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John

The decoupling cap on pin-6 of the LM386 would probably not be the culprit causing a short oscillation during the change over from Tx to Rx, as originally described.?

Take a good look at the Microphone Amplifier.? It's output is coupled directly to the input of the Rx audio section via the two 0.1 mfd capacitors.? If there is any residual voltage left on the Mike amp when the Rx audio section is turned on, a feedback path can be established from Mike to Mike Amp to Rx AF preamp to LM386 to Speaker...and back to the mike again.?

Now, how can there be a residual voltage on the mike amp when the transceiver has just switched from Tx to Rx mode?? Look at the Microphone Amplifier decoupling.? This 50 mfd capacitor is isolated from the rest of the Tx circuitry by a 220 ohm resistor.? That may be allowing some residual voltage to remain on the Mike Amp for a few milliseconds after Tx to Rx switchover.

A quick test to see if this is your problem might be to disconnect the speaker and use a set of headphones.? If the problem goes away, then it could have been caused by the scenario outlined above.? Using headphones should remove the acoustic feedback path, thus killing the oscillation.

Arv
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On Thu, 2004-08-26 at 15:29, market10gardener wrote: