[MRCA] Pondering Crystals for the BC-620 radio

[Mark K3MSB]

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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