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crite40 19th August 2012 10:58 PM

NAB 3100 problems
Good day all! Just joined this forum.
FYI my own stereo system is a home made copy of the Quad 405 series (50 watts
channel) with an upgraded Doug Self preamp. Wharfedale speakers and usual programme sources.
I still do a bit of servicing for various people though I mostly restore vintage radios etc for museums and private collectors these days.
My post is about an NAD 3100 which recently came in for repair from an old friend. Once I got the cover off, I was a bit dubious about the design. I do NOT
like all those power devices crammed together in a row.
Then I found the first fault. During manufacture a small solder bead had been left in the unit and it had managed to short out a driver transistor with serious results.
I downloaded a service manual and just happened to note that NAB recommended that after working on the output stages the unit should be first run with a variac transformer or a series bulb load!
Big warning sign! Over my many years of experience I have come across other amps which have dc stability problems, and they were often fatal!

However I took another look and realised that several small resistors in the power stages were "cooked". Then despite being very careful a meter probe slipped and more cooking took place.

Result. I won't be recommending this firms amps to anyone who asks.
Although very pretty, the board layout of the power stages and the stability is not up to par.

I have been involved in electronics for over 60 years.

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