This was very difficult to isolate...Checked all the components on the phono board and replaced several resistors, the diodes, some of the transistors, and the tantalum caps -- The only things that had some spec issues were a couple of the tantalum caps (higher esr), and that might've impacted the noise floor but I don't really see how it could cause the hum...Mostly just CYA because this is so painful to keep opening up. I checked all the grounds and rebiased the phono board, but I didn't find any specific issues. My test equipment showed input and output as clean, but it still hummed when hooked up to my test amp; out of desperation, I isolated the main outputs (not the phono stage inputs) because the hum was on both low and high level signal inputs. I then retested with the amp, and the hum was gone from the high level inputs but still heard on the phono. Ok, now officially puzzled, I changed the phono cable and hum is gone...Looks like a combination of ground loop, in my setup, between the pre-amp and amp, combined with some cable issue -- Cable issue could've been something to do with longer cable initially used, and routing that took it near my power conditioner. I do notice the noise floor if I crank this up without any input but not a hum...At normal volumes, the Thoebe output is clean, at very high levels with a quiet source (and inefficient speakers), there may be some background noise floor but you really have to listen for it. Bottom-line, this is a 40+yr old item, and short of a total rebuild, we have to be real with our expectations.
