Scott Philharmonic -- HiFi Control

[Jay Forbes]

When HiFi control is full clockwise, what are the positions of each of the IF Variable Trimmers?

Positions of these Variable Trimmers in the radio I am repairing are as follows, from Front to Rear of chassis (pic attached)

Open

Closed

Closed

Open 

Open 

Closed

Is this correct?

I recall that they would all be closed, but have no pics of it.

[Norman S Braithwaite]

That is correct.  In the full CCW position (sharp tuning), they are opposite and that is the position for which the IF transformers are initially peaked.  There are nine tuned circuits in the IF amplifier.  When rotating the selectivity-fidelity control CW, one third of the IF tuned circuits increase in frequency and one third decrease in frequency to widen the IF bandpass.  Final IF alignment requires sweep alignment to balance the aprons of the bandpass characteristic for proper audio reproduction by detuning one or both of the last tuned circuits slightly (will not impair sensitivity to a very significant degree).  Balance the bandpass aprons at the 1/2 to 2/3 CW rotation of the control, not the full CW rotation.

Norman

[Norman S Braithwaite]

Also, you will want to loosen and retighten the set screws holding the variable selectivity rotors.  The pot metal collars expand causing them to loosen and lose electronic contact with the shaft.

Norman

[Jay Forbes]

Norm, just to confirm, sharp tuning is when HIFI control is in CCW?  Seems it would be CW...

Norman S Braithwaite said:

That is correct.  In the full CCW position (sharp tuning), they are opposite and that is the position for which the IF transformers are initially peaked.  There are nine tuned circuits in the IF amplifier.  When rotating the selectivity-fidelity control CW, one third of the IF tuned circuits increase in frequency and one third decrease in frequency to widen the IF bandpass.  Final IF alignment requires sweep alignment to balance the aprons of the bandpass characteristic for proper audio reproduction by detuning one or both of the last tuned circuits slightly (will not impair sensitivity to a very significant degree).  Balance the bandpass aprons at the 1/2 to 2/3 CW rotation of the control, not the full CW rotation.

Norman

[Norman S Braithwaite]

Confirmed.  Narrow ("selectivity") is full CCW and broad ("fidelity") is full CW.  The high fidelity (full CW) bandpass is 32-khz, well beyond anything useful today hence the recommendation to balance the bandpass aprons at 1/2 to 2/3 CW rotation.

Norman

[Jay Forbes]

Thanks.