Re: Need help with Mitsubishi DA-F20


newaag
 

I will go ahead and order new oscillator parts and install, as I don't consider this job done. Why? 

I removed the adjustable cap for the 10 MHz oscillator, and it measured 10 pf. It is supposed to be 20 pF. 
This is probably right on the edge of even working as an oscillator. It is pulling the crystal about 300 PM higher.

Normally, you can adjust the load cap for a crystal +/- 100 PPM, at best, that is all designers recommend. At some point, it just stops working. At the edge of oscillation / not working, stability and jitter get much worse.

So in this case, it makes sense to simply replace the crystal, and load cap, bring them back to stock values with new parts. For the load cap, it need a 20 pf C0G spec part. For the oscillator crystal, a decent +/- 30 ppm crystal (or better). They are so cheap, it's best to order 10 of each, plus caps +/- 2 pf form 20, to fit the best combination.
The other option is to install a tightly specified TCXO oscillator, and eliminate the on-board one. This is more work, but for those with the chops, not too hard. Most of these are now CMOS running at 3.3V, which means it needs to interface with old tech TTL digital parts. Not sure if this would be a problem or not. So to keep it simple,going with parts replacment as stock. But you can also buy TTL 5V units, but they are either really expensive ($50+) or not TCXO, just can oscillators, and not tightly spec'ed, with no adjustment.

Some may wonder why this problem happened? Was it like this since new?
Probably not. Just like people, these crystals, depending on how they are made, "slowly age" and drift over time. Most data sheets say 5 PPM / year. Well this one is 40 years old!
If it was off +/- 100 PPM to begin with (not unusual), it could be normal aging, and alas, in 2020, off by almost 300 PPM.
Which is why I am documenting all this stuff. There may be other decent tuner units out there (digital tuned) that could use this cheap fix. This sort of instruction (like cleaning gangs) will NOT be in the service manual. :-)
Bob 
To be continued...

 

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