Bush VHF70 no AM

[PJL]

The oscillator is self-regulating by rectification of the signal on the grid. This pushes the grid -ve wrt the cathode, reducing the average anode current and hence higher anode voltage.

When not oscillating the symptoms should theoretically be, no -ve voltage on the grid and low anode voltage. The problem with this is in practice is the capacitance of the meter used to measure the voltage can be enough to stop oscillation.

In this case, as it is a VHF70, the most likely cause is the wavechange switch which are known to cause problems. I have a VHF70 here that works on AM and FM but not very well which I suspect will be the same cause. If you do a search you might be able to find some old threads with recommendations for rewiring the switch.

[McMurdo]

Finally found the real fault on this little set.

The AM oscillator coil was intermittently going open at the point where the winding is soldered to the spill on the top side of the former.

It's now survived a car journey and is playing on the kitchen worktop

[Billy T]

Originally Posted by Sideband

There should be a sizeable negative voltage on the grid and that will bring the anode volts down and the cathode volts up.

Surely that should read anode up and cathode down?

Negative grid voltage will bias the valve off, minimising anode/cathode current, so the anode voltage will rise and the cathode drop. In extremis with the anode supply open circuit, the cathode volts would read zero.

cheers

Billy