Last month had a bad fight with this amplifier. Has been in service, not repaired.After figuring out that without schematic will be shooting in the dark... also because of curiosity for their "no global feedback" design, decided to draw it off from the PCB. And there was no surprise - it have global feedback. Also distorting capacitors, wired like 69 on the input... they are there for nothing different than distortion.
Bad construction, difficult for repair - should remove front panel, knobs, input selector, power LED... after that the PCB can be slightly turned aside.
First problem - caused by previous service - was broken the power input screw terminals - the wires from transformers are single core (directly from the transformer winding) and are not flexible. It's very easy to break the terminals with them.
Second - 0,7V offset in the output. Was suspected transistor or diode... all measured good on the PCB. Started off-PCB check - gain, leakage... all OK. Moved to the other channel and others channel transistors on this - the problem remains. Checked all resistors... still not working. At that moment started to draw the schematic
After all... few hours later, found that C6* is shorted and can measure 100 ohms on it 
As we can see, there IS global feedback T8-T9 what are doing there? Soft clipping, current limit, active biasing?
Bad construction, difficult for repair - should remove front panel, knobs, input selector, power LED... after that the PCB can be slightly turned aside.
First problem - caused by previous service - was broken the power input screw terminals - the wires from transformers are single core (directly from the transformer winding) and are not flexible. It's very easy to break the terminals with them.
Second - 0,7V offset in the output. Was suspected transistor or diode... all measured good on the PCB. Started off-PCB check - gain, leakage... all OK. Moved to the other channel and others channel transistors on this - the problem remains. Checked all resistors... still not working. At that moment started to draw the schematic
After all... few hours later, found that C6* is shorted and can measure 100 ohms on it As we can see, there IS global feedback
T8-T9 what are doing there? Soft clipping, current limit, active biasing?Attachments
> T8-T9 what are doing there?
V/I limiting. Standard on most tranny amps since 1967. Here drawn a little different than usual.
Leach Amp Protection Circuit
V/I limiting. Standard on most tranny amps since 1967. Here drawn a little different than usual.
Leach Amp Protection Circuit
Attachments
