[R-390] 100kc crystals and 50kc Cal Tones

[Bob Camp]

Hi

The thing you have to worry about is the resistance of your 100 Kc  
crystals. They may be a *lot* different than the 200 Kc ones. Getting  
the thing to oscillate properly may take a bit of fiddling.  When you  
take the oscillator down in frequency you will also have to re-tune  
the multivibrator to get it to 45 Kc.

I once asked the guy who designed the 200 Kc crystals into the R390  
why he did it. His answer was pretty simple: "we were sold a bunch of  
hogwash about them". His opinion was that there was no real advantage  
to the 200 Kc crystals in the end. Neither one is really all that  
great a crystal.

Rather than rip out a bunch of stuff, why not just come up with a  
little drop in to generate a 200 KHz square wave off of a cell phone  
TCXO? Just plug it in where the crystal used to be. If you find a  
crystal later, pull it out and throw it away.

Bob
KB8TQ


On Jun 14, 2007, at 11:30 AM, Tim Shoppa wrote:

> Roger writes:
>
>> Some cap and resistor changes to move the multi vibrator time  
>> constant down
>> to a 50kc frequency from its 100kc frequency.
>
>> Does any one have any parts values?
>
> In the divide-by-two, doubling C313, C314 and C315 to about
> 470pF won't hurt any.
>
> I believe that all this is documented in an ER from a decade or two
> ago but I don't see it in the index.
>
> Tim.
>
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