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Old 27th Dec 2018, 2:39 pm   #1
Ian - G4JQT
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Default Yet another 'Pantry' tx

Hi all!

I got very board recently and drew out this circuit which I've been using for decades.

It's much simpler than it looks. The main circuit is U2 Q1, Q2 and Q3.

All the following are just embellishments:
  • U1 is tone control (mostly to increase treble modulation)
  • Q4 is a generates a low-distortion 400 Hz sine wave to help find on the transmission and adjust antenna tuning for best signal
  • U3/Q5 indicate over-modulation
  • Q6 drives a modulation Vu meter

It's appropriate now that I investigated a modern programmable and much cheaper substitute for the crystal oscillator.

Medium wave is going to get louder with interference, and quieter with stations. I think it's time to think ahead...

Season's greetings,

Ian
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Old 27th Dec 2018, 2:52 pm   #2
TonyDuell
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Default Re: Yet another 'Pantry' tx

Why is there no feedback on U1 (or U3, but the latter could be intended to saturate)?
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Old 27th Dec 2018, 3:02 pm   #3
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Default Re: Yet another 'Pantry' tx

If you upload the diagram as a PDF document, it won't be resized so will be easier to read.
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Old 27th Dec 2018, 3:05 pm   #4
Ian - G4JQT
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Default Re: Yet another 'Pantry' tx

Quote:
Originally Posted by TonyDuell View Post
Why is there no feedback on U1 (or U3, but the latter could be intended to saturate)?
Thanks for your thoughts. Unless I'm mistaken U1's feedback is via the tone control network, and U3 is just a voltage comparator - it conducts pulses to Q5 depending upon the setting of VR5. (Although it's quite possible I have miss copied from my old hand drawings.)

I shamelessly lifted those two circuits from '50 circuits using IC 3130'. Just found an online copy here: https://www.americanradiohistory.com...-ic-ca3130.pdf

Ian
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Old 27th Dec 2018, 3:09 pm   #5
TonyDuell
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Default Re: Yet another 'Pantry' tx

Is there supposed to be a connection from the output of U1 (pin 6) to the left hand end of C5/R3 in the diagram? It's not visible (but may have been lost in the re-sizing).
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Old 27th Dec 2018, 3:27 pm   #6
Ian - G4JQT
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Default Re: Yet another 'Pantry' tx

Think I've pdf'd it!
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Old 27th Dec 2018, 3:29 pm   #7
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Default Re: Yet another 'Pantry' tx

Quote:
Originally Posted by TonyDuell View Post
Is there supposed to be a connection from the output of U1 (pin 6) to the left hand end of C5/R3 in the diagram? It's not visible (but may have been lost in the re-sizing).
Yes, You're right! Thanks - my re-drawing error.
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Old 27th Dec 2018, 4:33 pm   #8
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Default Re: Yet another 'Pantry' tx

I missed the editing time window, so 've corrected my drawing error which Tony noticed. (Thanks Tony.)

New .pdf attached.
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