I bought this amplifier for my bass in http://www.thomann.de/es/ webpage. I choose refurbished item because i do not care and i save some money, but this amp malfunctioning from de beginning. It was not too much important because only affected the equalization: middle control not work. Because design of the eq, middle control is ressonant circuit in series with signal so, response is flat when circuit is here and not flat without it. response in a instrument amplifier is not as important as in a hifi amplifier therefore was not an important failure, but i ask thomann for schematics to fix it but nothing happens.. they asked me to send my amplifier.
Some weeks later my amp is here but, surprise!, with the same failure. So.. I began to examine it.
Shematic is very similar to Ampeg SVT3-pro, in this way i check with signal generator and oscilloscope and i finally found that problem is rotary switch. Is rare switch but didnt need to change it because i fix broken plastic part.
Amplifier is fixed!!