1. Time for some upgrades in server hardware and software to enhance security and take AK to the next level. Please contribute what you can to sales@audiokarma.org at PayPal.com - Thanks from the AK Team
    Dismiss Notice

Sansui 3000a protect light

Discussion in 'Exclusively Sansui' started by The Fuxtor, Aug 26, 2018.

  1. dr*audio

    dr*audio Fish fingers and custard! Subscriber

    Messages:
    18,774
    Location:
    Cleveland, Ohio
    If you get 1V offset at the speaker output with loads connected, either the output cap is bad or it doesn't have one.
    I don't have the schematic in front of me. What you want for idle current is 20mA through the emitter resistor. To calculate what voltage you should read across the resistor, multiply 20mA x the resistor value. If the resistor is 0.47 Ohms, you should read about 10mV across it.
     
    Hyperion, tom3 and The Fuxtor like this.

     

    Please register to disable this ad.

  2. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,811
    Location:
    Calgary, Alberta
    Ok, thanks for the info. I guess I'll try adjusting my bias closer to specs and see how the DC reads under load.
    I really think this unit is worth putting some new caps into, so hopefully I can get around to doing that later..
    I will post afterwards
     
  3. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,811
    Location:
    Calgary, Alberta
    Thanks again for this information, I am curious how you came to the number of 20 ma? The manual shows 80ma measuring at +/- rail fuses, I am just trying to learn how this works?
     
  4. dr*audio

    dr*audio Fish fingers and custard! Subscriber

    Messages:
    18,774
    Location:
    Cleveland, Ohio
    20mA per transistor is an accepted good value for idle current that gives you a reasonable low crossover distortion and safe operating area for the transistor. When I worked at Harman Electronics we did a test of bias vs distortion and found that 40 mA produces minimum distortion. Above that, distortion drops a very small amount and the heat rises. If your amp has a good heat sink, 40 mA is a good set point, but if your amp has a small heat sink or uses the rear panel as a heat sink, or if the idle current tends to climb over time, 20 mA is safe.
     
    Hyperion likes this.
  5. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,811
    Location:
    Calgary, Alberta
    Thank you for this!
     
  6. tom3

    tom3 Super Member

    Messages:
    1,950
    Location:
    So. Ohio
    One thing I try to do when checking the current across the fuse is to hook up the meter with the fuse in place, then pull the fuse for the reading. Keeps from interrupting the circuit.
     

     

    Please register to disable this ad.

  7. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,811
    Location:
    Calgary, Alberta
    That's a good tip ... Still kinda a pain to do , I suppose I could measure in the specified voltage at the fuse holder and then check what the mv reading is at the emitter resistor and write it down for the future.... Dr Audio's info is probably very close anyways I would guess. Still have to get back to this and see if cleaning those pots and resetting the bias is gonna keep that that protect from kicking in....
     
  8. skippy124

    skippy124 AK Subscriber Subscriber

    Messages:
    1,623
    Location:
    Sydney Australia
    Hi,

    Another suggestion re measuring the current at the supply fuses, I have done this with good results

     
    dr*audio likes this.
  9. dr*audio

    dr*audio Fish fingers and custard! Subscriber

    Messages:
    18,774
    Location:
    Cleveland, Ohio
    Yeah, the resistor across the fuse will work, but you already have emitter resistors in the output stage that you can measure the voltage drop across to get the current, so why not use them? As my algebra teacher used to say, "That's like going to Chagrin Falls by way of Outer Siberia."
     
    The Fuxtor likes this.
  10. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,811
    Location:
    Calgary, Alberta
    Well found some time to open this one again, I discovered the emitter resistors have been changed from a 0.3ohm resistor to a 1ohm resistors. Ok figured I had set the bias way high at 25 mv but that's actually pretty good with what's inside now..
    Checked the capacity of the coupling caps and found one reading 1600uf and the other reading 1430uf.... Drifted quite far from the 1000uf they once were. No esr function but I could only imagine..... So think some new caps are in order!
     
  11. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,811
    Location:
    Calgary, Alberta
    Update, I received 6 - 1000 uf bipolar caps and used 3 caps per clamp. They fit perfectly ! Ran the unit for 3+ hours at high volume with some cheap surround speakers and all was good. Temperature seemed not too hot on the heatsinks so switched it to the 4 ohm speaker setting and ran the little speakers at a high volume once again....
    While this was in testing in the basement ( monitoring the volume from upstairs) I again came down after some time had passed and the plastic on the little surround speakers were literally melting!! Checking the heatsinks also felt much hotter then last check. Protection never kicked in ,which is good but now I'm concerned that perhaps I need to check some other components out? When playing the shit out of some small speakers is it normal for them to become hot enough to melt??
    I rechecked the DC at the speaker outputs with the speakers hooked up to provide a load, and was still reading the same measurements as before?? 1.2 dcv... ?
    EDIT:
    Ok, so doing a bit of reading, I realize I must have been clipping those poor little speaks to death! I also realize I may need to really dial in the bias and doing so will bring the DC into specs...
    As mentioned earlier on, I will have to check the DC before the coupling caps.
    Thanks again everyone for a ton of awesome info!!
    Charles
     
    Last edited: Oct 12, 2018

     

    Please register to disable this ad.

  12. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,811
    Location:
    Calgary, Alberta
    IMG_20181012_125521.jpg Well, I narrowed the DC to around 15mv!
    Very tricky as both vr pots adjust against each other regarding the bias but also DC offset! Anyways what I ended up doing being I only have one dmm at the moment, was to just tweak each pot trying to minimize the DC measures and adterwards checking the bias , discovered that it was quite equal between channels! Very close to specs.
    Got it hooked up and it's been playing nice for a good couple hours now and sounding great!!
     
  13. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,811
    Location:
    Calgary, Alberta
    Well I need to revisit this topic, I have been using this receiver as a background music machine and I have left it on for essentially all day for a few weeks now, but I came into the room the other day and that blasted protect light is back!! So to recap, I replaced the coupling caps in the service bulletin and adjusted bias which in turn reduced the DC offset to nice low values.. thought I was set? What should I be checking out now? Maybe the bias pots are flaking out or maybe I need to readjust the protect circuit?? Any ideas??
     
  14. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,811
    Location:
    Calgary, Alberta
    Just thinking, but is there much point in the protect circuit now that the receiver is now cap coupled? Could the protection be bypassed?
    I wil need to look at the schematic, maybe the protect circuit is misaligned...
     
  15. kevzep

    kevzep Its all about the Music Subscriber

    I doubt the protection is the issue, but you could disconnect it if you wanted to.
    What I am seeing here is the 3000A up to its usual tricks, by nature they are an unstable design (with some great design ideas mind you) and you'll be forever going around circles with it... I got sick of mine and actually gave it away.....
     
  16. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,811
    Location:
    Calgary, Alberta
    I think Dr.Audio changed one to a different design .. I am thinking the bias pots may be flaky, and I don't think proper replacements are available anymore.. really sucks as the tuner and sound on this guy is amazing! Not to mention it's in A+ cosmetic condition.
     

     

    Please register to disable this ad.

  17. kevzep

    kevzep Its all about the Music Subscriber

    Your one sure does look mint thats for sure, and yes they have an excellent tuner onboard.
    I would almost go so far as remove the power amp and slip something from another Sansui in there, they are are a nice looking unit and a shame the power amp is so unstable.
     

Share This Page