SA-8100 protection circuit issue

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

  1. KeithD

    KeithD Active Member

    Messages:
    269
    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

    Messages:
    23,808
    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

    Messages:
    23,808
    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 Active Member

    Messages:
    269
    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

    Messages:
    23,808
    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

    Messages:
    23,808
    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 Active Member

    Messages:
    269
    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

    Messages:
    23,808
    Location:
    uk.. the middle bit
    go ahead and do the adjustments . unless you already did .
     
  9. KeithD

    KeithD Active Member

    Messages:
    269
    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

    Messages:
    23,808
    Location:
    uk.. the middle bit
    it should come out of protection in a few seconds .
     
  11. KeithD

    KeithD Active Member

    Messages:
    269
    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

    Messages:
    23,808
    Location:
    uk.. the middle bit
    maybe old capacitors charging up
     
  13. KeithD

    KeithD Active Member

    Messages:
    269
    Location:
    La Crosse, WI
    Well, after working perfectly for a couple of weeks, I again have the issue of the protection circuit tripping for a few seconds after having the amp on for a few minutes. Not every time, but once in a while. A new random problem has also appeared. Right channel is almost non existent. If I turn the volume way up, I can get some sound. This has happened once or twice in the past few days. The last time, I turned the amp off, and then back on, and the problem disappeared.

    Anyway, my thought is to continue with replacing the capacitors and transistors Matt originally suggested in post 27 on the protection circuit board, and do the two 1451s on the power amp board. Any thing else I should be looking at?
     
  14. QSilver

    QSilver Super Member

    Messages:
    1,093
    Might be worth looking into replacing the protection relay.. or at least cleaning its contacts.
     
  15. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    1,199
    Location:
    Victoria, Australia
    As suggested earlier on you will still have to figure out whether it is tripping the protection circuit due to high dc offset (protection circuit doing it's job) or whether it is a faulty component on the protection board itself. If it was mine l would rebuild the protection board (not a big job) and replace all of the 1451's, 726's (1312's if it has these) and small blue sanyo caps etc on the main amp board at a minimum. As QSilver suggested also check relay contacts.
     
  16. QSilver

    QSilver Super Member

    Messages:
    1,093
    The 1451's will eventually blow up the amplifier if left unchecked. One of mine arrived with a blown 1451, driver transistor, burnt out idle current trimmer, driver emitter resistor, blown outputs and blown output emitter resistor. So really, they need to be swapped over to the KSC3503's. The 756's get changed to KSA992FBU's and the 1312's are changed for KSC1845FTA's. But double and triple check the emitter collector base positions as they DO change compared to the old transistors.
     
  17. KeithD

    KeithD Active Member

    Messages:
    269
    Location:
    La Crosse, WI
    I already replaced the 726s on the power amp board with 992s. There are two more on the control amp board I'll need to do. I'll replaced the 1451s as mentioned, I've got the 3503 replacements. I've also got the parts to replace all the caps on the protection board and Q1, 2, 6, and 7. I don't exactly see where the relay is on the protection board. I replaced the speaker protection relay on my SX-780, but I don't see a similar relay on the 8100. Where is it, and what is the replacement part if I need to replace it?

    Thanks!
     
  18. KeithD

    KeithD Active Member

    Messages:
    269
    Location:
    La Crosse, WI
    OK, just located the relay on the schematic and in the service manual. Looks like it is next to the large electrolytic caps, so I'll take a look at that.
     
  19. QSilver

    QSilver Super Member

    Messages:
    1,093
    That's it. I think its an Omron MY4 or something similar... but it might screw into the chassis from underneath on a bracket that is part of the relay....
     
  20. KeithD

    KeithD Active Member

    Messages:
    269
    Location:
    La Crosse, WI
    Ok, here's the status. I was able to replace the 1451s on the power amp board, 726s on the control amp board, and all caps on the protection circuit as well as Q1, 2, 6, 7. Everything seemed fine. Tested at a few points during the process and each time when I switched on the amp came out of protection as expected. I cleaned the relay with some deoxit and then contact cleaner. Put things back together and turned on the unit to warm up for 10 minutes to then go through setting the offset and idle current. The amp came out of protection as it should. I went off to do a few things in the next room. When I came back after 10 minutes it was in protection mode. I followed the manual procedure to set the idle and neutral potential thinking maybe that would bring it out of protection. All voltages set within specification, but it never came out of protection. I did measure the voltages coming out of the protection unit going to the relay. Looks like pin 14 should be +8V, but it was measuring +34V. Pin 15 was also +34V, but that's what it is supposed to be according to the schematic.
     

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