Hello,
I replaced the relays in my NAD 2700 THX Power amp so I thought I would do an alignment while I had the box open.
The problem I'm having is with setting the Idling voltage. I did this a few months ago and had no problem getting the 7.5mV. Now the most I was able to get was 3mV maxing out the trim-pot. The amp was making a buzz when I got that high. I'm getting the same on both channels. I went ahead and set both channels to about where the trip-pots were before I started, this gives me about 1mV. I played some music through it, it sounds pretty normal.
Any idea's where to start?
I'll probably take it to a shop (any recommendations in San Jose California?)
Thank you for reading. 🙂
I replaced the relays in my NAD 2700 THX Power amp so I thought I would do an alignment while I had the box open.
The problem I'm having is with setting the Idling voltage. I did this a few months ago and had no problem getting the 7.5mV. Now the most I was able to get was 3mV maxing out the trim-pot. The amp was making a buzz when I got that high. I'm getting the same on both channels. I went ahead and set both channels to about where the trip-pots were before I started, this gives me about 1mV. I played some music through it, it sounds pretty normal.
Any idea's where to start?
I'll probably take it to a shop (any recommendations in San Jose California?)
Thank you for reading. 🙂
Attachments
check the trimmers-SVR302 and Q334 bias transistors. Also make sure D310/D312 have +/- 1.77v respectively, also check C312
I am one the SF peninsula
Home - QuirkAudio
Peter
I am one the SF peninsula
Home - QuirkAudio
Peter
Strange settings for dc offset.
Usually they look for 0.0mV or as close as possible.
The object of the exercise is to get no appreciable DC through speaker.
Are you sure your setting right pots ?
If your setting bias current instead the amp will start to complain if it gets too high.
On my amp transformer starts to vibrate if it takes too much current.
Usually they look for 0.0mV or as close as possible.
The object of the exercise is to get no appreciable DC through speaker.
Are you sure your setting right pots ?
If your setting bias current instead the amp will start to complain if it gets too high.
On my amp transformer starts to vibrate if it takes too much current.
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Looking at the schematic I would agree with you Nigel , there is a DC offset adjustment of 1K in the usual place but there is a SVR302 in the driver section for bias marked "idling adj."--aka bias adjustment .
This amp is full of all sorts of protection ( too much for me ) I like the simple life ) .
This amp is full of all sorts of protection ( too much for me ) I like the simple life ) .
SVR 302 adjusts the idling (and 7.5mv is correct), SVR 202 (10k), adjusts the DC (which should be zero and usually is +/10mv)- After about 15 mins the bias stabilizes.
The protection circuit is on the power supply board and uses a TA7137P IC- what does cause problems are the relays and the small value caps
The protection circuit is on the power supply board and uses a TA7137P IC- what does cause problems are the relays and the small value caps