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AM-FM Stereo Tuner TU-S707X ( Sansui )

[Outline] [Front view] [Inside view] [Trouble-shoots]

[What was happen?] [Cause] [Repair]

[Troubles]

When I got this tuner, there was no sensitivity on FM band. After IF amplifier was no problem. And 1st L.O. VCO was locked in the appropriate frequency. RF and MIX FETs were all OK. I was aware that VCO control voltage was lower than ordinary.


Fig.1 The front-end ( after trimmer capacitor was replaced ).

1. The capacitance of the VCO frequency trimmer was going low.
2. The VCO control voltage was going down to lock in the required frequency.
3. The tuning frequency of the RFA tank circuit was going down
   from the required frequency.
4. The sensitivity was going down because of the tracking error between
   the RFA and the VCO.

[Repair]

Replace the trimmer capacitor of the VCO.

Fig.2 Trimmer capacitor ( left : original , right : replaced ).