Greetings AK,
I'm currently attempting to fix a Mark Levinson ML-1 preamp that has a serious power supply problem. The ML-1 was given to me by a friend who refuses to bring it to a real tech (I already suggested that multiple times), so I'm attempting repairs. It came with the main preamp unit and a separate linear regulated power supply connected by a cable. My DMM tells me there is quite a bit of AC on the PSU's DC output, so the problem is pretty obvious.
I tested the bridge rectifiers with a DMM because this seemed like an obvious place for unwanted AC to enter the circuit. On every diode, I get a voltage drop both directions; none read open loop. Based on my limited knowledge of power supplies and reading some articles, this indicates a problem. At first, I suspected the diodes themselves were shorted, so I desoldered one and tested it with one leg off the board. Interestingly, once removed from the board, the diode gave a proper reading (~.5 voltage drop in one direction and OL in the other). What else could the culprit be? I think each side of the bridge rectifier is connected to the ground plane by another smaller diode and a few capacitors. Could these be shorted, allowing voltage to pass in both directions?
I have included some pictures of the bridge rectifier board and the schematic I have for the PSU; hopefully that helps a bit. I can take DMM readings of anything that would be useful but unfortunately I don't have a scope with me.
Cheers!
I'm currently attempting to fix a Mark Levinson ML-1 preamp that has a serious power supply problem. The ML-1 was given to me by a friend who refuses to bring it to a real tech (I already suggested that multiple times), so I'm attempting repairs. It came with the main preamp unit and a separate linear regulated power supply connected by a cable. My DMM tells me there is quite a bit of AC on the PSU's DC output, so the problem is pretty obvious.
I tested the bridge rectifiers with a DMM because this seemed like an obvious place for unwanted AC to enter the circuit. On every diode, I get a voltage drop both directions; none read open loop. Based on my limited knowledge of power supplies and reading some articles, this indicates a problem. At first, I suspected the diodes themselves were shorted, so I desoldered one and tested it with one leg off the board. Interestingly, once removed from the board, the diode gave a proper reading (~.5 voltage drop in one direction and OL in the other). What else could the culprit be? I think each side of the bridge rectifier is connected to the ground plane by another smaller diode and a few capacitors. Could these be shorted, allowing voltage to pass in both directions?
I have included some pictures of the bridge rectifier board and the schematic I have for the PSU; hopefully that helps a bit. I can take DMM readings of anything that would be useful but unfortunately I don't have a scope with me.
Cheers!
