[R-390] Calibrator tone every 90 Khz

[Bob Camp]

Hi

That's a little far for the crystal to have drifted. I suspect that the 
multi vibrator circuit has a carbon comp resistor in it that has 
shifted value.

Here's how to figure out if it's the crystal:

Crystals are pretty tough to tune very far at all. Fortunately they 
rarely drift further than you can tune them. A very wide tune range 
circuit will pull a crystal 0.25 % of it's frequency. When you do this 
the stability of the oscillator is degraded quite a bit. For stable 
operation you rarely see them pulled more than about 0.0025%. Note that 
in your case the percentages refer to the 100 KHz frequency. A 10 KHz 
shift is 10%, way more than the crystal can move without being 
physically damaged.

R-C circuits like the one in the mulitivibrator on the other hand can 
tune all over the place. That's why you see things like the HP 200 
series audio oscillators using R-C circuits to set up their frequency. 
They trade off stability for a wide tune range. At audio you never 
notice the stability, but at RF you would be bothered by it quite a 
bit.

The manuals are generally pretty good at describing what is going on in 
the radio, but here's how the two gizmos work together:

The multi vibrator runs as a frequency that is close to 100 KHz, but 
not quite on. The crystal oscillator runs at a higher frequency. When a 
pulse comes along out of the crystal oscillator *and* the multivibrator 
is just about to go from one cycle to the next the crystal oscillator 
pulse makes the multivibrator cycle. The net result is that you can 
divide frequency this way. The technical term for all this is an 
injection locked divider.  Back before digital IC's this was pretty 
common.

I don't know if that helps or not but I have to post a certain number 
messages in threads that do not involve or mention certain words or I'm 
back in the penalty box .... hhmmm I wonder if penalty box is one of 
the words ....

	Take Care

		Bob Camp
		KB8TQ



On Jul 8, 2004, at 12:22 AM, hdalexander at att.net wrote:

> Hi All
>
> I have a Motorola R-390(non-A). The calibrator makes a tone 
> approximately every 90 Khz., not every 100 Khz.. For example: 14000, 
> 14090, 14180,14270, 14360. Do any of you have any words of wisdom for 
> me? Thank you in advance for any help.
>
> Harold Alexander
> Modesto, CA
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