Motorboating on Zenith 6S632

I have recapped and replaced some resistors on my Zenith 6S632.   Also completely rewired the tone controls.   The radio works great with one exception.   It motorboats when the Alto tone control panel button is pushed in and only when the radio/tubes are first warming up.

I have checked tube shields and move the tone control panel away from the tubes.

Please correct me if I am wrong but pushing in the Alto button places a short across C13 and basically removes it from the circuit.   I have a replacement 7B6 detector-amp tube on order since the tone circuit connects to the grid of that tube.

Any thoughts on what to check would be appreciated.

Update:   Replacing the 7B6 tube detector-amp was no help.   Did find the RF tube very weak.  Only 500 micromhos instead of 3000.   The IF tube also was about 1/2 of the spec number.  Both on order.

Still wondering what the Alto tone button has to do with this problem....

Update:   Found the problem.   The negative side of the 5mfd C9 was completely mis-wired.   Amazingly I did not fry something.   The bass and alto tone controls were reversed as well.   No more motorboating with the Alto tone control selected or bass tone control.    Radio plays great!  :-)

 

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Da der Fehler am 04.Sep.2013 gefunden wurde, denke ich, kann das Fragezeichen entfernt werden.

Gruß

Hilmer Grunert

mein (dieser) Beitrag kann dann entfernt werden!

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