Direct Conversion Receivers, AF Transformers, and Motorboating

3:04 AM , , 0 Comments

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?  


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