[Hallicrafters] SX-42 BFO (Alignment saga)

[Rich Soennichsen]

Thanks to all for the great help with my ongoing SX-42 adventures.
The adventures are prolonged do to localized QRM (1 and 6 yr old
girls....)

I decided to try to fix the BFO before continuing.  I wasted some time
trying to find the oscillation on the plate of the BFO tube (7A4).
After looking at the schematic I noticed that the plate is not coupled
to anything and only carries the 150VDC.  I did see the oscillation on
the grid AND on the cathode with a DC offset.  (After finding an
intermittent on the socket)  The schematic shows the cathode coupled
to the AM detector so THAT must be the output.

There is a 7pF coupling cap; I could not see the signal on the
detector side so I suspected the cap and indeed, replacing it fixed
the problem.  It was a 22pF cap but I could still then adjust it
normally then.

I will order some low value caps so I can get the right value in there
and then continue the alignment, QRM permitting.

Thanks again guys.

Rich

W7SOE