Just a brief update: I never found what model the chassis was from – close to a Model 911 (as the cabinet would indicate), but not quite… having one fewer tube socket, so maybe a Model 811, though the layout still does not match (nor did any American Bosch models). Anyway, it was in such a state I removed all the old components, except power transformer, RF coils/tuning gang and tube sockets, almost all the old wiring, and rebuilt the circuit based mainly on the Model 811 (same tube line-up), but with some changes per the Model 911 circuit relating to the volume control/tone compensation circuit to best suit components to hand. I had originally left the IF transformers on the chassis, but these proved to cause instability that could not be tamed – changing these out for another pair from a scrap chassis (unknown model) cured that issue. There were some challenges – whoever had worked on the chassis previously had rewired the local oscillator and mixer stages in the RF sub-chassis. When I reverted it back to as per the schematic, I realized that there is an error on both the 811 and 911 schematics – two wires connecting to the local oscillator band change switch have been transposed on the schematic – wiring that was results in a non-functional oscillator (I know, as I built it that way before I realized there was an error!). So far I have the chassis roughly aligned on the Broadcast band, though it is working on the Police and Short Wave bands also. Currently doing some further checks, tidying-up the chassis and then will fully re-align. It sounds good – even with the speaker out of the cabinet – and appears to have good sensitivity. Some photos of the re-built chassis attached.
