achieve his plug-in TRF boards with any semblance of a $1.95
flea-market AA5 selectivity then I would be pleased.

These threads have grown to a point where I have not been able to follow
them all. I have been following most of the discussions like the
superhet, TRF, and segmentation of the MW band posts, but I have not yet
read the ones related to channel based receivers, which I hope to read
through as time permits. Perhaps that explains my confusion with relation
to the "thirty minutes on the bench", if you are referring to a
channelized TRF approach to receive all 117 or so MW channels, then I
would think you wouldn't need to go to the bench at all to realize it
isn't practical. A smaller number of channels, say half a dozen or so
might be practical.

I think the best approach for the all out audiophile would be the one
suggested by Randy, or was it Sherry? Gutting out a National NC-100, and
rebuilding the band selection assembly with 5 sets of 3 optimized band
pass filters to segment the MW band into 5 parts.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/

John Byrns wrote:


I'm not being the "devils advocate", I'm just saying that the problem is
more complex than you are making it out to be and a mere thirty minutes on
the bench is not going to resolve much.

The suggestion was not that the issue would be resolved in 30 minutes
but actually sitting down with a few components to make a plug-in BPF
would be very enlightening.

-BM

[Comment and question on NC-100 toward the end]

John Byrns wrote:

...if you are referring to a channelized TRF approach to receive all
117 or so MW channels, then I would think you wouldn't need to go to
the bench at all to realize it isn't practical. A smaller number of
channels, say half a dozen or so might be practical.

I think 15 or so channel slots will be fairly easy to implement, and
nearly all metropolitan areas I know of probably don't have more
than 15 stations of sufficient power and fidelity to make it worth
tuning any more (or the end-user will not want to listen to
conservative talk radio, but will listen to ordinary news, sports,
oldies, and progressive radio stations, etc.)

Again, the idea of the channel TRF approach is to be able to really
fine tune the bandpass filters for each channel. Several here say it
simply won't work, and I scratch my head on this one since the
simplest is to take an existing working TRF circuit using a variable
capacitor to tune the radio, remove the tuning capacitor, and replace
each gang with a fixed capacitor of the right value (probably with a
trimmer capacitor to fine tune the center frequency) -- it is now a
single channel receiver, and should work identical to the original
circuitry, but now it won't tune.

Now, can't one now extend this, and alter that part of the circuity,
adding LC components with the right value in the right ways to improve
the bandpass shape and proper electronic interfacing with the
properties of the RF section for that particular frequency? Obviously
the problem is figuring it all out, but one now has a lot of degrees
of freedom to work with -- no need to compromise any more as is needed
whenever one tries to continuously tune the circuit. I assume if one
can optimize it this way for one frequency (say the midpoint of the AM
band, around 950 khz), then one can then optimize it for each channel
in the 500 to 1800 khz range by simultaneously changing the values of
all the LC components as needed.

Of course, the question is how much is gained in performance taking
this approach. If several here believe it will make little difference
in real-world performance, then it makes no sense to even consider
the channel TRF approach, at least for high-fidelity purposes.

*****

Now moving to classic super-hets, the mention by John Byrns of the
National NC-100.

I think the best approach for the all out audiophile would be the
one suggested by Randy, or was it Sherry? Gutting out a National
NC-100, and rebuilding the band selection assembly with 5 sets of 3
optimized band pass filters to segment the MW band into 5 parts.

I recall last year a few people mention the NC-100. Is this radio
reputed to have excellent audio fidelity (I suppose when the variable
bandpass control is set wide) in addition to excellent selectivity and
sensitivity? And can the circuitry be modernized (e.g., modern tubes),
etc.? The idea of making it a 5 band AM radio is certainly
interesting.

Jon Noring

Jon Noring wrote:

I recall last year a few people mention the NC-100. Is this radio
reputed to have excellent audio fidelity (I suppose when the variable
bandpass control is set wide) in addition to excellent selectivity and
sensitivity?

No - not at all - nor the point. What the NC-100 (and NC-120) have is a
"sliding catacomb" band change mechanism. This is a cast metal box with
three compartments front to back - duplicated five times left to right.
Each compartment (front to back) houses the frequency critical coils
(and trimmers); etc.) for each RF stage of the radio (the NC-120 has an
additional one compartment deep five-wide box appended to the rear of
the radio to add yet another tuned RF stage). Each compartment has five
contacts that stick "up" towards the radio's chassis (15 total; 20 in
the case of the NC-120). The band-change mechanism is a rack and pinion
affair that "slides" the entire box left and right - so that one set of
compartments (and their contact fingers) line up with the mating
contacts sticking down from the chassis. In this way - each band has
it's own complete set of RF coils completely isolated and brought into
the circuit as needed. Much like a strip TV tuner - but done linear
rather than turret style. The advantages are extreme shielding - and a
good bit of room in each compartment to put all of the frequency
determining components (note only the tuning cap and tubes are above
chassis - and are the only "shared" RF components).

I think Randy mentioned that the NC-100/120 have a product detector -
but that's not to imply that the fidelity is any good - just that many
usable parts and ideas are already in place (guess you could count the
power supply and amplifier as well). They are communications receivers -
first and foremost - but they do offer some intriguing possibilities for
TRF or multi-band BCB experimentation.

And can the circuitry be modernized (e.g., modern tubes),
etc.?

Nothing wrong with the tubes they have- you could go with miniature
equivalents - but since the copper plated chassis is already huge - why
bother? I don't believe any significant performance gains (in the BCB)
would be realized by "more modern" tubes.

The idea of making it a 5 band AM radio is certainly
interesting.

That's the point - whether it would be the BCB divided into 5
continuously tuned bands - each optimized as best fit in five segments -
or 5 specific BCB stations... each tweaked to "perfection" - the
foundation is all there.

--
Sherry