Quote:
Originally Posted by Boater Sam
Diode to pentode grid is probably the pathway that this set suffered from: it oscillated in isolation whenever there was HT.
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Could the HT+ rail be a little too high?
I've also had I.F. amplifiers go unstable if their tuning was incorrect, especially if that tuning is substantially 'off-tune'. The way to tackle that is to damp the primaries and secondaries of all the transformers with something like 47 kΩ resistors across every winding: then tune them for correct freq., then remove those Rs and then make
very slight adjustments to their tuning, as required.
The secondary winding of the last I.F.T sometimes has one wire to the detector diode, the other to the AGC diode. If those wires are transposed, instability can result.
A defective AGC line can cause similar trouble, since it is usual for an AGC voltage to be applied to the control grids of the I.F. valves.
Al.