A
ant
Guest
hi all im a bit of a newbie, any help here would be greatly
appreciated.
i have old faithful yamaha emx 200 (bout 15 yrs old) powered mixer
which stopped closing its speaker protection relay (at power up) a few
days ago. to do a visual inspection of the solder side of the pcb i
had to remove one bank of output transistors from the heatsink.
resoldered a suspect looking joint and (big mistake?) thinking under
no load the transistors would not dissapate too much heat, briefly
powered up the unit without mounting them on the heatsink. maybe 30
secs later one of the 5w .47 ohm resistors goes red hot, one o/p
transistor smokes and the ac fuse blows... oops
closer inspection has revealed...
1) the pcb has pads and tracks where it screws to the h'sinks that may
provide ground, which i may have isolated by not screwing down the
pcb/ op trannies.
2) isolating the output stage power supply rails (B) on the red hot
and smoky channel stops the ac fuse blowing
3) the voltage at the bias test points (off 5w .47 ohm resistors) is
about 70v on the channel that didnt blow (the relay takes its sense
from here, and i think its meant to be about 0v)
4) isolating the output stage power supply rails (B) on the not blown
channel won't open the protection relay or bring the test point
voltage back to 0v, relay closure and 0v @ tp can only be achieved by
isolating supply rails (B2) which are about +/- 90v open circuit and
seem to power the driver stage electronics on the main power amp board
one thing id like to know is if i remove all of the op transistors, is
it safe to power up the unit?
i have schematics that i could scan and post on my website if needed.
(will yamaha care if i do that?)
ok many thanks in advance for any help or advice at all,
ant
appreciated.
i have old faithful yamaha emx 200 (bout 15 yrs old) powered mixer
which stopped closing its speaker protection relay (at power up) a few
days ago. to do a visual inspection of the solder side of the pcb i
had to remove one bank of output transistors from the heatsink.
resoldered a suspect looking joint and (big mistake?) thinking under
no load the transistors would not dissapate too much heat, briefly
powered up the unit without mounting them on the heatsink. maybe 30
secs later one of the 5w .47 ohm resistors goes red hot, one o/p
transistor smokes and the ac fuse blows... oops
closer inspection has revealed...
1) the pcb has pads and tracks where it screws to the h'sinks that may
provide ground, which i may have isolated by not screwing down the
pcb/ op trannies.
2) isolating the output stage power supply rails (B) on the red hot
and smoky channel stops the ac fuse blowing
3) the voltage at the bias test points (off 5w .47 ohm resistors) is
about 70v on the channel that didnt blow (the relay takes its sense
from here, and i think its meant to be about 0v)
4) isolating the output stage power supply rails (B) on the not blown
channel won't open the protection relay or bring the test point
voltage back to 0v, relay closure and 0v @ tp can only be achieved by
isolating supply rails (B2) which are about +/- 90v open circuit and
seem to power the driver stage electronics on the main power amp board
one thing id like to know is if i remove all of the op transistors, is
it safe to power up the unit?
i have schematics that i could scan and post on my website if needed.
(will yamaha care if i do that?)
ok many thanks in advance for any help or advice at all,
ant