[Boatanchors] National RAO-2 AVC problem

Wally Gibbons wally.gibbons at gmail.com
Wed May 27 17:42:17 EDT 2015

I'd like to start a thread which discusses the AVC in the National RAO2
receiver. I've got one (built by Wells Gardner) that I'm about finished
overhauling. It's very sensitive, dial calibration is perfect. A great
receiver except for one little problem. The audio is affected by modulation
peaks on the RF carrier. Yes, on an AM signal it should be to a certain
extent, but received audio causes big changes in the AVC voltage. Looking
at it on the scope I see up to 4 volts of change on the AVC line from audio
peaks. I've checked all the AVC 500k resistors. Checked all the .01uf
bypass capacitors, checked all the 6K7 tubes (and replaced a few weak
ones), and the 6C8G AVC detector and 6F8G detector too. The .1uf bypass in
the AVC rectifier circucit is also good. All these capacitors have been
replaced, and checked for correct value. The AVC voltage drops quickly on
audio peaks, and then you can see the discharge of the .1uf capacitor
through the 500K resistor until the next audio peak. It's very annoying.
S-meter bounces around too, as it should.

I've tried adding additional capacitance to the AVC line. As much as 2 uf.
It helps but the AVC rectifier can discharge it much faster than the 500K
resistor can charge it?

Any RAO2 owners out there who have seen this in their receivers? Any
suggestions as to a potential fault which is causing this?

My intent is to get this going as an 80 meter AM station with a Harvey
Wells Bandmaster transmitter, sure would like to use the old girl on the
air....but that pumping audio is pretty objectionable....

Wally Gibbons, AF7FH
North Logan, Utah

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