My pre-WW2 HRO's have a hole in the chassis just behind the panel and next
to the main tuning capacator. You can access the trimmer caps with the
coils plugged in. I'm pretty sure the later models also have this trimmer
access. I use a long 1/4" nutdriver. It doesn't seem to detune the coils,
but be careful. Some of those adjusting screws carry the full B+. I have
been knocked on my bum while aligning HRO's.
The low-end adjustment requires opening up the coil to move a one-turn loop
of wire, which changes the inductance of the coil.
The only way I could think to make this adjustment other than what you
mentioned, would be to build up a jig where you could mount the coil with
the cover removed and gain access to the coil, while it is connected to an
oscillator tube or an rf amplier tube with an external signal generator and
output meter, to indicate when the rf coil or oscillator coil is properly
aligned. Even that would be approximate, because the adjustment may be off
a little when the coil is re-mounted into the shield and the whole assembly
is inserted into the receiver. For the alignmentt jig, perhaps a spare
shield could be modified by cutting open the end to allow access to the coil
with at least most of the shielding in place.
This would be a good project for National affectionados who regularly do a
lot of HRO alignment.
Don k4kyv
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