Dear Peter, LF Group,
is the Gereral Radio 1656 covering our LF-Band?
I am not certain, but I believe this is an audio-frequency bridge - if
it does work at 136k, it is probably with reduced specification.
May be that any impedance bridge using as RF detector some selective
meter (pegelmesser) can help you for this measure.
Selective level meters work very nicely as RF bridge detectors - Since
it is only null detection that is needed, a receiver with "S" meter or
output meter would also work fine. All you have to do is make sure there
is enough attenuation between the bridge and RX/level meter so that the
local QRM does not overload the front end.
Cheers, Jim Moritz
73 de M0BMU