Help with FM Signal path circuit debug needed Technics SA-800

Discussion in 'Tuners' started by rocknroller, Jan 11, 2018.

  1. rocknroller

    rocknroller Active Member

    Messages:
    374
    Ok, using only the parts listed in the later rev I gets these voltages

    SA-400 regulator With voltages.jpg

    Still no voltage reduction. My assumption is that Q751 is bad, given the input voltages are correct, some of these passive components are brand new parts and the others measure fine. It's obviously not reducing voltages like it should. I don't have a replacement on hand. Will have to order.
     

     

    Please register to disable this ad.

  2. amptramp

    amptramp Active Member

    Messages:
    396
    Location:
    Mississauga Ontario Canada
    D751 is either a 14 or 15 volt zener and if it goes up to 21.3 it is open. Q751 may be shorted since the emitter voltage is above the base voltage.
     
  3. rcs16

    rcs16 Super Member

    Messages:
    3,800
    Location:
    Baldwin, Ontario, Canada
    I believe a OA99 diode is made from Ge, so it will measure 0.22.
    if the zener is fine then it should regulate at its breakdown voltage.
     
  4. rocknroller

    rocknroller Active Member

    Messages:
    374
    But if Q751 is actually shorted, wouldn't there be 21.3v at the zener anyway? Not because it's failed, just because of the transistor short?
     
  5. rcs16

    rcs16 Super Member

    Messages:
    3,800
    Location:
    Baldwin, Ontario, Canada
    Remove Q751, does the zener regulate?
     
  6. rocknroller

    rocknroller Active Member

    Messages:
    374
    Replaced Q751 with a new part and simplified the circuit to the later production design - no change. Base and emitter voltages completely wrong. I guess it's possible that something post Q751 emitter is providing that voltage instead, but the circuit touches so many things/others circuits past that point it's impossible for me to tell. I've triple checked all values in this part of the Q751 circuit. All in tolerance, the cap was already replaced and the diode checks out on a diode tester (.665/OL). I am stumped. The voltage coming in the circuit is correct but not when it leaves the Q751 emitter.

    SA-400 regulator With voltages.jpg
     

     

    Please register to disable this ad.

  7. Punker X

    Punker X Moderator Moderator Subscriber

    Messages:
    3,976
    Location:
    Panic in Detroit
    Double check you have Q751 oriented correctly, check continuity between all the components. Possible lifted or open foil runs or possible solder bridges.

    Did you replace D751? Emitter voltage should be clamped by the zener to the 14 ish volts.
     
  8. rocknroller

    rocknroller Active Member

    Messages:
    374
    I didn't have a 14V zener to replace with. I had a 13v and that didn't change anything when swapped. Same readings throughout (I also have a 15V but it think at this point it is a waste of time). The solder board paths are clean under high magnification and everything checks out when measured for continuity and ohms. Q751 is oriented correctly, as was the original. Every single component in that circuit has been replaced with no impact to the problem. The voltage must be coming from something else downstream but it's too difficult for me to diagnose further. At this point, I think I'm going to call it and direct my energy to another more valuable piece that has a more practical chance of being restored. This unit has several other issues anyway and wasn't worth pursuing unless the FM section was working first.
     

Share This Page