Doug,
Attached is a PDF of the Landwehr preamp "manual," though the BB copy will strip that off. As I mentioned, they do not give the component values, which was very typical of European products of that period. Landwehr is still is business, though they are no longer selling ham gear, and they may be able to provide some information. http://www.le-tronic.com/
You mentioned that it was switching between RX and TX in receive mode. TX mode is almost identical to unpowered, but there is a way to tell the difference. Hook up a ma meter in the 12 V line. In RX mode, it should draw about 100 ma. If it is in fact switching to TX mode, the draw should be much less, probably around 5-10 ma, as the TR relay is de-energized, and the preamp circuitry is (probably) unpowered. Only the switching electronics should be drawing. If the draw is 0, then you have a loose connection in the 12 volt line, most likely in the connector, which should be easy to locate and repair.
Hope this helps.
Alan WA4SCA