[HBR] BC-453, other than IF transformers

[N2EY at aol.com]

In a message dated 1/14/06 7:22:42 AM Eastern Standard Time, 
waltah at earthlink.net writes:


> Tim wonders ...
> 
> 
> 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. 

At least one design I saw solved that problem by mounting the cap behind the 
panel on spacers, and put a large pulley on the dial gear. The pulley drove a 
dialcord to move a pointer on a slide-rule type 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.
> 

If you do the tunable-IF thing, you only need one calibration.



> 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.  

I have found that with two tuned circuits at the signal frequency, images are 
not a problem on 80 and 40 meters. The trick is to use the 550 end of the 
dial more than the 190 end.

For example, to work 80 CW I use a crystal at 4050 kc. 3500 is at 550 and 
3750 is at 300. For 75 'phone, a crystal at 4300 puts 3750 at 550 and 4000 at 
300.

Since you need two xtals to cover 80 meters anyway, might as well get ones 
that do the job best.

For 40 I use a 7550 xtal, which favors CW over 'phone. A 'phone op might go 
for a 6750 xtal. 

Or have a bunch of them - with the right oscillator circuit, all you do is 
switch xtals.


 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.   
> 

W2PPL described a receiver based on the '453 which did something like this. A 
lot of stages, though.


> 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.
> 

There are xtal filters out there with a center frequency of 1400 kc - I think 
they were made for RACAL receivers. I use two of them in the Type 7. They 
show up on that auction place at reasonable prices from time to time - they're 
gray rectangular boxes. Input and output Z is 2000 ohms. I won't tell what I 
paid for mine at Gaithersburg back in 1989....

The most common are 500 Hz but other BWs exist. They work with 1415 
transformes.

But if you're going to build a custom 3.5-4 Mc. front end around the BC-455 
cap, why not go all the way and use a conventional IF like 455 kc? Doesn't have 
to be dead-on 455 - most IFTs will go 500-430 or so, and you can always add 
C. There are still "a few" FT-241A xtals out there.....

> 
> 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.  

And can deal with BC station feed-through!


 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.  

Trouble is, a -946 is going to be premium priced, and they're rare enough 
that it's kinda sad to tear one up.


> 
> 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.

Even junkers are prized.


> 
> 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.
> 

Same deal as a 946.


> 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.   
> 

Also puts a harmonic of the 1750 at the low end of each band.
> The output 
> transformer is useful only if you either run headphones or use another 
> matching transformer to get to a loudspeaker impedence.  

I would not try to run that much audio through them.

Those little audios have lots of uses. The primary is about 20,000 ohms and 
the secondary is 4000 ohms, which makes them useful for interstage audio and 
such. Transformers from a "B" model have a tap at about 300-600 ohms, making 
them very useful to drive LC audio filters. (If your audio trans has Terminal 6, 
it's a B model)

Many of my homebrew receivers use an Arc-5 audio output trans. The B tap 
feeds an external LC audio filter, and then to either headphones or an external 
"speaker driver" amplifier. Moving the LC audio filter and last audio stage out 
of the rx eliminates a lot of heat, space, and also permits using them with 
multiple receivers. 
> 
> Toss the paper caps in cans.   

Or open them up, clean out the insides, and refill with modern caps. Saves 
you from having to work in the cramped quarters of the ARC-5. Plus you can set 
up an assembly line and repack a bunch of them at once.
>  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.'   

Excellent points - TC was going for "reproducibility", I think. Making an 
exact copy was very possible in those days. It can still be done *if* you can 
find the parts.

I think the main point of the W6TC designs was to produce a receiver that 
could be made from all-new parts that were available back then (late 1950s - 
early 1960s) direct from catalogs and mail-order. I recall a picture story in QST 
showing the various stages of construction of one ham's HBR-16. He simply made 
up a parts list, ordered everything on it from a mail-order house, and then 
built the thing. You see it go from a pile of parts to a drilled chassis and 
panel to the finished rx. 

Today we face the challenge of building with what we have or can get easily. 
A very different game!

One thing I find amazing is that the whole series - HBR-14, HBR-16, HBR-8/11 
- were designed, built, written up, etc., over a period of only a few (5?) 
years. IIRC, you could get notes, templates and photos from TC for a nominal sum 
that probably just covered his cost of duplication.

Imagine!

73 de Jim, N2EY