AM Tuner Alignment

 


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Last Edited:
31-Jan-2002


Factory Service Bulletin Alignment Instructions for AM Sections:

Scott AM/FM  Tuner Types 300/320, 330/331-(series), 333-(series)

Scott AM/FM  Receiver Types 331-(series), 355, 380, 399

!WARN!NG! Do not attempt to "align" your tuner. Any attempt to "adjust" any coils, transformers, or capacitors without the proper test equipment and the experience to properly use this equipment will almost certainly result in performance degradation of your tuner! Having said that, here are the word-for-word instructions taken directly from H.H. Scott, Inc., publication number D-333B-2. All Scott AM tuners shared the same basic AM section design.

"Note: No attempt should be made to align the tuner or repair it unless the person doing so has extensive experience in tuner alignment and repair procedures and has the necessary laboratory equipment. Without proper experience or equipment, the repairman may seriously damage the tuner.

1.    Equipment required: Ferris AM Generator (or equivalent); VTVM (AC); and Oscilloscope.

2.    Connect scope and meter to tuner's recorder output jack. Switch tuner Selector to AM. Connect output from signal generator (455 kc, 400 cps int. mod.) to pin 7, 6BE6 through a .047 coupling capacitor. Connect one each 33k resistors across primary and secondary of both IF trans. Adjust primary and secondary of both IF transformers (cans) (455 D) for maximum output as indicated on VTVM. Input signal should be just great enough for good signal on scope. Remove 33k resistors.

3.    With 47-ohm load across output leads of signal generator, loop generator output leads around loop antenna. Set the tuner and generator to 530 kc with output from generator sufficient for output indication from tuner; adjust AM osc. coil for maximum output. Switch to 1630 kc and adjust oscillator trimmer for maximum output. Repeat until no noticeable improvement can be made.

4.     At 600kc., (using low input) adjust AM Mixer and AM Antenna (cans) coils (top slugs only) for maximum output. (Editor's Note: Can positions from front to rear are: AM Antenna can, AM Mixer can, AM IF Can, AM Whistler Filter Can (positioned out of IF lineup), and AM Detector.)

5.     Switch to 1400 kc. peak AM Mixer and AM Antenna trimmers for maximum output.

6.     At 20 uv input, output should be from .8 to 1.5 v With modulation off at 600 kc., output should drop -15 bB at 1600 kc. -20 dB. With 5k microvolts input, turn modulation off. Output should drop -35 dB at both ends of AM band."

Editor's Note: The typical (Type 330) Scott AM tuner section uses a four-tube/five-can lineup consisting of: a 6BA6 for the AM RF amplifier; a 6BE6 for the AM converter; a 6AU6 for the AM IF amplifier; and a 6AL5 used as the combined AM audio detector and AGC. All Scott AM tuners included a 10 kHz whistle filter. A 12AU7 was used for audio (line-level) amplification of the AM signal. Rectification varied depending on the model. Three AM sensitivity/bandwidth positions were provided on the Type 330: Wide-Range (highest audio bandwidth), Normal, and Distant.

 

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