Re: Need help with Mitsubishi DA-F20


rick.savas@...
 

Nice, thanks for sharing

On Wed, Mar 11, 2020 at 6:34 PM newaag via Groups.Io <newaag=yahoo.com@groups.io> wrote:
Quick update for those following along. Briefly, I removed the load cap on the crystal oscillator, and replaced it with an adjustable one, 3-22 pf. This cap looks like a very mini tuner gang with interleaved plates.
I adjusted it until the frequency at pin 56 of the digital board was exactly 1.25000 MHz. This moved the measured (at R215) IF frequency from 10.673 to 10.700. Nice!
more on what is going on -
The signal at pin 56 is the output of a divide by 8 counter that is fed by the 10.00 MHz crystal oscillator. It was originally 1.24967 MHz, or an error of 330 Hz. This 1.25000 Mhz clock is further divided down to 50 kHz, and then used in a phase locked loop (PLL) to sample the output from the local oscillator, to create an error signal that than feeds/tunes the local oscillator voltage tuned capacitor "gang".

This PLL feedback circuit, in essence, is supposed to lock the station on the tuned frequency, but also keep the IF at exactly 10.7000 MHz. Any error here is multiplied by the ratio of the local oscillator over this 1.25000 MHz clock. I was working at 98.00 MHz, so the LO was at 98.00 + 10.7 = 108.7 MHz. And (108.7 / 1.250) X 330 Hz error = ~ 28 kHz, which is almost exactly the IF error I measured. So by making this error zero, I was able to bring the IF back to 10.7000. 

Bob 

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