Re: Need help with Mitsubishi DA-F20

newaag
 
Edited

I wanted to update this topic with more information. Here's a quick recap of the problem. The tuner, a DA-F20, did not play in stereo, and reception was poor.
A quick alignment showed the both detector zero adjustments were way off. The narrow detector on this unit is used to enable both muting, and also stereo reception. If that detector is not at zero, stereo will not engage, regardless of any other settings (muting off, etc). So this was a quick fix.
But there was more wrong - the reception sensitivity was poor, and distortion measured high.
It turned out the crystal oscillator was off specification, (as per previous messages), tuning the IF to 10.67 instead of 10.70. It appears the previous tech who worked on this unit also saw this, and tried to correct it, but installed the wrong ceramic filter set in narrow (4 filters). They were marked, in handwriting, 10.72 on each filter, so they were obviously tested and sorted before installation. But the correction went the wrong way - 10.67 filters should have been used.  
It was likely an easy mistake to make, though. I frequently do a very low RF signal test, in IF narrow. Then tune the RF signal generator slightly above and below the station frequency (say 98.10 MHz), to see if noise goes lower and sensitivity improves slightly off frequency. It is a good test for crystal oscillator based tuners. 
For this tuner, initially the sensitivity improved, with lower noise, with the stereo generator tuned about 30 kHz above the actual target station - in this example, it was best at 98.13 MHz. 
This could give one the impression that the IF filters need to be moved "up" in frequency from 10.70. In fact, if the actual IF is measured, it would be where the oscillator told it to be, at 10.67. This is due to frequency inversion, as noted in previous messages. But...
The "fix" may not be using new narrow filters selected for 10.67 MHz. Because one needs to measure the other IF (wide) IF path, and see where it is "centered". In this case it was measured, and found to be correct, at 10.70 MHz. When going to all this trouble, you want the IF path centers to "match" each other, AND also match the oscillator. And in this tuner, the wide LC filter is a closed box and non-adjustable. So the wide IF drives which filters you select for narrow, and where you adjust the crystal oscillator.
Long story short - a new 10.000 MHz crystal was installed, and the 20 pf cap was selected so it ran exactly on frequency. Then 4 new narrow filters were fitted, all selected for 10.700 MHz. 
Then the tuner was completely re-aligned - this is a MUST after moving the IF frequency (from 10.67 to 10.70) and fitting new filters.
These changes completely transformed the tuner - it now is both sensitive, selective, and tests very low for distortion in both wide and narrow, with excellent stereo separation (over 50 dB in both wide and narrow).
That is basically the end of the story. I will document the alignment "how to" specifics elsewhere at a later time. The service manual could be a lot better for this unit.
Bob

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