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Receiver (tuner) signal fade-out ---heat related

Hello. I'm a tech with a tuner problem...I have a NAD 7140 receiver which, when powered up, loses the radio signal after just a few minutes. Next to the tuner module there are 3 transistors. I've traced the signal fade-out to there with a scope. However, I need a schematic or, preferably, the service manual to continue the hunt. Using a can of chiller comes to mind but that's risky due to condensation and other phenomenon that can occur--especially with a unit as old as this one is. (abt 25 years) I have to wait a while for the unit to cool back down so I can quickly start troubleshooting again before the loss repeats itself. Any ideas where I can obtain a service manual? Heh, I don't think an NAD service center is going to be gracious enough to sell me one of theirs (food out of their mouths and all that...). Any help will be greatly appreciated! Thanks in advance, ----Rick

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  • Richard C Cohen
    Richard C Cohen Mar 24, 2008

    Thanks for the input...the serial is 102335....I was told by a tech long ago that any NAD stuff manufactured in Hong Kong was basically junk because of poor quality control. Mine was made in Japan and is supposed to be ok. Never had a problem until now. There are 3 transistors located near the tuner module (Q102, 103, 104) I checked with the scope...the waveform of a few volts just drains away in 2-3 minutes down to nothing...the bar graph on front display correspondingly gets less and less until no signal is detected.There is an op-amp? located near the transistors (IC101). And, there is a power IC (IC104), located relatively nearby to the module towards the front. I assume it handles the incoming power to the front display? (Just guessing) Hope this helps...btw...the transformer mounted on the left side of the chassis which powers the whole upper pcb shows a LOUSY square wave--Looks more sinusoidal than square with the voltage drop-offs along the ramps--in other words, instead of a clean, vertical rise and decay, which would be a good quality square, the rise and decay looks delayed, forming a more sinusoidal shape. Don't know if this would affect anything--the rest of the receiver works perfectly.---Rick

  • Richard C Cohen
    Richard C Cohen Mar 28, 2008

    Hello, I found the defective transistor in the tuner module by using a chiller and O-scope. It's a C481. I also shotgun-soldered the entire upper pcb because of the aged condition of the solder pads. I uncovered a few "sizzlers"(near-cold joints) in the process. The board looks almost brand new now. In the meantime, I'm going to check the major parts houses for the transistor or a suitable substitute. If you know any sources for parts (that don't ask an arm and leg) please let me know...Thanks for the help!



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The main problem with the 7140 is that there are at least 3 versions that I am aware of. The varient is based upon the serial number. Can you post that here. I will look up the info for you. I have access to some NAD stuff. Post the part designations as well (Qxx) and I'll at least get you the exact part numbers etc.

Posted on Mar 23, 2008

  • ABRsvcs Mar 25, 2008

    I don't have the variant for your serial number at the moment. I've place the request with the appropriate folks to get a copy. I should have it in a few days. Stay tuned...


  • ABRsvcs Mar 28, 2008

    The C481 is a 2SC481. It is available at in the US for $7.44

    Hope this helps,




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