[MRCA] Pondering Crystals for the BC-620 radio

Wed May 30 12:42:40 EDT 2018

 Both the BC620 and the BC659 have standard suerheterodyne front ends.
The only tricky part to figuring out what frequency is knowing what harmonic they are using in the oscillator and what  IF frequency is being used.
On the BC630 they use the 3rd harmonic, so multiply the crystal frequency by 3The BC620 IF frequency is 2.88 Mhz .As posted above a 5.7067 mhz X 3 = 17.1201 Mhz + 2.88 = 20.001 mhz
to figure the xtal frequency for a particular center frequency , just rearrange and solve for the xtal.
XTAL freq =  (center freq - 2.88 mhz )/ 3

The BC659 uses the  FOUTH harmonic of the xtal and an IF frequency of 4.3 Mhz
to find an xtal frequency for BC659,:
Xtal freq = (center freq - 4.3 Mhz) / 4
Jim

    On Wednesday, May 30, 2018, 10:43:41 AM EDT, Dale Mecomber <dalen2dm at gmail.com> wrote:  

 Hi Mark,Please consider that CB(11 mtrs) is AM/SSB modulation. I realize "enforcement" is lacking, but use of FM on CB might cause some investigation.Good Day, 73, Dale, N2DM
On Wed, May 30, 2018 at 6:38 AM, Mark K3MSB <mark.k3msb at gmail.com> wrote:

Better yet....   Here's a web page to do it for you (since you, like I, don't have any graph paper handy....)
http://www.wolframalpha.com/ widgets/view.jsp?id= 82138b11a724b94d18df2e083d8b7b 55
For the two points you listed (5.7067 , 20.0)  and (8.3400, 27.9), the line equation is
 y = 3.00004 x + 2.87968 
 or
 BC620 Frequency = (3.00004 * Crystal Frequency) + 2.87968 

Using the above equation, check some of your crystals and see if the equation works. If it does, then a linear relationship exists and the formula holds true.    If it doesn't, then we'll have to use a polynomial fit and I'll need more crystal and BC-620 frequencies.
If a linear relationship doesn't exists across the  entire range of frequencies, it may be that a linear relationship does exist over a small range such as across the CB frequencies.  If it's close, then you may be able to use a linear approximation and tweak the non-linear part by the trimmer caps on the radio -- or similar;  I've never played with a BC-620.
73 Mark K3MSB

On Wed, May 30, 2018 at 6:09 AM, Mark K3MSB <mark.k3msb at gmail.com> wrote:

Mark
Graph the crystal frequency vs  BC-620 output frequency for several crystals.
If they are all on a line then a linear relationship exists.  Knowing two points on a line you can then determine the general equation of the line and thus compute the BC-620 output frequency for any crystal frequency and vica versa.
73 Mark K3MSB
On Tue, May 29, 2018, 11:36 PM WW2RDO via MRCA <mrca at mailman.qth.net> wrote:

Something to ponder:  The BC 620 radio is a 2 frequency, crystal controlled FM radio operating phone  between 20.0 to 27.9 MHz FM.  CB radio frequencies are set on specific frequencies between 26.9650 MHz to 27,4050 MHz FM.   Unless I'm missing something, if the crystals are cut correctly, the BC-620 should be able to operate on 2 CB channels if the crystals are cut precisely and correctly..  Here's the rub - the crystals made for the BC - 620 operate in an oscillator circuit. Is there a formula to cut crystals for the BC-620 to have the radio work on specific CB channels ?  Crystal # o in the radio set is actually rated at 5.7067 MHz, but when installed in the oscillator circuit in the BC-620, transmits and receives on 20.0 MHz.  Crystal # 79 is actually cut for 8,3400 MHz, but when put in the oscillator circuit in the BC-620, operates on 27.9 MHz.
Has anybody actually had custom crystals cut for a BC-620 to allow it to work on CB Frequencies?  If so, is there a formula used to determine what frequency the crystal should be cut for?  I know that running CB frequencies isn't exactly in anybodies wheelhouse, but if this could be figured out, perhaps the BC-620 becomes a popular player in the vintage military radio world, along with perhaps other vintage radio sets.
Mark D.WW2RDO______________________________ ______________________________ __
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