I purchased a nice Yamaha M-70 from ebay, knowing it was broken. I'm a bit of a rookie at repairs but I'm very tech inclined and I've built some of my own audio gear.
The rifa cap that plagues these amps was exploded and the three 6800pf rifa caps next to it were all spider webbed (self healing?) The protection board connected to the psu board was covered in dried glue, some of which was black and crumbly like charcoal.
I replaced the exploded cap with an X2 panasonic, and the 6800pf caps with matching Y2 panasonics from the same series as the X2. There were two other little can caps that got replaced as well.
Now on the protection board I replaced three large cans, one of which looked like it was a 1mm from popping its lid, and all the resistors (glue corroded them) plus one zener diode. (apparently the stock resistors are carbon film, i used metal film 1% tolerance)
When I turn the amp on (cold start), the protection light comes on, and a few minutes (5-7 estimate) it shuts off and the amp works fine, music is incredibly clear and bass is heavy, nothing wrong with the channels.
Here's a couple of strange things, if I play music while the amp is warming up, you can hear sound after 5 or so minutes and then a few seconds after the protection light shuts off.
Now if I play music for a bit, and shut the amp off then back on, the relay clicks within a couple of seconds, as I'm assuming it would under normal circumstances.
TL;DR all capacitors are replaced on the power supply and protection circuit, corrosive glue has been 99% cleaned off. Protection relay has a lengthy delay before shutting off and resuming normal amp function.
The rifa cap that plagues these amps was exploded and the three 6800pf rifa caps next to it were all spider webbed (self healing?) The protection board connected to the psu board was covered in dried glue, some of which was black and crumbly like charcoal.
I replaced the exploded cap with an X2 panasonic, and the 6800pf caps with matching Y2 panasonics from the same series as the X2. There were two other little can caps that got replaced as well.
Now on the protection board I replaced three large cans, one of which looked like it was a 1mm from popping its lid, and all the resistors (glue corroded them) plus one zener diode. (apparently the stock resistors are carbon film, i used metal film 1% tolerance)
When I turn the amp on (cold start), the protection light comes on, and a few minutes (5-7 estimate) it shuts off and the amp works fine, music is incredibly clear and bass is heavy, nothing wrong with the channels.
Here's a couple of strange things, if I play music while the amp is warming up, you can hear sound after 5 or so minutes and then a few seconds after the protection light shuts off.
Now if I play music for a bit, and shut the amp off then back on, the relay clicks within a couple of seconds, as I'm assuming it would under normal circumstances.
TL;DR all capacitors are replaced on the power supply and protection circuit, corrosive glue has been 99% cleaned off. Protection relay has a lengthy delay before shutting off and resuming normal amp function.