FMR RNC Repair

I bought a broken RNC online for $40 shipped. The seller literally tried every possible wall-wart with the unit except for what’s written on the back of it: 9VAC @ 500mA.

He mentioned smoke coming out at some point, so the first thing I did when I got it was to open it and see what burned, but nothing looked burnt. So I ran it on a 9VAC wall-wart I have, but before I plugged it into my interface, I decided to look at the outputs to see if spitting out a high voltage. Both were putting out 4VDC. I took another glance at the unit and noticed two bulging 470uF capacitors.

I tried to trace the power supply circuit, but it didn’t look like the usual rectifier and regulator circuit. I called Mark at FMR Audio and he explained that they use a charge pump to generate +/-15VDC. Now things were making more sense, especially when the two bulging capacitors are the rectifying caps. I replaced them with other 470uF 25V capacitors and no more DC at the outputs. The compressor seems to work fine, but the capacitors get kind of warm and the regulators get even warmer to the point where I can’t leave my finger on them for too long. I also noticed that the DC voltage on the two caps I replaced is a bit over 25V. Weird since the factory caps were also rated at 25V. Another weird thing I noticed is that my wall-wart puts out 10-point-something VAC even when it’s loaded.

I called Mark at FMR again and he explained that since my wall-wart is rated at 1.5A, it’s probably too stiff a voltage source for the RNC. We then calculated that the extra voltage is what’s causing the higher DC voltage I was seeing on the capacitors.  He also suggested swapping the 25V capacitors for ones rated at 35V so that they’ll last longer. He did say that the regulators getting warm is normal and expected considering the voltage drop and current through them. He also tipped me to the wall-warts they sell with their units (Jameco part no. 100061). Lastly, he said that the two distortion trimpots should be left alone. They dial in the distortion with an Audio Precision to .005%. He then mentioned that the units keep their calibration for the longest time.

As you can tell, Mark was extremely helpful. He is happy to answer question and help probe a broken unit, and never even said that I should mail it to them to repair it. If you’re looking for a new compressor, consider the RNC because it’s awesome and cheap, and their support is excellent.