[HBR] Capacitor question - terminology

Tue Mar 6 09:13:25 EST 2012

Tim, In you opinion, what is best, and why---high side local oscillator
injection or low side injection?
Don wb0rai

On Tue, Mar 6, 2012 at 9:03 AM, Shoppa, Tim <tshoppa at wmata.com> wrote:

> A typical superhet by its very nature, without front end preselection and
> ignoring higher order mixing, receives at two frequencies. IF+LO and IF-LO.
>
> It is the preselector that selects IF+LO or IF-LO in the front end (before
> the mixer), and prevents the other of IF+LO and IF-LO from entering the
> mixer. In most mid-20th-century radios the preselector is a sharply peaked
> high Q tuned circuit or more likely two or three.
>
> In a HBR-14 or -16 implementation, there is "single dial tuning" meaning
> that the same control that changes LO frequency is adjusted and padded to
> have tracking preselection.
>
> This "single dial tuning" was for a long time in the mid-20th century the
> holy grail of receiver technology.
>
> By the 60's though most ham radio equipment had moved past this
> preoccupation and the preselector was a separate control on the front panel.
>
> In many utilitiarian cases two different ham or SWL bands could be
> selected, with the same LO range, just by adjusting the preselector for
> either IF+LO or IF-LO sensitivity. This is intentional band-imaging.
>
> As to "bandspread" it is the counterpart to "bandset". This was a very
> common way of making a general coverage (say 0.5 to 30 MHz) receiver with
> two tuning controls. This technique is used less often (but still sometimes
> used) within a single ham band. Most (all?) HBR designs chose to not be
> general coverage receivers, but to have band by band coverage. There are
> very real tradeoffs that come with making a design be general coverage
> beyond just tuning rate that show up with modern synthesized general
> coverage rigs. This is one of the reason that even with modern rigs,
> upconversion general coverage receivers have markedly poorer specs than
> downconversion band-by-band rigs.
>
> Tim N3QE
>
> -----Original Message-----
> From: hbr-bounces at mailman.qth.net [mailto:hbr-bounces at mailman.qth.net] On
> Behalf Of Chris Howard w0ep
> Sent: Tuesday, March 06, 2012 8:26 AM
> To: HBR Receiver List
> Subject: Re: [HBR] Capacitor question - terminology
>
> I think I follow all of that except for your reference to "band-imaging
> techniques" and how that relates to a pre-selector control.
>
> I'm not sure what that means.  Are you speaking of an operator/using
> technique or a designer/builder technique?
>
> I think Walt made reference to bandswitching and band-imaging in a recent
> note also. (And so I kind of think your use of the term is from a
> designer/builder standpoint) I understand bandswitch just so far as it
> applies to the big knob on the front and the wafer thing inside.
>
> Can someone straighten me out?
>
> Another term that has always been difficult for me is "bandspread".  I
> understand that in such cases there are two knobs.  One does large
> excursions and one does smaller excursions, like the "fine tuning"
> on the TV set I grew up with.  I can never remember if "bandspread" means
> the coarse-change or the fine-change.
> I usually guess wrong.
>
>
>
>
>
> On 3/5/2012 5:50 AM, Shoppa, Tim wrote:
> >> I see a wide variety of 3-section air variables on ebay these days.
> >> I see a couple that have attractive gear drive systems on them.
> >> (see 370590238594 for example)
> >
> >> I was wondering how hard it is to take plates out.  The capacitors I
> >> see appear to have slots in the axle into which the plates fit and
> >> have a small bakelite piece which drives all of the rotors at the
> >> same time.
> >
> >> How hard is it to remove plates?
> >
> > It's straightforward but with close plate spacing and 3 sections would
> require a lot of care. It is a lot easier (hint hint) starting from a
> single-section unit with wider plate spacing.
> >
> > The types of caps where it's super easy: Single section. Here you
> disassemble the shaft and remove the unneeded sections and spacers, then
> reconstruct with the original spacers and put in a few handy extra spacers
> you have from previous cap reconstructions.
> >
> > For something like this, if you needed to just remove a few plates, you
> could go in with a Dremel or other good cutter and remove the plate or
> fraction of a plate you didn't need. But to turn this into a HBR tuning cap
> you'd have to remove 80% or more of the plates. What you picture is a nice
> cap suitable for a lot of homebrew uses... but is not a good cap for a
> carbon copy clone of the HBR-14 or -16 say.
> >
> > Some scandalous comments follow below:
> >
> > The original HBR design put a lot of attention on tracking RF and LO
> sections.
> >
> > This was back when the creme de la creme in any receiver was such
> tracking using a single tuning knob, it was an attempt to prove "this is as
> good as any commercial receiver".
> >
> > Unless the goal is to build a carbon copy of the HBR-14 or -16 or
> similar....
> > I see no reason why a new construction HBR-inspired receiver would do
> the same. A separate "preselector peak" control, there's nothing at all
> wrong with that, and it lets band-imaging techniques come in too
> (techniques common on the good old Drake and other well-respected vintage
>  receivers).
> >
> > Tim N3QE
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