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Thursday, September 21, 2017

Herring Aid Error: C14


Yea, take a look at C14 in the AF amp.   At 1000 Hz 10uF is about 16 ohms.  That would put a serious dent in the AF gain.  And indeed, when I went back to my Herring Aid 5 (38 years from start to finish) and pulled C14 out, there was a dramatic increase in AF gain.  

Mistakes happen, but I wonder if anyone else spotted and reported this one.   Could someone with access to the QST archives check to see if an Erratum was ever published?   The project also appears in the 1977 ARRL book "Understanding Amateur Radio" with the problematic C14.  My guess is that it was just an error in the value of the cap.  

3 comments:

  1. A quick search of QST archives finds no erratum or mention of the herring aid other than a quick snippet of someone putting it in a lunch box. Norcal has the same error in their rendition(update) of the same radio. Surprised no one found the issue after all these years.

    TF

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  2. Not just the value, Bill - also the symbol, emphasizing the error! A 10nF (my guess) would not be an electrolytic.

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  3. And more to my guess: Xc = 1 / 2 pi f C, so X of 10nF at that rate would be (about) 16k, giving a useful treble roll-off - and possibly adding to stability ... Justathort ...
    73

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