About Radio ... Part 14 Local oscillator for the AM frequencies by Andy Davies

In this fourteenth video 'About Radio' I show you the local oscillator for the AM frequencies and how the wave change switch selects the various components needed to change to operating frequency of both the receiving circuits and the local oscillator. If this helps you please let me know. Kind regards ... Andy gw0jxm Additional after posting I've had this comment posted on my vintage-radio.net account and I thought I'd post it here as it's very helpful, my thanks to Paula who signs as 'frsimen' Copy... Hello Andy, I don't have a YouTube account, so I'm not able to make a comment there. In your description of the aerial tuned circuits, you said that you weren't sure why there was a 100k resistor across the Long Wave winding. The reason it's there is to damp the Q factor of the long wave coil. Without the resistor, the Q would be 100 or more. With a Q of 100, using the trusty formula, Bandwidth = Resonant Frequency/Q, on 198kHz the bandwidth would be 1.98kHz, which in a double sideband system would effectively be only 990Hz for the modulation. Clearly that is going to spoil the attempts at giving a bandpass characteristic in the IF amplifier. The resistor will reduce the Q somewhat, widening the bandwidth to a more reasonable value. The unloaded coil Q would be the determined by the effective parallel resistance of the coil (from Q=R coil/wL) where w= 2 *PI * Frequency and L = coil inductance Thus R coil = Q*wL. The new Q is the effective coil resistance in parallel with the 100k resistor popped into the formula above. I don't think the resistor plays much part in the operation on FM. I must say that I have enjoyed your tutorial videos. They are very well presented indeed, and I'm sure that any beginner who looks at them would find them of benefit. Best wishes, Paula ...End of copy ~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~ In this series of 'About Radio' I want to show you a circuit diagram for a radio and talk you through what each component is there for. Here's the full link to 'About Radio ... Part 1. Removing the chassis.'    • About Radio ... part 1  Removing the chass...   Here's the full link to 'About Radio ... Part 2. Data sheet and DC components.'    • About Radio ... Part 2 Data sheet and DC c...   Here's the full link to 'About Radio ... Part 3. HT smoothing.'    • About Radio ... Part 3 HT smoothing by And...   HD Link to part 3    • About Radio ... Part 3 HT smoothing by And...   Here's the full link to 'About Radio ... Part 4. AC ripple on the HT DC.'    • About Radio ... Part 4  AC ripple on the H...   HD link to part 4    • About Radio ... Part 4  AC ripple on the H...   Here's the full link to 'About Radio ... Part 5. VHF input stage.'    • About Radio ... Part 5  VHF input stage by...   HD link to part 5    • About Radio ... Part 5  VHF input stage by...   Here's the full link to 'About Radio ... Part 6. Grid bias & tank circuits.'    • About Radio ... Part 6  Grid bias & tank c...   HD link to part 6    • About Radio ... Part 6  Grid bias & tank c...   Here's the full link to 'About Radio ... Part 7. A closer look at the tank circuit.'    • About Radio ... Part 7  A closer look at t...   HD link to part 7    • About Radio ... Part 7  A closer look at t...   Here's the full link to 'About Radio ... Part 8. Intermediate Frequency.'    • About Radio ... Part 8 Intermediate Freque...   HD link to part 8    • About Radio ... Part 8 Intermediate Freque...   Here's the full link to 'About Radio ... Part 9. Ratio Detector AVC feature'    • About Radio ... Part 9 Automatic Volume Co...   HD link to part 9    • About Radio ... Part 9 Automatic Volume Co...   Here's the full link to 'About Radio ... Part 10. Ratio Detector, AF Detection'    • About Radio ... Part 10 Ratio Detector, AF...   HD link to part 10    • About Radio ... Part 10 Ratio Detector, AF...   Here's the full link to 'About Radio ... Part 11. Cleaning the variable capacitor'    • About Radio ... Part 11  Cleaning the vari...   HD link to part 11    • About Radio ... Part 11  Cleaning the vari...   Here's the full link to 'About Radio ... Part 12. The wave change switch'    • About Radio ... Part 12. The Wave change s...   HD link to part 12    • About Radio ... Part 12. The Wave change s...   Here's the full Link to 'About Radio ... Part 13 inside the IF transformer'    • About Radio ... Part 13 Inside the IF tran...   HD link to part 13    • About Radio ... Part 13 Inside the IF tran...   In my videos, inevitably there will be things that I forget to mention and maybe some things I get wrong, please feel free to comment and put me right if I go wrong. It's some 40 odd years since I was involved with domestic radios, that was before I qualified as an electrician and subsequently moved on into high power high frequency electronic engineering. I'm not setting out to tell you how to repair radios, the fact that you've read this far probably means you're Interested in radio and I'm merely helping to confirm what you already know.