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.)
