[HBR] Another HBR Project -- Chapter 9

[hbr@mailman.qth.net]

I guess you could call this edition 'starting to wrap it up.'   There are 
odds and ends, but as of Sunday night, a month from the start date, 
I'm declaring the 'one week HBR,' finished.   Guess I should have 
called it a 'one month HBR,' huh?

Performance numbers; all measurements at 3900 kcs:  Drift 
something like 500 cps from cold, stable after 1.5 hours; can be 
further improved.   Sensitivity about 0.2 uV for 10 db s+n/n which is 
good.   Interference free dynamic range (IFDR, a measure of ability to 
hear a weak signal among many strong signals) 60 db; about as 
good as you're going to get with a high-gain RF stage.   

The modest IFDR is the result of an explicit design choice to spread 
the gain over as many frequencies as possible in order to minimize 
construction 'challenges' but use only the bare minimum number of 
tubes.  An advanced version would have an IF stage (instead of just 
back-to-back transformers) at 1665 kcs and much reduced gain (or a 
wide range gain control) for the RF stage.  That would require a 
beefier plate supply but there's room to install the necessary larger 
transformer.

A 60 db IFDR is 'ok' -- the FT-101 receiver is around 50 db, the 
Collins 51S about 60 db.   The military R-390A, designed for spare-
no-cost performance in a strong signal environment (nearby 
transmitters and possible jamming), 80 db.   At last check, the 
HBR2K project stood at 70-some db.

Selectivity -- I haven't measured it yet.   With all the 85 kcs coupling 
rods pulled up (loose coupling) it seems to be just about right for 
SSB on 75 -- comparable reception but of course 'peakier' voice than 
the 2.4 kcs crystal filter in the HBR2K.   It's a lot better on AM with 
the rods pushed down -- the carrier is just too far down the curve in 
the sharper position, and unfortunately that mechanism is very poorly 
suited to repeated changes between up and down.   Since I copy 
mostly SSB, I'll probably use it with the rods 'up'.  

A sophisticated builder would figure out how to switch the coupling 
caps between the back-to-back transformer pairs; that would give 
about the same effect as moving the rods.

The BFO/calibration oscillator saga wound up as follows:  I moved 
the grounded plate tickler feedback version of the BFO to the 
pentode half of the 6U8.  It's wired as an ECO although technically 
not one, because G3 is (internally) connected to the cathode rather 
than ground.   But it gives significantly better isolation from the signal 
than a triode, meaning essentially no distortion caused by oscillator 
pulling on voice peaks.   The BFO in the original HBR-series is an 
ECO and that was the right choice!  To improve the waveform I put a 
low-Q tuned circuit in the plate circuit -- harmonics there can lead to 
distortion in the mixer (AKA product detector) circuit.    

That left the triode half as the 100 kcs calibration oscillator.   With 
the BFO power level reduced and lead dress improved, the 
modulation effects are tolerable -- after all, a calibrator isn't used that 
much.   

The next step was a 40-meter coil set.   Boy, what a job.   Even 
when you have calculated values for all the parts there's no really 
easy way to do the 'first edition' of a front end like this.   There is one 
trimmer for all bands for each tuned circuit, so at least you can tell 
easily which way you have to adjust -- but except for the 'align input' 
trimmer, all the trimmers have to wind up in the same place for all 
bands.   In the end you install a coil in the coil can and take it out 
maybe 10 times to get all the values right.   Repeat once for each 
coil in the set.  

When I got the 40 meter coils working, the gain was too low.   There 
were two parts to that:  (1) The antenna I'm using (full wave horizontal 
loop for 160) isn't very close to resonant on 40 so the antenna coil 
can't be peaked on that band.   (2) Evidently the Q of the coils is 
somewhat low, compared to 80m.   I got satisfactory performance by 
replacing the RF/mixer coil with a toroid.  

Warm up drift with two command set 3 mmf/N750 caps is acceptable 
although not as good as possible.   The coil sets (fixed caps for each 
band are also in the cans) get up to about 90F after several hours; 
the chassis between the oscillator tube and tuning cap hits about 
100.   I don't think a well designed cabinet would hurt this much -- 
the underchassis ventilation should be by fairly small holes under the 
coil set -- say 2x1/2" holes.

I'll wait until everything else is done to revisit the temp compensation 
issue.   Probably it will be necessary to add a compensating cap in 
each oscillator coil can.

I finalized the output audio stage as a pair of 6AQ5's submounted 
through a clearance hole in the chassis.   Not only does that do a 
good job of cooling the tubes, it pulls more air through the coil area in 
the front of the set which will help stability.

I replaced the internal capacitors on a couple more IFTs that were 
not perfectly resonant (command set IFTs have just enough tuning 
range for normal variations in wiring and tube capacitance in the 
those sets -- they have nothing like the range of commercial 'build 
your own' parts).   I believe there's one more 85 kcs IFT that's not in 
the middle half of its range.

Several details remain:  The STANDBY switch; coils for 20 and 
maybe 160; maybe even an SWBC band.   The dial needs to be 
calibrated.   There should be an accessory/muting connector on the 
rear.   I'll poke around the detector and audio stages with a scope; 
you nearly always find something that can be improved.   

And since the project has been successful, I'll probably build it some 
kind of simple wooden cabinet.   Maybe put a drawer under the 
receiver to hold the coil sets?  There's no place to put them that's 
exactly the right temperature but at least they'd be dry and wouldn't 
get lost.

There was a request for plans to duplicate the project.   I'd be 
inclined to do it, I guess, if there is enough interest.   On the positive 
side, the use of Radio Shark transformers and the extensive use of 
command set parts makes the parts finding phase much easier than 
for the original HBR designs.   And it's an excellent performer -- 
because I used later tubes, it's probably better than the original 
HBRs.   Electronically it wouldn't be hard to duplicate -- the only 
oddity was the interaction between the BFO and calibrator and I 
intend to fix the layout to separate those oscillators.   Or you could 
just leave the 100 kcs calibrator out.   Using a 1750 kcs crystal as a 
second oscillator gives you a band edge marker for free and except 
for the initial calibration of each coil set, that's really all you need.  

You do need a *bunch* of command set stuff -- the tuning cap is from 
the 6-9.1 Mcs receiver, the two 1665 kcs IFTs are converted 1415 
kcs units from a 3-6 Mcs receiver, and the four 85 kcs IFTs and BFO 
coil are from the 190-550 kcs set -- since there are only three IFTs 
per set, you'd need a second set or an extra transformer.   Plug in 
coil sets are a dime a dozen when you find them but you need one 
per band of coverage.  You'll need to solve the classic command set 
knob problem -- the tuning caps have a splined shaft.   

In my opinion the main thing on the negative side is that the layout 
and construction for the plug in coil sets is difficult.   The coil 
sockets (actually, male plugs) mount inside the chassis on a sturdy 
bracket facing a rectangular hole in the chassis front/panel; the hole 
is flanked by a pair of machine screw studs.   Everything has to be 
perfectly aligned if it's to work well.  It can be built with nothing 
fancier than a drill press (that's all I've got) but must be laid out and 
the work done with great care and some skill.   This isn't a project for 
the beginning mechanic.   

Second to that, tweaking the 85 kcs IFTs (if necessary to get them 
on the same frequency) is annoying and slow.   Mine almost surely 
came from four different radios, quite likely built years apart, so if you 
start with one radio and a spare IFT, you might have a much simpler 
problem.   I don't recall seeing this issue discussed in any of the 
construction articles using these IFTs so maybe it's a self-inflicted 
wound.

Making up the coil sets is a real task for me, but I think they'd be 
easy to duplicate.   With the number of turns and parts values known 
and the ability to use a GDO to set up the coils before putting them 
in the cans (because the only parts of the tuned circuit that don't 
plug in are the tuning cap and 'strays'), they should be very close to 
right the first time.

The coil sets require that all of the possible six pins be present, both 
on the socket and the coils themselves.   This is because the series 
padder cap is part of the coil set and the antenna coil has both a 
separate winding for the antenna and (on some bands) a padder cap. 
On the RF coil, there's a pin for the padder and another to disconnect 
the HV to the screen if the coil set isn't installed.   I *believe* all the 
Navy receivers (ARC-5 and ARA) have six pins on all coils but the 
more plentiful Army (SCR-274N) have only 3 or 4 on the antenna and 
RF coils.  This might or might not pose a parts-finding problem.

You also need to find the right mica (or NPO ceramic) caps -- 
'something close' from the junkbox won't work in this area.   You can 
make up the right values from multiple smaller caps, but space is 
quite limited.   

If you're interested in plans, let me know.   In a couple of weeks I'll do 
a circuit diagram for my own purposes, but the chassis drawings and 
stage by stage construction notes (typically 20 pages for something 
this size) are another few weeks of work and I won't do those unless 
others want them.  The notes not only go stage by stage through the 
construction process, they discuss the philosophy, possible 
problems and variations.   Diagram will be $5, the notes and layout 
drawings (8-1/2 x 11 sheets with full size chassis and panel halves) 
$19 (including the diagram) if I do them.

I'm sure I have some leftover parts -- one or two 1750 kcs crystals, 
crystal sockets, a spline fitting or two, and various other small items 
that I'll throw in for the first few drawing/notes orders if we get that far.

Walt 
KJ4KV