[HBR] Another HBR Project -- Chapter 7

[Eddy Swynar]

Hiya Walt,

Many thanks for providing us with these wonderful installments & snippets as
to your progress there with the homebrew receiver...they certainly make for
good reading, & to someone who is still light years behind your progress,
really quite inspirational.

My 1929-style superheterodyne project here is now in the lay-out stages: I
have all the parts, & after making like the proverbial carnival
"...hidden-pea-under-three-shells" pitchman in what seems like "...umpteen"
different parts moves, finally arrived at what I believe to be the best
possible configuration in terms of both function, & final appearance---i.e.
can the various controls be situated on the front panel neatly if the
stage(s) controlled by same are situated on the chassis here, or there...?
Does each succeeding stage logically follow the preceding one...? Will the
big hardware components FIT if I do this, or that...? Etc. Etc. Etc.!!!!!

This part of the exercise always reminds me of a segment I saw once on the
old Jonny Carson Show, wherein Johnny was commenting on a rubber "doggy
barf" item he saw for sale in a novelty/joke shop: "Did you ever give any
thought to the fact," said Johnny, "that somewhere, in some factory, a
university-educated design engineer actually had to sit down, design, then
draw up a blue print for that doggy barf, and then spec out all the tools
and proper materials in order to properly manufacture it & to make people
actually think that it was the real thing?!".

Whenever I ponder that, I always gain a renewed appreciation for the trials
& travails of "...build-it-from-scratch" designers...!

The hardware for a 1929 set is LARGE: I'm afraid my two chassis bolted
side-by-side are full---but not quite to the point of overflowing. I managed
to locate two other aluminum chassis of the same specs as the first pair,
but which are twice as deep---this is good, as it'll allow me more
flexibility in "burying" those pieces of hardware that aren't quite kosher
1929 items, like the two audio output transformers that are NIB Zenith TV
items which I plan on hooking-up in series/parallel for the p-p 245 final
stage...

The big old Meissner IF transformers are just the ticket---they all have an
extra hole punched at the top for the grid lead of succeeding tubes like the
224A that had grid caps atop the envelope...nice black wrinkle finish on two
of them, too. I have to employ two 2.5 volt filament transformers to satisfy
the lot of 10 tubes. Man, this thing should look like a Christmas tree when
it's fired up!

I don't know how effective TWO series stages of *regenerative* IF amplifiers
will behave, but I guess I'll find out soon, as that's what I'm planning to
do (they didn't have crystal filters in '29, but regeneration was "de
riguer" in more ways than one!). By my calculations I should be able to get
away with but TWO pairs of RF amplifier & detector plug-in coils, i.e. one
set for "lo band" (1.7 - 8.0 MHz), & one for "hi band" (8.0 - 30.0 MHz),
using a pair of old STAG-brand 365 pfd. air variable capacitors at each
stage. Tuning may be REALLY sharp on the high bands, but I'm most anxious to
play the rig on 80- & 160-meters, anyway.

Thank goodness for my old ARRL L/C LIGHTNING CALCULATOR in this regard! It
also showed me the optimum capacitance values for the local oscillator
fixed/variable capacitors, which I intend to be configured on a
"band-to-band" basis, as opposed to general coverage. Luckily, the junque
box here yielded a wonderful old Hammarlund three stage air variable, with
25 pfd. in each stage: I intend---with judicious useage of a shorting wire
inside specific 4-pin coil plugs that are to be used in the local
oscillator---to put all three sections in parallel on the low bands, with
but one single section on the higher frequencies. In fact, I may even have
to remove some of the plates off the single section for 7-MHz & higher, in
order to get proper bandspread across the dial.

The one really good thing that has me confident Ican get this beast done by
the end of November is the fact that a non-Ham buddy of mine says he has a
bunch of Greenlee punches that he can lend me---that should make the
otherwise tedious, BORING, and time-consuming process of
drilling/reaming/filing 1-1/4" holes for those big ceramic tube sockets a
one session breeze here, rather than a multi-day nightmare!

Wish I had a digital camera so that I might show you all the stuff sitting
atop the chassis! Maybe when it's done I'll borrow one from somewhere & take
a few shots...

Until then, keep us posted as to progress, & I'll do likewise here...

~73!~ Eddy VE3CUI - VE3XZ




----- Original Message -----
From: <waltah@earthlink.net>
To: <hbr@mailman.qth.net>
Sent: Saturday, September 13, 2003 6:31 PM
Subject: RE: [HBR] Another HBR Project -- Chapter 7


> It seems to be working.
>
> The RF stage oscillation was cured by (1) adding a tinplate shield
> across the socket and over to the coil bracket so that no part of the
> grid circuit could 'see' the plate side; (2) Changing the cathode
> bypass from 0.01 to 0.1 mfd; and, (3) Changing the bypass on the
> AGC feed to the grid from 0.01 to 0.1 mfd.   The reason the cathode
> bypass needs to be so big is that the 1st mixer is the 'push-push'
> circuit, with a cathode that's 'hot' for RF and thus the filament picks
> up some RF stage output.   And what stage filament is in parallel
> with the 1st mixer?   Yep ... the RF stage.   At an earlier time I
> considered splitting the two to opposite halves of the filament
> transformer winding, but that seemed likely to cause other problems
> due to other powerful oscillators being on that side ... harmonics of
> 85 kcs, for example, wouldn't be too welcome in the RF stage.
>
> There was some audio distortion, particularly on strong signals.   I
> increased the BFO injection to the detector by switching from
> coupling from the cathode of the oscillator to the plate.   Also
> worthwhile due to the better waveform at the plate.
>
> The S-meter worked as soon as it was hooked up.  That's a genuine
> vintage 'S-meter' with a 1 ma movement, a hamfest find from maybe
> ten years back.  It's marked 'Calrad,' made in 1958, s/n I-294,
> probably sold to someone planning an HBR project.   Remember
> when meters had serial numbers?
>
> I aligned the RF stage plate 'on the money' just by adjusting the coil.
> Time consuming trial and error, what with pulling the coil set,
> removing the coil from the sheild, removing the lock from the core,
> and moving the core, then reversing the process -- but it took only
> two tries.   Got to do the same to the antenna circuit next, although
> that's less fussy due to loading by the antenna.   Also move the
> shunt caps (currently tacked on the bottom of the coil sockets)
> inside the coil can.
>
> I was concerned that removing/replacing the coils might be too
> difficult but that does not seem to be a problem -- a little wiggle and
> they come right out.   For now I am turning the set off when changing
> to eliminate the possible shock hazard.
>
> Surprisingly (to me) the frequency comes back accurately enough
> that there's no change in the pitch of voices after removing/replacing
> coils.
>
> The audio sounded clean -- perfectly readable, but definitely narrow --
> with all the 85kcs transformers at minimum coupling.   I pushed two
> of them down to max coupling, and it now sounds okay to me.  It's
> not great on ham AM though -- not broad enough at that setting to
> pick up enough carrier, so you really need to zero beat and receive it
> as SSB.
>
> My impression is that the overall gain is adequate with only with one
> IF stage.   I haven't made any actual measurements yet.   The AGC
> controls only the RF and single IF stages but it seems to be
> acceptable -- will know more after an evening of use on 80 and yet
> more after winding the 40 meter coils.
>
> I'd expect the AGC to run out of steam on extremely strong signals.
> If that does happen, it may be possible to put the 2nd mixer on the
> AGC line.
>
> The scheme of using a 15-watt filament transformer with the
> secondary replaced with the primary from another such, is okay.
> The transformer reaches about 120F after long operation while
> delivering 135 VDC at about 65 mA.   The two 5902 subminiature
> pentodes deliver adequate audio, although certainly none too much.
> I'll probably make that arrangement permanent, rather than installing
> 6AQ5's.
>
> I was concerned that the second harmonic of the 1750 2nd oscillator
> would be strong enough to cause problems but it's about S-5 -- a
> good band edge marker but not in the least a problem.
>
> Remaining work:  wiring the 100 kcs calibrator, winding coils for 40
> and 20 -- probably won't bother with 10.   Need to hook up the
> STDBY switch.
>
> Overall this is an *impressive* receiver for being so simple to build.
> No wonder they were so popular -- it really is way better than
> anything the average ham could have bought in 1960.  What a shame
> that the ARRL couldn't get motivated to seriously promote home
> brewed equipment as the RSGB did.
>
> Anyone out there have a spare set of command set front end coils in
> the junkbox?   I have only one more set and want to do two more
> bands.  Any of the HF coil sets would do fine, though the mica
> bakelite forms are easier to use because new holes can be drilled as
> needed.  They don't even need to have windings, since I start by
> stripping them to the bare form.
>
> Incidentally, I have a pretty complete selection of homebrew-related
> parts -- way more than I'll ever use -- if someone is stuck for this or
> that item.
>
> Walt
> KJ4KV
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