Repairing a NAD 2200 Poweramp. Advised needed.

[owens]

I have a NAD 2200 power amp (2-channel, 150w each) which I've had since the 80's. It's served me very well up until just recently when the left hand channel blown.

After closer inspection I found a few burnt resistors and ceramic capacitors. Since this is a pretty old amp its no surprise as it seems alot of the resistors are starting to corrode.

I pulled out the left channel board so I could replace the burnt out components. I also want to replace all the mosfet transistors while I'm at it aswell, even though they have no sign of physical damage, I just want to be on the safe side.

The main mosfets are Toshiba 2SA1302 and 2SC3281. After doing some research, they haven't made these since 2000. People around here have mentioned the 2SC5200 and 2SA1943 are replacement models for the older ones.

I looked at the datasheets and and all the properties seem all the same. Although a couple value variations have got me a little worried.

For example:
Older 2SC3281 datasheet has "Collector-base and Emitter Voltage = 200V"
Newer 2SC5200 datasheet has "Collector-base and Emitter Voltage = 230V"

Older 2SC3281 datasheet has "Collector Output Capacitance = TYP. 270pF"
Newer 2SC5200 datasheet has "Collector Output Capacitance = TYP. 200pF"

My main concern is, can I replace the old 2SA1302/2SC3281 with the newer 2SC5200/2SA1943 directly? Will this effect anything because the values mentioned above are slightly different?

Here is a photo I took earlier. I'm also a little unsure about the "S6K20 Diode Rectifier" (labeled in red). Are these just generic and can use any brand I can find?



Sorry for all the newbie questions. I'm cluey with electronics and have lots of experience with soldering, but this is the first time I've tackled trying to fix an amp.

[bob91343]

First, there are no MOSFETs in there. All bipolar transistors. Second, replacing transistors involves more than voltage rating; you have to consider speed too.

The diodes, well they are used as temperature compensation for idling current, and while most diodes will work, you will need to see how stable the idling current is once it's working. Some of those diodes are multiples, several in series, so you need to have the right number or it won't compensate. Check the specifications. I have replaced those with generic types successfully but they have to be on the heat sink.

You should test all transistors on the board for shorts and opens. One bad part and you will have disaster when you apply power. Further, use a variable autotransformer to bring up the power gradually as you monitor the current drawn. It's too easy to blow a bunch of new parts.

[djk]

The diode is just a high speed rectifier on the low voltage rail, any generic high speed rectifier will work. Looks like 6A 200V.