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SA-8100 protection circuit issue

Discussion in 'Pioneer Audio' started by KeithD, Jul 2, 2017.

  1. KeithD

    KeithD Member

    Messages:
    80
    Location:
    La Crosse, WI
    Well, that was interesting. Monitored voltage at pin 3. Initially it was at -35V. I started turning Vr1. Voltage was jumping around a lot, but did not really seem to correlate with moving Vr1. Over time the voltage came down, but still swinging a lot. Eventually after about 2 minutes it came down low enough to come out of protection briefly. Again voltage was swinging from -35 to +7, but would settle in a smaller range for as little as 3 seconds to 15 seconds. Then after about 5 minutes the voltage settled down below +/-1V, and it remained OUT of protection. :) At this point adjusting Vr1 did seem to have an effect, and things seemed stable.

    At this point, it really does seem like a temperature issue, such that things seem to settle down when components warm up.

    I really appreciate all the help you are giving me Pete! Any idea what is happening here?
     
  2. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    22,730
    Location:
    uk.. the middle bit
    the circuits take some stabilising after each small turn of the vr . so turn it ever so slightly and wait a few seconds then nudge it again if needed . get as close to 0v as possible .. it might be the vr is dirty and needs replacing ..
    things are looking good now ..
    try same thing with other channel vr2 and monitor pin 23 .
     
  3. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    22,730
    Location:
    uk.. the middle bit
    p.s you will find the zero crossing point when turning the vr you should see it go negative from positive as it crosses 0v .. it doesn't matter if its plus or minus just needs to be close to 0v .ideally under 30mv ..it may well get better as the circuits warm up .
     
  4. KeithD

    KeithD Member

    Messages:
    80
    Location:
    La Crosse, WI
    OK, was able to adjust pin 23 to +/-5mV of zero. Pin 3 varies more. I've gotten it to about +/-10mV of zero, but it will spike up momentarily to 20mV.

    I had done the idle and neutral potential adjustments previously and had both pins very close to zero, about +/-2mV. It seems like now the pins have a lot more variation, especially pin 3.

    HOWEVER, it has not gone back into protection mode after about 15 minutes or so. When I switched it off to change pins with the DMM, it came out of protection as normal after turning it on. So..... I'm really hoping that replacing the 726s on the power amp board and that one resistor solved the original problem. However, I agree Pete, Vr1 is jumping around more than it seems it should an may be dirty. Can that be cleaned with deoxit or something instead of replacing?
     
  5. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    22,730
    Location:
    uk.. the middle bit
    try deoxit . if it works all is good .
    best to check idle bias next .. i dont have the manual here just the schematic .
    will be back shortly with bias adjustment .if you have the instructions already follow those .
     
  6. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    22,730
    Location:
    uk.. the middle bit
    first measure voltages from pin 2 to 34 and 31 to 28 . expect around 25mv maybe higher .
     
  7. KeithD

    KeithD Member

    Messages:
    80
    Location:
    La Crosse, WI
    The manual does have the procedure for setting the idle and Neutral voltage. I'll type it here since you don't have the service manual:

    1. Remove output load. Terminate input terminals of power amplifier unit with 4.7k resistance. (others have said this is unnecessary)
    2. Turn VR1 and VR2 to approximately center position. Turn VR3 and VR4 completely counterclockwise. Then turn on power.
    3. Allow 10 minute warmup. Then adjust VR3 so that voltage between terminals 1 and 2 becomes 20mV (I see pins 1 and 34 are connected, so that is the same as your instructions)
    4. Adjust VR4 so that voltage between terminals 24 and 25 becomes 20mV (I see pins 24 and 28, 31 and 25 are connected, so that is the same as your instructions)

    It then has you adjust neutral potential using VR1 and VR2 measuring pins 3 and 23, respectively with reference to ground.
     
  8. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    22,730
    Location:
    uk.. the middle bit
    go ahead and do the adjustments . unless you already did .
     
  9. KeithD

    KeithD Member

    Messages:
    80
    Location:
    La Crosse, WI
    Made the adjustments last night. Turned it on this morning listening to a CD and all is well. 25 minutes and it has not fallen into protection, so looks like the original problem is solved! Thanks again for all the help. It appears the 726 transistors on the power amp board and/or that one bad resistor were the issue. Just curious as to why I was getting such high voltages on pin 3 after I replaced the transistors and the one resistor? Is there a "burn in" period on these transistors and/or the resistor? As I mentioned, it took about 5 minutes or more for the voltage to settle down and come out of protection.
     
  10. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    22,730
    Location:
    uk.. the middle bit
    it should come out of protection in a few seconds .
     
  11. KeithD

    KeithD Member

    Messages:
    80
    Location:
    La Crosse, WI
    It does now, but yesterday when I was initially measuring the voltage on pin 3 and adjusting VR1, it started out at -35V and then over the course of about 3-5 minutes it came down to less than 1V and came out of protection mode.

    Now it works fine and as expected, so I was just curious why the voltage on pin 3 was so high and came down like that. In the tests I did after putting in the transistors, I only left the power 5 or 10 seconds and then shut it off since I did not want to potentially damage something when the amp would not come out of protection. Maybe, if I had left the amp on longer for all those tests it would have eventually come out of protection, as it did yesterday.

    Not a big deal, just want to learn as much as I can.

    Thanks,
    Keith
     
  12. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    22,730
    Location:
    uk.. the middle bit
    maybe old capacitors charging up
     

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