[HBR] Capacitor question - terminology

[Shoppa, Tim]

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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