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Wednesday, March 12, 2014

Direct Conversion Receivers, AF Transformers, and Motorboating

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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  1. Except it changes with the volume control, I'd wonder if it's audio feedback around the mixer transistor.

    The RC circuit wouldn't radiate, but the AF transformer might inductively couple to the antenna, thus providing a feedback path.

    I once played with an inductive loop around my room, feeding it with an audio amplifier. I could pick up the signal with an amplifier and inductive pickup, but if I turned up the amplifier feeding the loop, I'd get oscillation.

    What seemed to be happening was the loop signal was getting into my turntable cartridge, thus providing a feedback loop.

    It is somewhat possible the af transformer is doing the same.

    Michael VE2BVW

  2. In addition to Michael's comments on the transformer radiating, look at the power supply bus filtering and at the capacitor locations. The bus may soft or it may not be filtered close enough to the transformer and output transistor. As for shielding, you could be inducing currents into your ground plane so don't overlook that possible path.

    I can get some transformer lamination steel if you want to try to make a shield. Look for an email from me on this topic.


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