Model 16B Squealing or Motorboating

Model 16B Squealing or Motorboating

Double check that the power supply electrolytics that have un-grounded negative leads (insulated fiber sleeve around can) have not been accidentally grounded to the chassis through their cases, or inadvertantly mis-wired to ground beneath the chassis. Also, some sets may be sensitive to electrolytic values. It’s usually okay to put in larger filter caps, but there may be some circuits where the extra capacitance adds unwanted low frequency gain that may push the audio circuit into oscillation.

Check that the audio output tubes (type 42’s) are at least roughly balanced in gain. If one tube is much stronger than the other, that will induce uneven currents in the power supply. Check the signal going to the grids of the 42’s. The two signals should be very close in amplitude at both grids.

Make sure there is proper plate voltage at each output tube as well. An open circuit in one half of the primary winding of the output transformer will unbalance things too.

By George R. Gonzalez (11/96, R+P message thread exerpt.)