klitzy13
New Member
I'm hoping one of the Mac experts here can help me with a MA5100 I am working on for a friend of mine. He was using it on Aux and the sound dropped out completely and he couldn't get it back.
I brought it home and put it on my bench, after cleaning the unit and reflowing a few solder joints I got the sound back on the Aux but while checking the other inputs on the unit I found an issue with the phono stage. I found some resistors and a capacitor that were bad and replaed those and everything sounded beautiful.... for about 3 minutes....
Then the right channel dropped out completely and produced a hum. I checked the power transistors and Q220 was shorted out. I replaced all four power transistors and powered the unit back up and the issue is still present.
The right channel output transistors get very hot to the point that the thermal cutout on that heatsink cuts power from the amp. Resistor R238 on the power output PCB is also quite hot compared to the left channels R237.
I have checked the all transistors on the power output board except for Q206 and Q210 so far along with all of the diodes except for D206 with all of them testing good. Also where D202 is shown in the service manual there are two diodes and both tested good. I have also tested Q212, Q214 & Q216 and all three tested ok.
I will post voltage measurements that I have taken tomorrow as well.
Any insight or suggestions?
I brought it home and put it on my bench, after cleaning the unit and reflowing a few solder joints I got the sound back on the Aux but while checking the other inputs on the unit I found an issue with the phono stage. I found some resistors and a capacitor that were bad and replaed those and everything sounded beautiful.... for about 3 minutes....
Then the right channel dropped out completely and produced a hum. I checked the power transistors and Q220 was shorted out. I replaced all four power transistors and powered the unit back up and the issue is still present.
The right channel output transistors get very hot to the point that the thermal cutout on that heatsink cuts power from the amp. Resistor R238 on the power output PCB is also quite hot compared to the left channels R237.
I have checked the all transistors on the power output board except for Q206 and Q210 so far along with all of the diodes except for D206 with all of them testing good. Also where D202 is shown in the service manual there are two diodes and both tested good. I have also tested Q212, Q214 & Q216 and all three tested ok.
I will post voltage measurements that I have taken tomorrow as well.
Any insight or suggestions?
