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Old 22nd August 2006, 10:54 PM   #1
dis is offline dis  Canada
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Default Need the help of some electronic whizzes

I have a NAD T742 that is has an issue when it heats up. When the amp heats up enough, distortion rather than audio will output on all speaker in any of the DSP modes.

This does not happen with 2ch stereo mode. I traced the problem to the AC3 board, specifically what is happening is the components responsible for the 5V supply to the dsp are heating up and causing the problem.

I found a NAD service bulletin (for the T761 but still relevant) that talked about changing a 85 degree cap to a 105 degree cap because at higher temps the lower-rated cap wasn't filtering the voltage ripple.

Here's the Problem:

The 85 degree cap was swapped for a 105 degree cap by a NAD service technician. I don't know if it completely fixed the problem but the amp was trouble free for about 2 months. It just recently started making the noise again.

So I'd thought I try to fix it myself this time a save a few bucks (paid $109 last time)

I power cycled the amp to have it stop making the distortion noise, it was clean souding when the measurements were taken

I checked the voltage on the components:
- Capacitor C107
- 5V 7805 Regulator U123
- Diode Bridge Rectifier RB152

Results
---------
Vcap = 8.12 - 8.18 V (it's a 16V cap so I don't think it's broken)

Vrectifier (DC)= 8.7V (soon as I touched it to measure the voltage the distortion noise started, I also noticed it was very hot to the touch)

Vinput_regulator = 7.75V
Voutput_regulator = 4.99V (regulator appears to be functioning fine)

So I believe the culprit is the bridge rectifier. The trouble is I'm not sure if something further up the daisy chain could be causing the problem or whether it is only the rectifier's fault (an easy fix if this is the case)

Any ideas or insights?

I wanted to post the entire t742 service manual but it is too big (5mb) I'll post the relevant sections instead
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File Type: pdf nad-svbt761_heatedcapacitor2.pdf (53.5 KB, 45 views)
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Old 22nd August 2006, 11:02 PM   #2
dis is offline dis  Canada
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Old 22nd August 2006, 11:02 PM   #3
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take the measurements again when the fault occurs..
check all the solder joints around that area.. who knows, mabey its just a dry joint..
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Old 22nd August 2006, 11:04 PM   #4
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are you sure that the soldering is solid on that filter cap? That 8.something volts may be lower than expected. It might be cause the filter looks open, due to poor soldering, etc. It also may be that the cap is mechanically intermittent, where a little physical movement is causing it to re-connect. Hmmm, the rectifier is getting hot? It could be leaky, causing the filter to fail prematurely. Good time for an o'scope.
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Old 22nd August 2006, 11:04 PM   #5
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Old 22nd August 2006, 11:05 PM   #6
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Old 22nd August 2006, 11:07 PM   #7
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Old 22nd August 2006, 11:09 PM   #8
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Old 22nd August 2006, 11:11 PM   #9
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Old 22nd August 2006, 11:12 PM   #10
dis is offline dis  Canada
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Here's the Spec Sheet for the RB152 bridge rectifier
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