[HBR] Another HBR Project -- Chapter 4

[hbr@mailman.qth.net]

Swimming in molasses ... 'talking book' speed ... those are the 
metaphores that come to mind.  In other words, things are going 
pretty normally.

A problem I should have seen going in -- I picked a modern 'wide 
range' automotive speaker but those are invariably very low efficiency. 
I may have to use something else to get decent volume.  There's 
plenty of room for any of the usual 3" 'communications' (i.e., cheap) 
units.

I couldn't get enough output from a 12BH7 operating class 'A' at this 
low plate voltage -- 140 volts.   Currently I have a pair of 5902 
subminiature beam power tubes on a 9-pin plug stuck in the audio 
output socket; the long run will either be the same in sockets or a 
pair of 6AQ5's in sockets, depending on what the space permits.

The plate detector worked with little trouble.    The AGC detector 
(basically the same circuit but with the cathode at -170 volts and the 
plate at 0 with no signal) was also easy.  I used both of these 
circuits in the HBR2K. The 85 kcs IF stage took a bit of work 
because of the need to tune the IFTs; one of them required an 
internal capacitor swap to resonate.   All had to be opened, 
inspected and the adjustable coupling freed up, but no real 
difficulties.   

One of the questions at the start was whether one IF stage would be 
enough gain.   At the moment the answer looks like 'yes.'   
Sensitivity at the input to the IF stage is better than 1 mV, the two 
mixer stages will have some gain and the 6EH7 RF stage should 
take us 'over the top.'   Of course the great advantage of having the 
gain distributed over several frequencies is that it's easy (okay, 
*easier*) to avoid multi-stage instability.    

The BFO was a whole 'nuther story.   Pretty much the gang that 
couldn't 'wire straight' at first -- filaments on the wrong pins of the 
6U8, then I had the (command receiver) transformer hooked up 
backwards.

When I fixed that, it still didn't work.  I checked everything, swapped 
tubes, but no oscillation.   Took the transformer out, disconnected 
and checked the caps, replaced the mica bypass just to be sure -- 
still didn't work. The transformer winding resistances (it's a tuned 
plate -- tickler feedback circuit) were okay and no measurable 
leakage.   I had another junk transformer so I decided to swap it in.  
I'm mounting this coil like the IFT's, that is with the terminal end to 
the chassis, so the first step is to drill and tap each of the four corner 
posts #3-48.   Two holes okay, third hole I twist off the drill flush with 
the top of the hole.   So much for that.

I decide to swap the guts to the frame I already modified.   Half an 
hour's work, looks okay -- oops -- I broke one of the coil leads.   
Yeah, you can splice that Litz wire, but I'm not desperate enough for 
that just yet.   

Back to the archives.   No more junk 85kcs transformers ... wait, 
what's this?   Looks like someone stepped on the top of the can, but 
the insides might be okay ... corrosion has wiped out the markings 
so not sure of the frequency.   Fifteen minutes getting it apart without 
damaging the innards -- I'm in luck; it's an 85kcs unit.   This time I'm 
more careful than ever doing the drilling/tapping operations and an 
hour later (total about 6 hours) an oscillator that any 1950's novice 
could have wired in five minutes using only his teeth, started to perk.  
I have no idea what was wrong with the original transformer -- maybe 
a very few shorted turns?   

Walt's law of the junkyard:  "Sometimes when you buy junk real 
cheap, it turns out to be just junk."

As I hoped, the internal trimmers serve perfectly to set the frequency 
range and a front panel mounted HF-50 is exactly right to tune 85 
kcs +/- 2.6 kcs or so.   Right now, that doesn't seem like a small 
blessing at all.

Next the 2nd mixer and 2nd (1750 kcs) oscillator.   How hard can it 
be?   

Hehehehe .... giggle ...

Walt 
KJ4KV