[Catkins]

Originally Posted by mark pirate

Indeed, the set I am currently restoring (RGD 166 from 1939) has a 1,200 ohms.
If the speaker is working, there must be continuity on the field coil.

Curiously on the set I just restored, I found an exception to that.

After restoring it, and soak testing it on the bench, the sound would be as expected, but, occasionally the sound would drop to about half or quarter volume after a while, and sometimes it would be half/quarter volume on turning on.

This set (being a TRF with reaction) is quite fiddly to tune in, and it can drift after a while. So I initially assumed the sound drops were drift, and/or being unable to get the tuning and reaction "just right".

But, after a while it became clear it must be due to a fault somewhere. When in it's "low" sound mode it would also become unstable.

Tests showed the HT in "low" sound mode was about 30 volts too high, which was causing the output valve to become unstable (235 volts on anode with voltage drop across output transformer primary, normally it would be about 203 volts).

I traced the fault to a bad connection on the field coil. With the field coil completely disconnected the radio would continue to work, albeit in "low" sound mode.

The key to the "mystery" of how this was so is because the field coil in this set (GEC mains universal 3 of 1934) is not involved in smoothing the HT, and it is connected directly across the rectifier output.

So the disconnected field coil didn't cause lack of HT, in fact the reduced load meant it rose by about 30 volts.

But it still meant the loudspeaker was working without the field coil being energised, but at a reduced volume. I assume this was due to residual magnetism.