[HBR] Navy Guy and (slight) progress report

[waltah at ntelos.net]

Tom wrote:
> Thanks for the compliment, Kees (hope it was one).
> 
> Tom - WØEAJ - former RMC, USN

HEY!  I thought that was for me!   <G>

I guess my Navy service is the reason I like the gray paint.   It's 
certainly the reason for my enduring affection for one of the biggest 
wastes of government money ever -- the RCA SRR-11, 12, 13 
series of receivers.   A much modified SRR-13 is the only general 
coverage receiver here -- the R-390A's have stayed ahead of my 
price range for three decades now.   

Study the SRR-13 closely and you'll cease to wonder why RCA is 
now a subsidiary of GE.   

I've had to switch to darker grays though -- can't find spray cans of 
the 70's Ford 'dove gray' any more.   The few finished projects 
these days look more Canadian military than U.S. Navy.  

I continue chewing on the General Coverage HBR project.   The 80-
40 meter band imaging receiver (six tubes plus rectifier, half lattice 
crystal filter, push-pull 12AT7 mixer) turned out so well that it 
inspires use as a tunable IF in a set along the lines of the G2DAF 
Mk II.  I will use a ladder filter rather than the 2-crystal half lattice -- 
squarer sides and higher ultimate rejection.

With the IF section needing only the tuning range to be tweeked, 
the project SOUNDS simple, and I'm hoping that at some point it 
will be.   But details remain elusive.   

The plan is to take the 120 crystals of the BC-1335 WW-II portable 
FM set, 5675-8650 kcs (25 kcs spacing) and multiply the 
frequencies x10 to give 120 frequencies 56.75-86.50 Mcs.   
Subtract 49.2 Mcs yielding 7.55-37.30 Mcs by 250 kcs steps.   
Adding a tunable oscillator 1.704-1.454 Mcs to a fixed IF of 4.096 
Mcs (the ladder filter with 4.096 microprocessor clock crystals) 
gives a tunable IF of 5.800-5.550 Mcs.   The difference between 
that and the synthesized band crystal frequency is the tuning 
range for the band:  for the lowest band, it's 7.550 Mcs - 5.550-
5.800 Mcs = 2.0 - 1.75 Mcs.  

The 1st mixer input must be tuned to cover the whole range; I plan 
three ranges, roughly 1.75-5.5-16-32 Mcs.   Each encompasses 
two 'real' ham bands and there'll be two crystal sockets selected 
by the bandswitch for each, plus a single socket on the front panel 
to allow any one other 250 kcs segment to be covered.   The 
bandswitch will be marked 1 A, 1 B, 1 external, 2 A, 2 B, 2 
external, 3 A, 3 B, 3 external.   Extra crystals store in a drawer 
under the receiver cabinet.  I have a nice nearly complete set and 
quite a bit of junk.

Somewhat along the lines of the Collins S-line.   In theory I could 
use an overtone scheme for the crystals but it would have to be 5th 
overtone x2 to get the x10 and using WW-II fundamental crystals 
on 5th overtone and dealing with the displacement of the 5th 
overtone from the 5th harmonic of the marked frequency, 
discouraged me.  Overtones from overtone crystals are fine but 
getting them from fundamental cuts can be tricky, unstable, and 
require slightly oddball tuning settings -- not good for a control 
that's ganged.  

The synthesizer must be tuned; that's another knob and half or 
more the sections on the bandswitch.   I'm hoping to gang only two 
tuned circuits in the synthesizer -- maybe the x10 frequency can 
be generated with a single envelope triode-pentode or an ECO-type 
circuit with the crystal frequency untuned and the plate tuned.   
Then only the x10 - 49.2 Mcs mixer output (7.550-37.30 Mcs) has 
to be ganged with that.  That would be two tuned circuits with three 
coils and a couple of trimmer caps each.   A little messy but not 
too bad.  

This does not sound terribly complex -- an extra mixer plus three 
tubes to generate the crystal mixing frequency.   However more 
plate current is required so I'll use two 17H3 (miniature damper 
diode) rectifiers instead of the 35W4 of the 80-40 design.   A 
crystal calibrator is also needed to align things on each band -- 
another tube.   So six tubes for the IF section, four more in the 
converter section (1st mixer and crystal synthesizer), a calibrator, 
and two rectifiers -- 13 tubes.   

The calibrator will tweek the 49.2 Mcs crystal oscillator.   Tuning 
range may be an issue -- crystal freq. errors of a couple kc are 
common and they are multiplied by ten.

The mechanicals are more challenging than I expected.   A general 
coverage receiver of this type cannot dodge all internal spurs just 
by careful choice of conversion schemes.   I think I have put the 
worst of them outside the six real ham bands but the rest must be 
minimized by filtering (fancy/numerous tuned circuits are not 
popular in a simple home design), by use of balanced mixers 
(complexity takes off) and shielding.  

But better shielding means boxing stuff up, while bandswitching 
three tubes and tuning a total of five air variables (1st mixer input, 
synthesizer, LO, calibrator, and VXO BFO) means some shafting 
must be fitted in.    Then there are the crystal sockets/switching 
that go with the bandswitch.   

Two boxes, both below the chassis:  One housing the 1st mixer 
and calibrator, the other with the LO/2nd mixer.   Boxes take up 
space by forcing space-wasting arrangement of parts.   But they 
stiffen the chassis which is particularly useful for the LO.   With 
thin shims under, the Bud open side boxes will go in a Bud 
chassis of the same nominal height with the flanges level with the 
sides, allowing a single flat bottom plate.

The sheilding also bites on front panel space -- and this is a 
receiver with more than the usual number of knobs.   I had to go to 
a wider chassis (14") with a 16" panel because so much was taken 
up by the LO/2nd mixer box and LO dial that there wasn't enough 
room on the chassis and panel I had planned.

I think the front panel is going to be a bit untidy.   There's just not 
the freedom there that you'd like.

A freebee discovered on the 80-40M design is that you can put 
push-pull mixers driven from a very low impedence mixing 
frequency source on the AGC line without trouble.   (Mixer drive for 
that set is taken across 0.01 mfd caps right into the cathodes.)   

The oscillator must develop a lot of power -- ~80V p-p on the plates 
of the 80-40M design -- but with careful shielding and filtering, 
chassis currents minimized, and in a tight box, it may be okay.   
The LO tunes in the opposite direction from the receiver so it's  
tweets will go by quickly.   

The front end is in another box and push-pull signal feed will also 
be favorable.  And LO harmonics are not in the real ham bands.

(If it has only ever been denominated in mHz, it is not 'real.')   

The 80-40M set is as flat AGC-wise as you could want, and the GC 
one can have both mixers on the line.  This helps the 'working' 
dynamic range even more and simplifies things considerably in an 
IF section using sharp-cutoff tubes and plate detectors.  

There seem not to be any remote or semi-remote cutoff tubes in a 
pentode-triode combination.   I'm using 7HG8's for IF stages and a 
few other jobs.   

There's way too much other stuff going on here; I don't ever work on 
the set in a concentrated way, but I'm still in thinking things 
through mode with parts sitting on the chassis on the kitchen 
counter, so that's not a big loss.   

Huh?  Oh, next to the scope.   Isn't that logical?   

Walt Hutchens 
KJ4KV