Hi,
I recently acquired a boat with an old VDO depth finder, that didn't work. I connected a cheap handheld fish finder to the transducer (thru hull mounted), and got reliable readings, so obviously it is working. I also tried with a modern entry level lowrance unit, but it didn't work with VDO transducer.
In thread Lucky fish finder similar fish finder is partially reverse engineered, I'm going to try something similar.