NAD 705 tuner weird behavior

Discussion in 'Tuners' started by Chapmro, Aug 10, 2017.

  1. Chapmro

    Chapmro New Member

    Messages:
    5
    I hope this is the right place to post this, since it deals with the tuner section of a receiver. I have an NAD 705 I've owned since new. The amplifier part still works fine, and it plays all sources except the tuner perfectly, good audio quality, all switches and dials and display works. What happens with the tuner is this: pressing the tuning up and down buttons cause the displayed frequency to go up and down appropriately, but the tuner audio bears no relationship to what frequency is displayed. On power-up, the tuner starts at 87.5Mhz or so, and increses over the course of about 30 seconds up to 108 Mhz (you can tell by what station it is receiving -- the display doesn't change) and then stops. The audio quality is good when it has a station tuned in, but of course it tunes past every station except the highest one pretty quickly. All the presets have 197.0 Mhz stored in them and you cannot change the value by storing a new one. Changing the mode from search or tune to preset causes the speakers to turn off. So that's what's happening from a symptoms point of view. I have the schematic and service manual, but there's no theory of operation and I am a bit at a loss how to proceed in debugging this. I am guessing at some point that a digital value is converted to a voltage that is fed to the analog tuner. I have access to a bunch of test equipment, including a meter and scope (and even a logic analyzer) and I know how to use them, but I have no documentation on the digital part of the circuit beyond the schematic and don't know where to begin. My gut from fixing a lot of 70's stereo receivers is that when something just dies, the most likely culprit is in the power supply, so my first thought is check the voltages going from the PS into the tuner board. Thanks!
     
    Last edited: Aug 10, 2017
  2. Chapmro

    Chapmro New Member

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    5
    The PS voltages are all good. Checking the test point voltages against the schematic now, and reading the datasheets on the LM7000 and LA1266 chips.
     
  3. rcs16

    rcs16 Super Member

    Messages:
    2,820
    Location:
    Baldwin, Ontario, Canada
    I'd have to take good look at the schematic to understand the design type. I know the LM7000 is the PLL ic and a LA1266 is
    FM : IF amplifier, quadrature detector, AF preamplifier, signal meter, IF count outputlifier, tuning indicator drive output (common with stop signal, muting drive output)
    You can read up about theory in the device datasheets and in other tuner service manuals that are close to the same design.
    For T/E you could use a proper RF SG and and a THD analyzer.
     
  4. Chapmro

    Chapmro New Member

    Messages:
    5
    The schematic and service manual are available free at hi fi engine. The third chip in the tuner is LA3401. So far the weirdest thing I have found is that one of the test points, TP7, labelled "tuning" seems to increase linearly with the frequency tuned, going from 0v at 87.5Mhz to 30.5v at 108Mhz. When you turn the receiver off, it resets. It takes 30 seconds or so to make it the whole way. I have two theories -- the chip generating the tuned frequency is receiving a "search up" signal all the time from the processor that is never cancelled in spite of stations being found (the lights and signal strength and stereo indicators in the display comes and go appropriately as the tuner tunes upward), or some big capacitor is being charged all the way up. It's weird because when digital electronics fail they generally fail dead, not with this weird behavior, which makes me think it is something analog. The bad frequencies in the presets also leads me to think something wrong in the digital side.
     
  5. Chapmro

    Chapmro New Member

    Messages:
    5
    So, it looks like the frequency displayed in the LCD comes from the microcontroller, the same signals that get sent to the PLL freq. synthesizer chip. Which explains why the audio and the display don't match -- the error is downstream from the microcontroller. The PLL intializes in an autotune/search mode, and the mechanism for stopping it is that the IF chip signals when a station is found, and the PLL chip counts that freq. from the IF chip, then signals back to the microcontroller that it is stopping. Hard to believe that the IF chip or PLL chip could be bad in this small specific way, yet okay in all other ways, so the search goes to the small capacitors, resistors, solder joints in the bit of circuit responsible for stopping the search. Time for the logic analyzer.
     
  6. Chapmro

    Chapmro New Member

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    5
    Fixed the tuning problem -- it was the LM7000.
     

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