Hi,
this is my first post, so please bear with me, if this is far from perfect.
I am currently attempting repair of a PA Amplifier MWL-5E/200 and at the same time try to
at least half-way understand the background of the design.
The Amp has several pre-amplifiers / equalizers for microphone inputs, a main amplifier section (where the magic smoke has escaped)
and an output transformer to allow connection of 50V / 70V / 100V transformer based speakers or normal 4 ohm / 8 Ohm speakers.
As no schematic has been available, I read back most of the circuit of the main amplifier board.
(I am still working on some part of the protection circuits driving the output relay.)
1.) Repair attempt
I cleaned off the burn marks, checked most components and marked up in the schematics what seems ok and what is definitely broken.
Please check / comment attached:Schematic_Amp_Final_Stage_defects.pdf
Schematic_Amp_Final_Stage_defects.pdf (1839.18 kB - downloaded 83 times.)
Schematic_Amp_Input_Stage_defects.pdf
Schematic_Amp_Input_Stage_defects.pdf (102.46 kB - downloaded 65 times.)
PCB.pdf
PCB.pdf (1626.39 kB - downloaded 53 times.)
I’d like to have your help / comment regarding the following:
- What is likely to be the cause of the damage?
What could have been the starting point?
Prior to the damage, it has been reported, that the RCD has been set off when the Amp has been switched on. However there are some outdoor circuits on the same RCD and the behaviour could not be reproduced….
Later the glass tube fuse of the Amp has blown and been replaced by some user with a higher rated one leading to black smoke….
- Which components would you exchange even though they seem ok?
I ordered replacements for everything broken and the components directly next to the burning marks.
- What other parts need checking? Could it be that the (seemingly) intact input section drove the
rear part into desaster?
thanks a lot for all help.
If any info is missing, please let me know.
Johannes