13-02-2015, 11:45 AM
(13-02-2015, 10:34 AM)Mark Hennessy Wrote: There's nothing stopping you from putting beefier transistors in - I recommend using MJ15003 and MJ15004. Farnell stock them for a not-unreasonable price.
As to what caused the failure, quite a lot of NAD amps lack active current limit protection, relying instead on the fuses, so it's possible that an intermittent short in the 'speaker wiring might be a factor (surprisingly common in my experience).
Or perhaps it was thermal runaway? Some of the NAD amps I've serviced had frighteningly small heat sinks, and didn't seem to pay attention to close-coupling the VBE-multiplier transistor to the heat sink. Definitely make sure the quiescent current is correctly set, and is reasonably stable over a few hours...
Have a really close look for dry joints. Again, a common problem area with these in my experience. I use a x7 magnifier to be absolutely sure. I often re-do joints that are hot-running, even if they look OK (remove old, put on new). Basic, I know, but easily overlooked.
What sort of pre-set resistor are they using for the Iq adjust, and if it were to develop a poor contact, what would be the effect? On a sensible amp, the preset should be placed between base and emitter of the VBE multiplier, so the worst-case scenario is crossover distortion. But others use arrangements whereby a poor contact can lead to the VBE multiplier being biased off, which clearly will cause the output pair to conduct heavily with the inevitable results.
Does the owner like the music loud from time to time? A good trick is to retro-fit a bimetallic cut-out to the heat sink - perhaps one set to 70C - wired in series with the mains (well insulated!). When the heat sink gets hot, the power is switched off. After a few minutes, it'll come back on again. Hopefully no damage results, and the owner gets the message that he needs to turn it down, or remove all the stuff that's blocking the vent holes! It's a trick that saved my bacon on several occasions - something like this might do the job: http://uk.farnell.com/multicomp/t23a070a...dp/1006843
Oh, and it's probably at the age where some of the capacitors must be past their best.
Apologies in advance if I was stating the obvious in the above. Good luck with the repair
Mark
Points noted, Mark. Dry joints and/or thermal runaway, plus external causes like faulty speaker wiring may all be factors in the failure. Uprated transistors and a bimetallic cutout may also be worth considering. I'll give it a thorough check over before fitting new transistors in any case, though 15 years before it failed again is quite good going, IMHO, and perhaps external problems are the cause, given that the RH channel has never failed.
The amp is actually a 3130, btw, and I don't see a pre-set for iq adjustment on the circuit diagram. The resistor between the base & emitter of the Vbe multiplier is actually two fixed parallel types.
