Help with DC offset on NAD 7125.

[Domsmith1970]

Hi guys, posted before about DC offset on my NAD 7125 receiver. Please bear with me because I am new to this and love to learn. Recently I noticed my right speaker was lacing some bass/mid. And my left speaker was sounding a bit to loud. I trouble shot the speakers and it's not them. Read the sicky "DC offset and you"...whipped out my multimeter...checked terminals L(-2) R(1.7) seems very good. I downloaded the manual off here and proceed to adjust center voltage and idling current. The schmatics/service manual is pretty straight forward...but are a little confusing. It has pots you can adjust the speaker terminals with....which I did. To make this easier I will bullet point my questions...I apologise if this post is long winded but am trying to learn and really appreciate the help. 1) Manual states "adjust to 0 V+30mV" am I adjusting to 0,15, or 30 mV? I the ultimate goal to adjust all to 0? 2) Idling current "test and adjust for 2-4 mV. It says to test across the output transistor emitter resititor. Now...the transistor is a large rectangular 3 pronged transistor. I put the black(g) on the left prong, and the red(+) on the far prong. Didn't touch the middle prong. Is this correct? I adjusted to 4 mV. 3) Now...the manual states "leave amp on for 5 mins then recheck everything. Here's my confusion. The last step states "and adjust VR102 (left channel) for 4mV" earlier steps said to adj. between 2-4 mV?? also why is the last step only adjusting the left (VR 102) to 4mV and not the right? If anybody wants to actually download the manual to better help that would be amazing! I know I may not be explaining myself correctly but am kind of new to this and really appreciate the help! Cheers.........Dom

 

[petehall347]

set the dc offset (centre voltage ) (dc balance) all same thing ..to as close to 0v as you can . a few mv here is ok .
bias is done by putting your meter across one of the large resistors that reside next the output transistors that reside on the large heat-sink . set as per manual .
if you still dont understand post a picture or 3 and someone will explain in more detail .

 

[Goldie99]

Hi - to answer some of your questions:

1) If you check the service manual, it should say 0V +/- 30mV (mine does) - so you're adjusting to get as close to 0V as possible - within a range of +30 millivolts to -30 millivolts. It's not critical, and makes no difference if it's + or -, just in that range if possible. Be sure to only adjust with the correct trimmers - VR101 (Left) & VR201 (Right)
2) My copy of the service manual says to connect a millivolt meter across R150 ("output transistor emitter resistor") - EMITTER RESISTOR are the key words here. You measure the voltage drop across R150 - so one probe goes on each side of the R150 resistor, for the Left channel - and VR102 is adjusted to give 2 - 4 mV reading. This is then repeated with the voltmeter across R250, for the right channel - and VR202 is adjusted to 2 - 4 mV reading. R150 & R250 are located just in front of the main output transistors - they should be marked on the pcb.
3) Now you leave the amp on, for 5 - 10 minutes, with no input, and all controls set to zero (no inputs, no volume, balance / bass / treble in the middle, etc.) - so it can reach thermal equilibrium. The bias (idle current) especially is very sensitive to thermal changes (blow on it when you adjust it - you'll see). Repeat the above settings after 5 - 10 minutes, but now aim for 4 mV across R150 and R250 - now you want to be as accurate as possible.
4) The manual only lists repeating for the left channel - it's a mistake, it means repeat for both channels.

Finally, I would strongly recommend that you get some meter leads with mini-grabbers before making the idle current checks - so that you can 'clip' them onto the relevant positions in the amp before turning the amp on !! If you accidentally touch the wrong parts while making these adjustments (especially the bias / idle current) you run a high chance of blowing the main output transistors + others.... so put the mini-grabbers in position, and move / remove them only when the amp is turned off..... it's takes longer, but it's a lot quicker than sourcing and fitting new outputs....

Alan

 

[Domsmith1970]

Thank you Alan and Pete! Big help....did all the adjustments. All seems good. Will double check the speaker terminals tonite after a few days...see if anything has changed. Amp now does actually sound a lot better.

 

[refoamwizard]

Hi - Old thread, but I wanted to clarify for the next readers...

The emitter resistors are indeed double 0.1 ohm resistors with three leads. The service manual images of the schematics for the 7120 and 7125 say "30 mA IDLING". That would correspond to 3 mV through half the resistor, or 6 mV through both halves. Confirmation of this is silk screened on the PCB and shown on the board image in the service manual which shows "6 mV" and lines indicating that this is across both halves of the resistor.

I think the service manual should be more explicit and indicate "2-4 mV across half of the resistor" or "4-8 mV across both halves" in Step 2 of the Alignment Method.
Then in Step 5 of the Final Adjustment after the unit is warmed up, it should say to dial VR102 to 3 mV across half of the resistor, or 6 mV across both halves.
I believe that the "4 mV" in Step 5 is a typo. And the manual neglects to prompt for adjustment of VR202 for the other channel.

Hope this helps!