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NAD 7240PE Receiver "Idle current adjustment" manual clarification/help.

MB13

Aug 16, 2011
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Aug 16, 2011
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Hello all,
I'm currently working on the finishing touches to an NAD 7240 PE receiver and during the "Main Amplifier Alignment"
procedures I've run into a problem during the "Idle current adjustment" procedure as described in the service manual.
The problem is, well first maybe I should show what the manual describes as the procedure in question. It's pretty short so:

B. Idle Current Adjustment. (A. was "Center Voltage Adjustment")

1.Remove solder short across R-471 and R-472
2.Connect DVM across R-471
3.Turn on and adjust VR403 for a reading of 28mV +/- 2mVDC.
4.Repeat using R-472, adjust VR 404.

C. Final Adjustment
1.Leave power on a minimum of 5 minutes.
2. Repeat Center current (A) and Idle current (B) adjustments.
3. When finished replace solder shorts across R-471 and R-472

...and that's that.
The problem is:
There are NO SOLDER SHORTS ACROSS R-471 and R-472!
And no other shorting method that I can see that might accomplish the same thing. (A) the center current adjustment went just fine
but now I'm stuck at (B) the Idle current adjustment because I have no idea what to do at this point.
Should R-471/2 which were not shorted when I received the receiver, be shorted to accomplish the "Idle current adjustment?(..and shorted how?)
And once this question is answered should the solder shorts be "replaced" as instructed in the last step (C-3) when they didn't exist in the first place?!!
I'd really appreciate any input anyone has to give on this. This is weirding me out, seriously.
The unit was blowing fuses left and right and I noticed that two of the output trans were shot.
So I replaced all 4 and hooked it up to a "current limiter" and all seems quite well, to make a long story short.
But these adjustments are holding me up and I can't just neglect them.
So any help on this issue would be appreciated. I have the service manual with schematics and block diagrams for anyone that'd like them.
Thanks to all,

MB13
 

Rleo6965

Jan 22, 2012
585
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Jan 22, 2012
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585
Pls. post picture of mentioned R471 and R472 pcb connection also include schematic diagram. Surely, we will able to help.
 

MB13

Aug 16, 2011
6
Joined
Aug 16, 2011
Messages
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NAD 7240PE - PC board pics & Schematic

Seems that the attachments I need to upload are too large at the moment so I'll get them right and send them again.
Thanks for your patience.

AAVA
 

MB13

Aug 16, 2011
6
Joined
Aug 16, 2011
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NAD 7240 PE Instructions/Schematics/ Pics

Pls. post picture of mentioned R471 and R472 pcb connection also include schematic diagram. Surely, we will able to help.

OK, here are some pics of the circuit board along with excerpts of the service manual:
i.e. amp board schematic; 1 whole schematic (may not enlarge appropriately) therfore;
the same schematic segmented into 4 sections; upper left/right; bottom left/right at a higher resolution.
There are also pics of the circuit board positions of R-471 and R-472 from the top and bottom of the circuit board.
Hopefully this will be of assistance to all assisting me :)
Assistance for which I am truly grateful.
Thanks again,

AAVA
MB

PS. Pic uploads will have to be in 2 installments.
 

Attachments

  • R-471-1.jpg
    R-471-1.jpg
    93.3 KB · Views: 804
  • R-472-2.jpg
    R-472-2.jpg
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  • R-471-Und.jpg
    R-471-Und.jpg
    87.9 KB · Views: 729
  • R-472-Und.jpg
    R-472-Und.jpg
    70.2 KB · Views: 1,356
  • 7240 PE Schematic.jpg
    7240 PE Schematic.jpg
    103.9 KB · Views: 2,268

MB13

Aug 16, 2011
6
Joined
Aug 16, 2011
Messages
6
Pls. post picture of mentioned R471 and R472 pcb connection also include schematic diagram. Surely, we will able to help.

This is the 2nd installment of pics. The schematic in the first installment was, as I suspected, at a pretty low resolution. So here are 4 segmented images of the same power board schematic. If these aren't good enough please let me know and I'll try to make better images of the schematic.

Thanks again,

AAVA
MB
 

Attachments

  • 7240 PE Schematic bottom-left.jpg
    7240 PE Schematic bottom-left.jpg
    111 KB · Views: 2,219
  • 7240 PE Schematic bottom-right.jpg
    7240 PE Schematic bottom-right.jpg
    103.5 KB · Views: 1,447
  • 7240 PE Schematic Up-left.jpg
    7240 PE Schematic Up-left.jpg
    99.6 KB · Views: 1,169
  • 7240 PE Schematic Up-right.jpg
    7240 PE Schematic Up-right.jpg
    105.5 KB · Views: 1,901
  • 7240 PE Schematic.jpg
    7240 PE Schematic.jpg
    103.9 KB · Views: 1,124

MB13

Aug 16, 2011
6
Joined
Aug 16, 2011
Messages
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...One more submission...

Pls. post picture of mentioned R471 and R472 pcb connection also include schematic diagram. Surely, we will able to help.

I actually submitted the whole schematic image twice when I meant to upload other images.
So here are the other images.


AAVA
MB
 

Attachments

  • 7240 alignment instructions.PNG
    7240 alignment instructions.PNG
    55.6 KB · Views: 649
  • Amp PCB parts location.jpg
    Amp PCB parts location.jpg
    90.1 KB · Views: 1,199

Rleo6965

Jan 22, 2012
585
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Jan 22, 2012
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585
This is the way I understand the instruction. Pls see portion of schematic diagram from your uploaded photo 7240 PE Schematic Up-right.jpg (4 of 5)

The instruction was that you have to measure “reading of 28mV +/- 2mVDC”. This voltage was so low to measure on +43V supply rail. So, I think that this small voltage would be measure the voltage drop on "shorting etch" in parallel with R471. See photo with Blue arrow.
Try removed or unsolder one end of R471 shown in" red X "and placed your dvm on etch where R471 was connected or soldered. You measuring the voltage drop on shorting etch pcb for R471. dvm testpoint shown by 2 red arrow.

Hope you understand my instruction.

R471dcmeasurement.jpg
 
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Rleo6965

Jan 22, 2012
585
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Jan 22, 2012
Messages
585
I can't find the shorting etch across R471 from uploaded photo. . You can confirm if there's really a shorting etch across R471 by measure a 0 ohm resistance once R471 was unsolder or removed.
 

MB13

Aug 16, 2011
6
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Aug 16, 2011
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Shorting R-471/2 re: Rleo6965

I can't find the shorting etch across R471 from uploaded photo. . You can confirm if there's really a shorting etch across R471 by measure a 0 ohm resistance once R471 was unsolder or removed.

Hi Rleo6965, thanks for your response and advice.
What you mentioned is pretty much the way I was heading in my interpretation of the instructions.
I think I see what you mean by "shorting etch" on the schematic and I'll look for it on the circuit board.
Funny that I didn't notice this paralleled circuit trace and derive what it might be for.
So at this point I'm going to try your suggestion as it sounds quite reasonable in light of the circuits configuration. I'll let you know what readings I get.
Thanks much Rleo6965, I greatly appreciate you taking the time to give me a hand.

Take care,

MB13
 
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