[HBR] BC-453, other than IF transformers

[Walter A. Hutchens]

Tim wonders ...
> OK, I got a couple of BC-453B's, and I see the real nice IF
> transformers. BUT... I also see some other stuff, like a nice dial and
> drive and probably some other stuff I do not yet appreciate.
> 
> It's obvious I could use the BC-453B as a kind-of tunable IF but the
> band (190-550kHz) may be less than ideal (a different command set
> receiver higher up in the HF band might have been more appropriate there.)

This is one of the great design questions of the 'old technology' receiver 
game.  

The tuning capacitors are outstanding.   Stable, solid, smooth tuning, no 
backlash.  Disadvantages: You have to deal with a dial covering ~330 
degrees to use the full swing of the cap and the knob shaft interferes with 
the space for the dial.   If you recalibrate the orginal dial it works fine but 
you've got a single band.   If you put a pointer on in place of the dial disk 
you can do multiple bands but the inner bands get cramped.

The BC-453 cap is very high capacitance.   Better matches to tuning the 
RF/mixer section of the usual HF ham receiver are the BC-454 or 455 caps, 
the latter especially if tuning a narrow range like 200 or 250 kcs.   You can 
use series caps to make adjustments but may wind up with a very non-linear 
scale with the -454 cap.   I used a BC-455 cap for my HBR-type design.

At least two other approaches would work:  

1.  Use BC-453 receiver (or transplant the converter/IF stages) as a tunable 
IF.    You'd have to put two stages of conversion ahead of that, because 190 
kcs is definitely not a high enough IF to avoid images.   For example you 
might use a BC-455 cap in the front end, converting to a broadband IF 
roughly 2650 kcs-3010 kcs based on 2830 kcs IFTs from the BC 455, then 
use an osc. fix tuned at 3200 kcs to convert that to 550 kcs-190 kcs for the 
tunable IF.   

2.  Use a BC-455 cap with padding to tune a 250 or 500 kcs range -- maybe 
3500-4000 kcs -- and use a crystal controlled converter ahead of that.   
Use either 1415 kcs or 2830 kcs IFTs with either a crystal filter -- you'd 
have to order those crystals at ~$25 each -- or a second conversion, which 
is the HBR design.

You can't go with just the 1415/2830 IFTs unless you're willing to accept a 
very wide bandwidth.   

There are two other IFs in the command series, one used in the BC-946 
which tunes the broadcast band; those are 239 kcs and might be 
satisfactory in a double conversion design if you want to do mainly AM.   Or 
you could use the BC-946 converter/IF with a crystal controlled front end;  
since the -946 covers a 1 mcs range this would be a good start for a general 
coverage receiver, although image rejection would be a bit 'soft' at the 550 
kcs end of the range.   You might modify the -946 front end to cover 1-2 
Mcs -- that would work a lot better, image wise.  

The -946 was much prized as a broadcast band tuner for use with a high fi 
and they weren't as common as the other sets.   However you might find a 
junker with the parts you need.

The Navy used a receiver covering 1.5-3 Mcs; I don't recall the number.  
That IF was 705 kcs I think but I can't see a particularly good use for it 
and these receivers are pretty uncommon compared to the others.

Multiple conversion designs need a lot more than this sort of back-of-the-
envelope thinking.  Like, does one of your oscillators show up right in the 
middle of one (or every!) band?   

The IF transformers are good quality although not designed for high 
ultimate rejection or great skirts.   The BC-453 units have excellent 
selectivity -- four of them will get you down around 2 kcs BW.   However, 
they're 85 kcs, meaning that your receiver needs to be double or possibly 
triple conversion.  

They are a natural fit to the HBR series since those are double conversion 
to start with.   My HBR-type used BC-454 IFTs (originally 1415 kcs) retuned 
to 1665 kcs with a 1750 kcs crystal for the second oscillator to match the 
85 kcs '453 units.   That design works fine.   

Getting your selectivity at 85 kcs forces you to a double conversion design 
because you have to have enough selectivity ahead of this IF to get rid of 
the image which is only 170 kcs away.   You'll have image trouble even on 80 
with such a low IF unless something else is used ahead of it.   

I have retuned the command receiver BFO coils for the second converter 
job, but they're not terribly stable so they're less than ideal.  

Other useful parts -- the 3H filter choke is adequate for any small receiver 
design but either have spares or check carefully before drilling any holes 
as some of them have suffered from moisture over the years.   The output 
transformer is useful only if you either run headphones or use another 
matching transformer to get to a loudspeaker impedence.   The little antenna 
trimmer caps (~15 mmf I think) are fine for uses like that -- I used one for 
the calibrator cap on my HBR-type and a BC-453 trimmer (higher 
capacitance) for an antenna trimmer on that set.

The command series equipment has several different kinds of knobs that 
can be used.   I use the antenna terminals; if your panel is thicker than a 
command receiver you can use a command TX antenna terminal -- it has a 
longer shank.   You have to run a 4-40 die down the shank to thread the rest 
of it.  

Toss the paper caps in cans.   Many (the majority?) are defective.   Some 
will test okay but fail within a few hours.   The ARC type 12 sets that were 
Aircraft Radio Corp's follow on product for the civil aviation market after 
the war use a very similar design, however, and I've never seen one of those 
caps with a problem.   

If you enjoy activities like beating your finger with a small rock, you can 
even use the plug-in front end coil assemblies.   My HBR-type (with the -455 
tuning cap) takes coil sets through the front panel right below the chassis 
level.    This is NOT a very practical design; the metalwork has to be fairly 
precise and mechanically sturdy if it is to be satisfactory.  Getting good 
performance requires higher-Q coils than the command coils on the higher 
bands so you have to put toroids in those cans and you have to precisely trim 
each coil by cut and try.  

The result works well -- probably better than the W6TC design in that you 
change only one coil assembly instead of three individual coils for band 
changes and because the coils are under the chassis and get some 
ventilation through the gap in the front panel, there's less drift.   But the 
purpose of the W6TC design was a sound BUILDABLE receiver that would 
be a good performer by the standards of the day; my HRO-like adaptation 
gives up a lot of 'buildability.'   

I use the 3-48 machine screws EVERYWHERE.  You have to come up with the 
corresponding nuts and/or a 3-48 tap but the 'net makes that easy.

I have a pretty complete set of command receiver metalwork saved to 
provide drilling templates for the parts.

Walt
KJ4KV