Thanks for the replies. I was wrong regarding TO-99 package for the MJE3055, it's actually a TO-220, which is smaller than the original part, but the replacement part number is MJE3055TG. Anyway, this is the part I replaced it with: MJE3055TG ON Semiconductor | Mouser
As of now, I've narrowed it down to definitely the driver amp or the line input stage.
When I use my hafler amp to drive the transducer the reverb works fine (the voice coil was blown on the driver, which I successfully replaced)
I don't have a scope, so. . .
(my rudimentary method of replacing things where I 'think' might be the problem hasn't gotten me anywhere.) But fwiw, I replaced A1 on the schematic, which is the uA759HC in the driver stage with the same part.
And, I've also replaced both LF351's chips in the line input stage (w/ the same part)
When I use the Echoplate's driver amp, the thing that's different now is that there is no thumping sound, instead I'm getting nothing, just the noise of the pickups on the output. No sound is going to the transducer.
Couple more questions. Radardoug, I have a bunch of TLO71's sitting in my parts box. Are we talking about replacing the LF351's?
Jim, which chips are you talking about specifically if you don't mind sharing?
And, also, anybody have a link to an appropriate driver amp/amp kit for that little transducer that's clean, cheap, and doesn't overload, if I end up needing to go that route? (not gonna spend a whole lot more time screwing with this thing) Unbalanced input is fine, as I have a balancing transformer in front of it already.
(Edit: Okay, I checked voltage at the center node between the transistors and I'm getting about 6.0VDC. So, yeah, that theer seems to be a problem)
Is the 5K trimmer at the output of the line amp stage the way to adjust the voltage to 0VDC at this spot?