[HBR] Another HBR Project -- Chapter 7

[hbr@mailman.qth.net]

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