[Milsurplus] BC-669 Project: Xtal Subber Update

[David Stinson]

Still working on a herd of BC-669s.
One of the issues is the need for crystals for the six 
pre-set channels.  While usable crystals from good
suppliers like AF4K are always preferrable, most of the 
frequencies I need are either unaffordable custom-made
($45 each for International to make the for you 
and you need 12), the freqs needed are not available 
or are not a useable crystal type.

Those tiny little xtals so common today will die a quick 
and tragic death in the 6L6 transmit oscillator
but they will work OK on the receiver side's 6J5 oscillator.
So as with earlier projects, I'm working with
Programmable Oscillators which can be ordered from 
Digikey for any frequency from 1-125 MC in 100-cycle
steps for a whopping $4 each.

Receiver Xtal Subber.
This simple circuit will drive the receiver side very well.
It works with most types of oscillators which have 
a tuned-circuit in their plate circuits, i.e. the 1625 
oscillator-multiplier in the ARC-5 VHF transmitter 
and also in the ARC-5 VHF receiver.
I've heard the objections about "jitter" and "noise" etc.
Used them in several projects and these
 have not been an issue.  Here is the circuit:

https://goo.gl/photos/8pEso7ra9sp26msz6
https://goo.gl/photos/prgaQMniksRHVqq16

The .004 caps are for DC blocking.  I used them
because I had them on hand, .01 is too big
and .001 is too small.  Physically smaller caps 
rated 50V or more should work fine.
The little 4:1 UNUN came from a local surplus house
(shout out to Jim at Tanner Electronics in Carrolton);
he's got a zillion of them for about 25 cents each.


The IF of the BC-669 receiver is 385 KC.  While the
manual specifies IF+385 for channels 1-5 and IF-385
for channel 6, I've found either formula works as long 
as the oscillator frequency falls below 4.4 MC.
I selected oscillator frequencies outside the 80 meter
band to avoid the need to switch the oscillators on
and off.  Examples:

RX Channel Freq     RX Oscillator Freq.
3890     3505, 4275
3885     3500, 4270
3880     3495, 4265
3870     3485, 4255
3547     3162, 3932
1885     1500, 2270

Transmitter Xtal Subber.
This is still a "work in progress."  
Here's the original TX oscillator, mislabeled as 
the heterodyne osc. in the manual:
https://goo.gl/photos/ZknAK2d9KJLQD57B7

A strong crystal will drive the 807s to 6 mA of 
grid current.  This circuit will deliver 2.5-3 mA, 
but that seems to be enough to 
drive the PA to within 10% of full output.  
I began the project with a circuit derived from 
WA6FWL's VFO buffer published in QST.  
I've moved-away from the Class A
amplifier to a Class-C and replaced the broadband
transformer on the output with a slug-tuned coil also 
from Tanner.  It's about 60 turns on a quater-inch
form about 3/4 of an inch long.  Tune the slug until 
the 6L6 stage breaks into oscillation and peak drive 
to the PA.  The Prog. Oscillator locks the oscillations
to the desired frequency.
Need to find a stouter transistor and work on the
coupling to the 6L6 Oscillator.
Photo of the transmitter oscillator flanked by two
receiver versions and one original crystal:

https://goo.gl/photos/PiXAUBALiCnvnvQb9
https://goo.gl/photos/gkWYm9YD1dgX4YsA7

Current prototype diagram:
https://goo.gl/photos/zuiPq2VZfvS2Gp61A

Power

Power is supplied to all boards by a cute little DC-DC
buck converter mounted on the transmit board.
Size of a postage stamp.
They take the unregulated 12 VDC derived from the
filament string and produce clean 5 VDC.  It can 
source up to 3 Amps.  Available on Ebay for the 
big price of 75 CENTS each.  Crystal mount plate
glued to the top of the converter's inductor deletes
birdies in my application.  Buy 10 of these little 
beauties for $7.
http://www.ebay.com/itm/162064959588

Grateful thanks to Nick Broline, Arden Allen,
Mike Hanz and others for help and encouragement.

73 DE Dave AB5S