Need digital tuner expert

Discussion in 'Tuners' started by audiomaniac2, Jun 23, 2018.

  1. audiomaniac2

    audiomaniac2 Member

    Messages:
    63
    Location:
    Modesto Cal.
    I'm working on a NAD 7140 that has a defective tuner. Unfortunatly I hav'nt found a schematic with voltages listed on it. The tuner will scan but never pick up a station on fm, it works ok on am. Test point 102 voltage never changes as it scans stations.This the variable voltage going to the tuner for changing capacitance in the different tuner sections.On the am i can see the voltage change on test point 101. Dual op amp (feeds tp- 101 and 102) Whats interesting is I can't read any voltage change on the input of the op amp with my volt meter as the stations are advanced up or down but I can read voltage change on the output of the op amp when it's in am mode.
    ic112 (upc4558) has the following voltages on it's pins.All voltages a positive to chassis ground.
    1- 1.31 volts
    2-4.99
    3-3.07
    4-0
    5-3.07
    6-5
    7-1.3
    8-34
    The big pll chip tc9147p chip drives this chip.
    Any good advice as to how to track down the fm problem would be appreciated.
     

     

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  2. rcs16

    rcs16 Super Member

    Messages:
    3,656
    Location:
    Baldwin, Ontario, Canada
    TC9147 will contain pulses on pins 34,35, IC112 is an integrator/LPF that will filter these pulses in order to create the DC tuning voltages.
     
  3. audiomaniac2

    audiomaniac2 Member

    Messages:
    63
    Location:
    Modesto Cal.
    That would explain why I didn't get a change in dc voltage on the input of ic112. So i hooked up my scope to pin 34 on ic 110 and on AM I picked up a good pulse .It was a quick upward spike.On pin 35 on Fm I picked up a weaker inverted pulse that looked different than the am spike pulse.I couldn't see any change in frequency or amplitude on the am pulse as I changed frequency. Am is working fine though.Thanks for the info. Do you think that ic 110 is defective?
     
  4. rcs16

    rcs16 Super Member

    Messages:
    3,656
    Location:
    Baldwin, Ontario, Canada
    Look at page 8 in the service manual, see if you can get the voltages as in steps 1-3. if not check the power supply voltages.
    it might be ic110, but I am not sure yet.
     
  5. audiomaniac2

    audiomaniac2 Member

    Messages:
    63
    Location:
    Modesto Cal.
    I found the problem. I pushed down on L6 to adjust it and the voltage on tp102 started working properly. The voltage on tp102 would come and go as i pushed down on L6. As I was pushing down it bent over and one leg of the coil came out of the solder joint . Now I need to figure out how to get to the bottom of the tuner board. When I got the correct voltage the fm did work.
    Thanks for your help I appreciate it.
     
    Last edited: Jun 24, 2018
  6. audiomaniac2

    audiomaniac2 Member

    Messages:
    63
    Location:
    Modesto Cal.
    I was able to solder L6 back in so the only problem now that I know of is, when you tune to a station after the receiver is shut of for a while it looses the last station you were tuned to and goes to a default station. I'm guessing that c213 may be defective it's the memory back up capacitor.
    It's interesting that this receiver only puts out 40 watts per channel into 8 ohms,yet it has parallel push pull output transistors. I'm guessing that's why it can take all the way down to a 2 ohm speaker load.
     
    Last edited: Jun 25, 2018

     

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  7. rcs16

    rcs16 Super Member

    Messages:
    3,656
    Location:
    Baldwin, Ontario, Canada
    Sounds like the memory backup cap as you say. Looked like a decent design when I glanced at it, glad to hear you fixed the problem. Not too many have paralleled o/p's at 40W/ch.
     
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  8. jlovda

    jlovda Things I loved from the 60's and 70's Subscriber

    Messages:
    3,400
    Location:
    NE Ohio
    If you are dealing with a memory backup cap, be careful. My Technics ST-G7 uses two "super caps," about 4 farads each. Before fiddling with them, bleed them off with a resistor. A short will cause a spark that will destroy a solder pad.
     
  9. audiomaniac2

    audiomaniac2 Member

    Messages:
    63
    Location:
    Modesto Cal.
    I'll make sure the cap is discharged first. This cap is only .047 farads (47000 mfd ) @ 5.5 volts.
     

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