klitzy13
New Member
This is my first post on the forums so go easy on me... and I'm gonn start off with a novel of a post but I want to get all the info out there in hopes that someone can give me some advice on what I'm finding on this Mc...
So I'm working on a MA6100 for a friend and I am at a dend end. The two issues I can't seem to figure out are that the left channel volume is lower than the right and the comp switch (pres/flat/loud) has no effect on the right channel. Both issues change channels if I connect the right channel preamp output to the left channel power amp input and the left channel preamp output to the right channel power amp input. The issues do not change channels if I swap preamp or power output pc boards. I also get a pretty wicked hum in both channels when I use the LF switch.
Some background.... When I got the amp the left channel didn't work at all. I believe someone shorted the positive and negative speaker terminals together taking out one of the power transistors, Q417. After replacing that and the 5A fuse that is off of the emitter of the transistor that blew because Q417 was shorted I still did not get any output on the left channel except for when the volume was turned to the max. (I replaced all 4 power transistors Q415-418). After some investigation I determined the volume pot was bad. I then replaced that and had output on both channels, except the volume was lower on the left and the right had some decent distortion. I then completely recapped the whole unit, replacing all electrolytics. I still had the same two issues so I started pulling transistors and testing them. I replaced Q113, Q114, Q115 & Q116 on the preamp boards, Q115 was shorted out and I replaced the rest as they are all the same part and from some forum reading seem to be unreliable. Still the same issues presisted. I then started troubleshooting the output transistors I just replaced to again verify the voltages were correct when I realized the diode that went from the collector to the emitter on Q417 was blown open. So I replaced D419, D410, D411 & D412. The issue was still there, so I went back to the power output PC boards and took Q408, Q410, Q409 & Q411 off and tested them. Realizing three of the four were shorted so my tester thought they were resistors I replaced them based on what I found in the forums here. This resolved the distortion issue on the right channel but the volume issue remained. And that brings me to where I am now.
I should also mention that I thoroughly went through the connectors for the boards, cleanign with DeoxIT. I also reflowed almost every solder joint on the unit and went through the grounds to make sure there were not issues there as that was another thing I found in the forums. I also verified the 9300uF caps were good. I know the multi-cap is out of range a little and I believe it is causing a hum on startup and shutdown but haven't gotten around to installing the individual caps I have to replace it yet. The voltages off of that cap are off from what it shows in the service manual, for instance at the junction of C312A & R313 I have 90V and the junction of C312B and R313 I have 78V. I also have around 80V (I can't remember exactly and it's getting late
) at the junction of R314 & C312C.
Well, that's my story, anyone have any suggestions? Any help will be greatly appreciated.
So I'm working on a MA6100 for a friend and I am at a dend end. The two issues I can't seem to figure out are that the left channel volume is lower than the right and the comp switch (pres/flat/loud) has no effect on the right channel. Both issues change channels if I connect the right channel preamp output to the left channel power amp input and the left channel preamp output to the right channel power amp input. The issues do not change channels if I swap preamp or power output pc boards. I also get a pretty wicked hum in both channels when I use the LF switch.
Some background.... When I got the amp the left channel didn't work at all. I believe someone shorted the positive and negative speaker terminals together taking out one of the power transistors, Q417. After replacing that and the 5A fuse that is off of the emitter of the transistor that blew because Q417 was shorted I still did not get any output on the left channel except for when the volume was turned to the max. (I replaced all 4 power transistors Q415-418). After some investigation I determined the volume pot was bad. I then replaced that and had output on both channels, except the volume was lower on the left and the right had some decent distortion. I then completely recapped the whole unit, replacing all electrolytics. I still had the same two issues so I started pulling transistors and testing them. I replaced Q113, Q114, Q115 & Q116 on the preamp boards, Q115 was shorted out and I replaced the rest as they are all the same part and from some forum reading seem to be unreliable. Still the same issues presisted. I then started troubleshooting the output transistors I just replaced to again verify the voltages were correct when I realized the diode that went from the collector to the emitter on Q417 was blown open. So I replaced D419, D410, D411 & D412. The issue was still there, so I went back to the power output PC boards and took Q408, Q410, Q409 & Q411 off and tested them. Realizing three of the four were shorted so my tester thought they were resistors I replaced them based on what I found in the forums here. This resolved the distortion issue on the right channel but the volume issue remained. And that brings me to where I am now.
I should also mention that I thoroughly went through the connectors for the boards, cleanign with DeoxIT. I also reflowed almost every solder joint on the unit and went through the grounds to make sure there were not issues there as that was another thing I found in the forums. I also verified the 9300uF caps were good. I know the multi-cap is out of range a little and I believe it is causing a hum on startup and shutdown but haven't gotten around to installing the individual caps I have to replace it yet. The voltages off of that cap are off from what it shows in the service manual, for instance at the junction of C312A & R313 I have 90V and the junction of C312B and R313 I have 78V. I also have around 80V (I can't remember exactly and it's getting late
) at the junction of R314 & C312C.Well, that's my story, anyone have any suggestions? Any help will be greatly appreciated.