NAD 356BE AMPLIFIER REPAIR
NAD 356BE AMPLIFIER REPAIR
My beloved NAD C-356BE amplifier has gone silent. A quick look reveals that too much thermal stress in the auxiliary power supply cooked the PCB. Current in the power supply was normal, so there is no other fault. It's just a very poor linear supply design, were a hand-me-down transformer from an older model with a 55V output was used in a 21V power supply, and the excess voltage is just dumped in heat across power resistors and power zeners. My amp was on for years in my studio, which eventually got too hot for the PCB. I just repaired the PCB, and put beefier components so they would heat up less, and the amp was up again.
Here is the service manual with schematics:
NAD C-356BE Service Manual.pdf
For reference, the section that's bad is the power supply in this part of the schematic. It's 3.3V 1W zeners D110, D111, 3.3 Ohms 0.5W Fusible Resistor R45, and 51 Ohms 1W Fusible Resistor R46:
I replaced the 1W zeners by 3W zeners, the 3.3 Ohms 0.5 by a 3.3 Ohms 3W Fusible, and the 51 Ohms by a 5W equivalent. I could not find a fusible in that power range, but the power supply has good fuse, crowbar and temperature protection, so I decided to live with it. If you want to can install a fuse in series with it, probably something beefy and slow blow in the 3-5A range so it does not blow no in-rush. Below is the list:
Unfortunately the heat had cooked all the nearby electrolytic capacitors. I replaced with same value, same voltage, 105C Nichicons electrolytic radial capacitors. I ended up changing most of the electrolytics while I was a it as they all had started to deteriorate. Here is a picture snapshot of my order from Digikey. That probably does not cover everything since I had some in my component box, but you get the idea:
Here are the links from the image in clickable form
Oh, and contrary to YouTube comments very misguided but very vocal misinformation, the amplifier is properly repaired. The current on both rails of the power supply was checked. As you can see below, using my trusty HP 428A Milliammeter - hey, using a tube instrument is proper form for audiophiles, I am told. The current in the +55V rail is 247 mA, and in the -55V is 170 mA. Which is perfectly normal, and all the components including the original ones were working within their rated power envelope, with room to spare. But too hot for the PCB. There is no other hidden fault in the amplifier that I can detect. The amp has been working fine since then. Replacing the caps did not change the power consumption either, of course. Not even 1 mA. Don't trust what you read on the interwebs...