| Systeme_D |
Hi Everyone,
This is my first post and would like to seek the advice of the wise out here. I am trying to fix a friend's NAD302 integrated amp. I have searched the net and forum for a schematic I couldn't find one. The schematic for NAD3020 seems to be different.
It has exactly -3.3v dc offset on both channels. There is still amplification but severely distorted, as seen on a scope. It is still able to amplify a test tone and output to a speaker (a test speaker), but not music.
The +- power supplies seem to be ok, and all the presets are working. No shorts on the power trans, I have not desoldered them to measure HFE though.
Is it common to have both channels fail the same way at the same time. My past attempts were usually limited to one failed channel and use the working channel as a reference to detect faults.
I would appreciate any advice or suggestion of possible failure points.
thanks;) |
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| MikeB |
Hi !
The 1st Idea would have been the powersupply, not supplying
a symetrical Voltage. Are the +/- Voltages at the same Level ?
As both channels show the same behaviour, it must be a part
shared by both channels. This nearly excludes a fault in the
amps itself.
Another possibilty might be the preamp, most integrated
amplifiers use a 4x opamp, amplifying both channels in the same
chip. Maybe a cap or a zener for the voltagesupply of this opamp
is defect ?
Michael |
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| li_gangyi |
| a quickie check would be to unplug the shorting links that enable the preamp section to work...then just jack the CD player's or wadever source you are using to the "amin" ins...becareful though...might go really loud if u had speakers connected... |
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| slowhands |
| I don't know the model, but it is an old number, so I assume an older design. I'll take a long shot. I suspect a part of the power supply that is common to both channels has failed. That is assuming they use multiple voltages, and one has a defective regulator. |
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| Systeme_D |
Hi Folks,
Thanks for the suggestions. I have checked that both the supplies are symmetrical within a reasonable range i.e. 0.5v The offset is taken any preamp input i.e. the link bars between pre and power taken out.
The preamp also shows heavy distortion on a scope using a signal generator. I would probably have to look for voltage references zeners etc.
This NAD is quite old, at least 7 years, not high-end stuff. It is built with standard components, a la NAD style.
cheers:) |
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| soma |
1)Pls check not only voltage is correct or not,but also whether has oscillation.The relevant voltage values are as below:
Q415 c +33V/8ohms +28V/4ohms
Q417 c -33V/8ohms -28V/4ohms
Q507 e +38V
Q508 e -28V
Q511 e +28V
2)Pls check the following listed voltage is correct or not
Q401&Q402 c +36.6V
Q403,Q404,Q405,Q406 e +37.3V
Above is what I can think until now.If there is any new founds,Pls let me know. |
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| li_gangyi |
| maybe it's only the preamp section...check the voltages to the opamps...have you tried using just the main amp part??? |
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| Systeme_D |
Hi Guys,
Thanks for all your suggestions. I am quite tied down at the moment, haven't gone back to the board. If anyone has a schematic for NAD302 to share, it would be much appreciated.
gangyi: the pre and power were disconnected. both behave strangely on scope
Will post again when I have further results. |
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| li_gangyi |
| strangely...what do you mean by strangely?? perhaps it might be the input section...nad amps are easy to fix...even without schematics...but my friend's 304 continues to baffle me as well... |
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