[MRCA] Pondering Crystals for the BC-620 radio

Wed May 30 13:20:32 EDT 2018

Hi Jim - I stand corrected..The formula I noted is for the BC-659
(different IF and multiplier apparently).  Sorry for any confusion..

Tim
N6CC

On Wed, May 30, 2018 at 9:42 AM, J Mcvey via MRCA <mrca at mailman.qth.net>
wrote:

> 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 3
> The 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
> <http://www.wolframalpha.com/widgets/view.jsp?id=82138b11a724b94d18df2e083d8b7b55>
>
> 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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