The Herring Aid 5 is a direct conversion receiver (scroll down for details). It is a minimalist design from 1976 using parts available at Radio Shack stores. One of the parts no longer carried by Radio Shack is a 10K-2K ohm audio transformer. Following NORCAL's 1998 design update I ordered an equivalent Mouser part (Xicon 42TU002-RC). I had been running the receiver with simple RC coupling instead of the transformer.
Yesterday the Mouser part arrived and I put it into the circuit. An increase in AF gain was immediately apparent, but the thing went into AF oscillation as soon as I turned up the AF gain.
I tried beefing up the AF decoupling. But I think the real problem is just the presence in the middle of the board of a rather large (1 inch x 1 inch) audio transformer. I moved it around a bit to get it away from the toroid of the preceding stage. This helped a bit, but it still breaks into oscillation if I turn up the AF gain.
Any suggestions? Or is this just part of the minimalist 1976 lifestyle?
In response to popular demand, "SolderSmoke -- Global Adventures in Wireless Electronics" is now available as an e-book for Amazon's Kindle.
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For the print version:
For shipping from a printer in the U.S. (probably better for N. American buyers) Click here: SolderSmoke USA Version
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The two versions are identical, except for a minor difference in the paper used. That's why the prices are a bit different.
Bill's OTHER Book (Warning: Not About Radio)
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