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Practical FM Alignment

Practical FM Alignment. Al Klase – N3FRQ 9 April 2013. Frequency Modulation. RF. AF. IF. LIM. IF. LIM. Typical FM Receiver Block Diagram. Frequency Changer. BP. BP. Local Oscillator. FM Detector Discriminator or Ratio Detector. AVC. IF Amplifier. Remote-cutoff Pentode

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Practical FM Alignment

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  1. Practical FM Alignment Al Klase – N3FRQ 9 April 2013

  2. Frequency Modulation

  3. RF AF IF LIM IF LIM Typical FM ReceiverBlock Diagram Frequency Changer BP BP Local Oscillator FM Detector Discriminator or Ratio Detector AVC

  4. IF Amplifier Remote-cutoff Pentode 6BA6 etc.

  5. Limiter Sharp-cutoff Pentode – 6AU6 etc.

  6. AVC Detector for FM

  7. Foster-Seely DiscriminatorFM Detector

  8. Foster-Seely DiscriminatorFM Detector

  9. So, What’s the Problem? • Varying circuit topologies for FM detectors cause bewilderment. • Each radio seems to have a unique alignment procedure. • Factory: Usually sweep generator and oscilloscope. • Repair Shop: Simple test equipment. • Vintage service-grade test equipment had sever limitations. • Can you say CLUSTER SHTUP?

  10. Foster-Seely Discriminators AF OUT Generic 19T8 19T8 SCHEMATICERROR! AF OUT Emerson Zenith AF OUT

  11. Ratio Detector AF OUT

  12. RCA 6-RF-9 AF OUT V-REF

  13. Improvise, Adapt and Overcome • Secure service documents (Google is your friend.) • Electrically restore the set • “Don’t even try to tune junk.” • Read the manufacturer’s FM alignment instructions. • Identify alignment frequencies • Identify test points and adjustments • Pay attention to this presentation!

  14. Zenith H723Z2

  15. “Alignment” Instructions

  16. Adjustment Locations

  17. Locate the Test Point and Adjustments INJECT 10.7 MHz HERE

  18. Locate the Test Points and Adjustments X DISC SECONDARY B LIMITER GRID A AUDIO OUT C

  19. On the Bench

  20. Classic Sweep Alignment AMPLITUDE VERSUS FREQUENCY DISPLAY AVC OUT RADIO UNDER TEST H V DET. OUT SWEEP GENERATOR SWEEP OUT RF OUT

  21. My Equipment • HP 141-T Spectrum Analyzer • HP 8443 Tracking Generator • 15 Grand in 1980 dollars • Took 3 days to get it working properly

  22. Sweep the IF • 10.2 – 11.2 MHz swept output from tracking generator to radio mixer grid. • Sawtooth from the spectrum analyzer to the scope horizontal input. • Radio limiter grid circuit to scope vertical input. • Display show approximately 75 Khz per division. VIDEO NEXT

  23. Discriminator Alignment • Scope to discriminator output VIDEO NEXT

  24. On-The-Air Test • Let’s try it! VIDEO NEXT

  25. AlignmentUsing Reasonable Test Equipment • Signal Generator • Accurate 10.7 MHz CW with output attenuator • DC-blocking capacitor in test lead • Oscilloscope • DC coupled • Bandwidth not critical • Hi-impedance DC meter • VTVM: HP-410, RCA VoltOhmyst, etc. (1-meg resistor in probe) • DMM: bar-graph is a plus

  26. Modest but EffectiveTest Equipment

  27. Universal Simple FM Alignment • Align the IF amplifiers • Inject CW signal on mixer grid (Signal Generator) • Monitor limiter-grid voltage (VTVM or Scope) • Tune for maximum • Align the FM Detector (Off-Air Signal) • Tune secondary to center frequency • Tune primary for max. output and min. distortion • Align the Front End (Off-Air Signal)

  28. IF Alignment Alignment • Signal generator at IF frequency to mixer grid. • VTVM/scope on limiter grid – S-meter VIDEO NEXT

  29. Discriminator Alignment • Scope on DC output of the discriminator. • VTVM on limiter grid – S-meter • Disconnect signal generator • Tune to strong station. VIDEO NEXT

  30. RF Alignment • Use the signal-strength indicator for RF alignment. • Follow published instructions. • Frequencies • Adjustment locations • “Trim high – Pad low” • Use a TV balun when connecting a signal generator to balanced (300-ohm) antenna terminals. • Use a digital receiver to identify station for on-the-air RF alignment.

  31. Multiplex Alignment • Use Oscilloscope • Peak 19 and 38 KHz circuits per manufacturers instructions.

  32. In Review • A signal-strength indicator is crucial. • VTVM, Scope, or DMM • Connected to limiter grid circuit • Align IF w/ accurate signal generator • Use oscilloscope to setup the discriminator. • Connect to AF output before the decoupling cap. • Use a strong local station as signal source. • It’s possible to make this adjustment by ear if the radio is accurately tuned to maximum signal. • Center the disc. secondary adjustment midway between the points of noticeable distortion. • Adjust the disc. primary for maximum clean output. • RF alignment: sig-gen or OTA w/reference receiver

  33. RF AF IF LIM IF LIM Typical FM ReceiverBlock Diagram BP BP Local Oscillator

  34. RCA 6-RF-9 AF OUT V-REF

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