Hello,
I bought an older MicPre SX202 from SYMETRIX. Unfortunately it doesn't work properly. If I plug in a dynamic mic everything is fine. But when I plug in a capacitor mic, it hums very loudly with the phantom power turned off. When I turn on the phantom power it hums even louder. This is on both mic channels. On suspicion, I replaced all the electrolytic capacitors on the board, but that didn't change anything. Since this hum only occurs with condenser mics, but not with dynamic mics, I suspect a problem in the phantom power supply. I tested various condenser mics, which I also plugged into other MicPre for comparison. The cause of the humming lies in the MicPre SX202.
Can anyone tell me what this could be? Is there a "typical" component (other than capacitors) that could be the cause? Unfortunately I don't have a circuit diagram for this MicPre.
Maris
I bought an older MicPre SX202 from SYMETRIX. Unfortunately it doesn't work properly. If I plug in a dynamic mic everything is fine. But when I plug in a capacitor mic, it hums very loudly with the phantom power turned off. When I turn on the phantom power it hums even louder. This is on both mic channels. On suspicion, I replaced all the electrolytic capacitors on the board, but that didn't change anything. Since this hum only occurs with condenser mics, but not with dynamic mics, I suspect a problem in the phantom power supply. I tested various condenser mics, which I also plugged into other MicPre for comparison. The cause of the humming lies in the MicPre SX202.
Can anyone tell me what this could be? Is there a "typical" component (other than capacitors) that could be the cause? Unfortunately I don't have a circuit diagram for this MicPre.
Maris
The company is still in business, and the manual is online, but says you will have to get the schematic from them directly or trace it out yourself. http://www.symetrix.co/kb/SX202_ug.pdf. How old is it?
The SYMETRIX company responded to my request. They no longer have a schematic for this MicPre, at least that's their answer. I had already read the manual, but there was no circuit diagram in it.
When you turn on the phantom supply with no microphone connected, what voltage do you measure between pins 2 and 1 and between pins 3 and 1 of the microphone input?
I was about to say, the schematics are on post #7, here: https://groupdiy.com/threads/upgrading-from-4560s-in-an-sx202-mic-preamp-in-2023.85536/#post-1115422
Hello.
Unfortunately, replacing the suspected relevant components did not eliminate the hum. That really irritated me. That's why I looked for other sources of error. The hum only happened when the circuit board was firmly installed in the case. When removed, no hum could be heard. There are a total of 4 holes on the board for the mounting screws. 2 of them create a conductive connection between the metal housing and the circuit ground. After trying out a few things and comparing them with the circuit diagram, I cut through a conductor with a sharp knife so that of the 2 holes, only 1 hole was directly connected to the housing and ground. When installed, you can no longer hear any humming. If my "research" was correct, there may be a hum loop on the board, which is eliminated by cutting one conductor and thus interrupting the hum loop. The result is clear. A star-shaped layout is often referred to for ground layout. In my opinion, this is now guaranteed by just 1 direct connection to the housing base.
This information may be of interest to you if you are facing a similar problem.
Maris
Unfortunately, replacing the suspected relevant components did not eliminate the hum. That really irritated me. That's why I looked for other sources of error. The hum only happened when the circuit board was firmly installed in the case. When removed, no hum could be heard. There are a total of 4 holes on the board for the mounting screws. 2 of them create a conductive connection between the metal housing and the circuit ground. After trying out a few things and comparing them with the circuit diagram, I cut through a conductor with a sharp knife so that of the 2 holes, only 1 hole was directly connected to the housing and ground. When installed, you can no longer hear any humming. If my "research" was correct, there may be a hum loop on the board, which is eliminated by cutting one conductor and thus interrupting the hum loop. The result is clear. A star-shaped layout is often referred to for ground layout. In my opinion, this is now guaranteed by just 1 direct connection to the housing base.
This information may be of interest to you if you are facing a similar problem.
Maris