[MRCA] Pondering Crystals for the BC-620 radio

[Dale Mecomber]

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=
> 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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>>
>
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