I have an old Heath HD-1250, which is a solid state equivalent, and I have
never been convinced that it works well. Seems like the dips are vague,
unless coupling to the circuit is awfully tight.

BF-

How tight is tight?

I think my newer GDO is the HD-1250, but I haven't touched it in several years.
I also have the old GD-1B. As I recall, they do have to be coupled fairly
tightly. Once you find the dip, you should be able to back away some and get a
more "accurate" dip. Tight coupling seems to pull the oscillator's frequency
somewhat.

It is also possible to use them in an absorption mode, where energy from an
energized circuit will deflect the meter when it is tuned to the frequency.
Have you tried that mode?

73, Fred, K4DII