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Some Z3801A receivers exhibit loud annoying high pitched sounds that come from the outer oven controller. Located on the power supply circuit board, this circuitry sometimes has a tendency to oscillate. This problem does not cause any receiver malfunctions.
However the noise is definitely noticeable and can be substantially reduced by the addition of a simple component.
Thanks to Kaoru Suzuki /ex- 7M3LGB of Japan for this information.

There were two possibilities regarding the low (audible) frequency annoying tone generation by the circuits around U104 (LT1270A). One was sub-harmonic caused by insufficient inductance of L102. The other was oscillation caused by inadequate damping.

The wave form was recorded from TP103 (probe GND was connected to TP5). The wave form implied that inadequate damping might be the cause.

Waveform before modification

An additional 0.01 uF ceramic capacitor was connected between pin 1 and pin 3 of U104 as shown in the photo below.

Added .01 uF capacitor

Then the low frequency oscillation was stopped as shown below. Of course, the audible annoying tone was also stopped.

Waveform after modification

I hope that this information might be helpful for the other Z3801A users bothered with the same tone.

Silence is Golden

Just wanted to send you a very happy "thank you!" for posting the article on correcting the Z3801a oven control whine.

My receiver is now happily silent, whereas before it was guaranteed to give me a splitting headache after about 5 minutes of operation. Now I can actually let it sit around and stabilize for a while.

Thanks again,

Dave.

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