NAD 2200 current draw rises

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pwdiya12

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Joined 2012
quirkaudio.com/
#1
I have a NAD 2200 PE that I am repairing/upgrading. When I got it the right channel had a number of fried resistors, and three output transistors (Q320,Q318,Q314) and R368- the emitter resistors were fried and the main PS caps were bulging. I have recapped the unit, and replaced the output transistors (I had some spare originals that I had pulled form another 2200, replaced all the fried transistors and any that looked that they have heat damage, and also replaced Q302/Q304 and Q306/Q308 as although they were of on the transistor tester I thought it better to replace them. I have also replaced SR 302. The rest of the transistors test ok on the right channel and the unit fires up and comes out of protection, and both channels pass signal. However, these units on idle (no input no speakers connected) should draw 0.4A approx. This unit starts there and then creeps (in small jumps to towards 0.75A- which is when I noticed a "warm" smell- and turned it off. I use a Senore PR57- which does not have current limiting. I tested the NAD with cold bias (Collector Q302 to collector Q304) to see if it was the output transistors, and the result is the same- the current creeps up. So although I have "fixed" the unit, I don not think I have found the underlying cause as to why the right channel self destructed. any thoughts or guidance would be greatly appreciated Peter
 
#2
No that's utterly out of spec. No wonder it burnt.

The manual says adjust for 7.5mV across each 0.22ohm emitter resistor, about 35mA per output pair, 70mA per channel.

Check the Vbe multipliers, Q333/Q334, and adjust the idle correctly - perhaps it won't adjust? Check all the resistor values in the Vbe sections if not.
 
#4
Hey Mark, thanks


well my assumption was that is was the right channel and whilst I was adjusting the bias to around 7.0mv- (the current draw was about 0.55amp)- C301 just went bang- so I will investigate and report back


On the right channel Q334 tests fine and i have tested all the resistors- I think my assumption that the left channel (which worked) is OK- just proved to be false


thanks


Peter
 
#7
So replaced the cap on the left channel and R329, left channel is fine.


I removed the power to the right channel (via the cable) and amp fires up, comes out of protection and stays stable at about 0.4A AC draw- no creep- so the problem is on the right channel.



Another observation is that it appears heat driven, as when the amp warms up, the current draw increases and the idling comes up form about 1mv towards 7.5mv- which I think is normal


so something on the right channel is causing the overall current draw to increase.


any thoughts?
 
#14
all the outputs have been checked- Q320/Q316 E=0v, B=-0.5v C=58v- same for the positive voltage rail. I tried "finger test" on the transistors, and nothing is getting hot.
I agree that it has to be going somewhere- I will check Q326 and Q322 on the +ve side (i have focused on the negative side as this was where all the damage was).


The cold bias test, which should turn off the outputs, still saw the current rise.


I think it has to be a transistor-I will try the finger test and let the current rise a bit (but dont want to fry things again)- now I see why I want a current limiting AC supply.


@rayma- thanks for helping with this- it has me puzzled


I will report back on Q326/Q322- I tested Q330 and it tests good
 
#15
Hook up a 10R load resistor and make sure it is not hot from HF oscillation.
Is the Zobel resistor hot (or open)? Do you have a thermal gun to check for hot spots?
Rule out as much as you can.
 
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#18
 
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