I got this to work on from a friend. It is a 4 zone, 2 source whole house amp with keypads in each zone. The problem description was static on all sources in all zones. I surmised that the problem was a bad audio filtering cap. Upon cracking the cover I found over 50 10uf 50v caps. No other values which is weird. I checked them with my cap analyzer and found on really dodgy one that was a different brand then all the rest. I replaced it and 4 others near the input stage.
Now I go to test it. Well there is no static but a new problem. A hum on all zones/channels. It sounds sort of like a 60hz hum. So I double check all my work, even pulled the caps I did one at a time to try to get it to stop. No luck, so I look next to the power supply. It is linear with a large center tapped transformer. I shotgunned the 4 small caps on there just for the hell of it. No change. So now I try to use my thinking cap and attempt to determine if there is some sort of hum originating at the power supply. I pull out my scope meter which I am good at turning on and not much else.
So eventually I figure out the right button to press and I am off probing voltages. The supply puts out +25v, -25v and +12v. See the pictures, but I think the -25v has a 120hz ripple on it which could be the source of the hum. Its a little strange because the voltage was lower (-18v) probing just the power supply with the connection to the amp removed (pic 2&3). With everything in place for the amp to work I got the proper -25v (pic 4&5). Both however show a 120hz ripple. The +25 and +12 rails were normal. What causes this? One of the filter caps going bad maybe? They look like high quality items (Topcon!).
TLDR: I have a 120hz hum on a whole house audio amp. I have isolated it to the -25v rail. Any suggestions on how to fix this are most welcome.
Now I go to test it. Well there is no static but a new problem. A hum on all zones/channels. It sounds sort of like a 60hz hum. So I double check all my work, even pulled the caps I did one at a time to try to get it to stop. No luck, so I look next to the power supply. It is linear with a large center tapped transformer. I shotgunned the 4 small caps on there just for the hell of it. No change. So now I try to use my thinking cap and attempt to determine if there is some sort of hum originating at the power supply. I pull out my scope meter which I am good at turning on and not much else.
So eventually I figure out the right button to press and I am off probing voltages. The supply puts out +25v, -25v and +12v. See the pictures, but I think the -25v has a 120hz ripple on it which could be the source of the hum. Its a little strange because the voltage was lower (-18v) probing just the power supply with the connection to the amp removed (pic 2&3). With everything in place for the amp to work I got the proper -25v (pic 4&5). Both however show a 120hz ripple. The +25 and +12 rails were normal. What causes this? One of the filter caps going bad maybe? They look like high quality items (Topcon!).
TLDR: I have a 120hz hum on a whole house audio amp. I have isolated it to the -25v rail. Any suggestions on how to fix this are most welcome.
Comment