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Sansui AU-X901 Protection Mode

Discussion in 'Exclusively Sansui' started by sajjad amer, May 18, 2018.

  1. sajjad amer

    sajjad amer Active Member

    Messages:
    255
    Location:
    Lincolnshire, United Kingdom
    I have purchased this amplifier in protection mode. I have followed the instructions below from this very forum, to adjust the offset and Bias after recapping the amplifier board and changing trimmers.

    the 5-pin plastic terminal,
    Connect the DC voltmeter between "out" and "out", on the each channel (those two points are derived vrom NPN-PNP emmiters of each channel)....adjust at minimum possible. Do it for both channels
    The adjustment trimmers are located on the power amp PCB left and right, on the corners.
    They are two: HOT-COLD and COLD - EARTH

    Pins 1 and 4 should be adjusted with hot-cold one
    If those values are OK (<500mV DC), then check between pin 1 and 3, and pin 4 and 3 (Ground) and, also adjust to minimum using the second trimmer (COLD -GND)

    At the time of less than 500mV, the relay should be released, and then proceed as per manual, if you want.



    1. the service manual is wrong in printing; you should measure between HOT (red) and chassis; put the DMM on the highish voltage selector, in volts range (e.g, 2V, or 20V). Some modern DMMs has also VU meter, similar to many decks, using LEDs or something like that.

    2. adjust DC between the HOT and GND and HOT/COLD; although the leakage is present in milivolt-range on the DMM, it can be done stable using volt-range mode.

    3. adjust bias, as mentioned previously; 8 mV is OK but only if that value is between central and any lateral leg of the big rectangular emitter-resistors

    If you are following exactly the service manual description, then you are measuring between two LATERAL legs on the rectangulars, and in this case, the bias should be 17-19mV (= 36-40mA, since 17/0,44 = 8,5/0,22=38,6mA (the later is between central leg and any lateral).

    4. All adjustments must be done with the volume pot at minimum. This is important, since we are adjusting only power amp, and we don't want the DC from the preamp section to be measured now. (This, different part of adjusting is done in the opposite side - volume pot at maximum, and adjusting some other VRs, which are usually on the PCB behind the front plate)

    5. recheck the DC HOT/COLD and HOT/GND

    6. recheck again the bias several time (no signal, no music after first warming up, just left it for some 15-30min in steady state), and recheck DC after any correction for bias. Adjust as necessary.

    However the amplifier is still stuck in protection mode on DBT. Would someone be kind enough to guide me to next steps?
    Many Thanks in advance
     

     

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  2. Beau Geste

    Beau Geste Rione Traiano Subscriber

    Messages:
    17,200
    Location:
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    Personally I never touch the trimmers after having replaced the capacitors and the final transistors. I did it once and I regretted it very much. The trimmers have the setting of the workshops of the manufacturer. They are necessary when the appliance is switched on for the first time and needs to be set up. After that they must not be touched, since the interventions are to replace the components only. In your case ,I think ,the final transistors are burned. :)
     
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  3. sajjad amer

    sajjad amer Active Member

    Messages:
    255
    Location:
    Lincolnshire, United Kingdom
    Thank you , I will start to remove them and check one by one and report back
     
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  4. Beau Geste

    Beau Geste Rione Traiano Subscriber

    Messages:
    17,200
    Location:
    Italia
    A work of simple replacement does not require a new calibration of the device because you have not altered anything, you have only replaced one piece with another identical. The new calibrations are necessary only for those parts that are subject to movement and which tend to go out of register over time.
    The problem you now have is to guess the original value of the trimmers. You can ask someone who has the same Sansui . If the problem persists, excluding the endings, the problem is in the protection circuit or in some faulty welding :)
     
  5. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    48,631
    Location:
    Hertfordshire, UK
    If you replace ANY components on an amplifier driver board, you should ALWAYS check the settings of the trimmers, so often the trimmers themselves have suffered with age and also need to be replaced, do not bother to clean them, replace them. If you are not sure about reading schematics ask here for instructions on where to set the new trimmers before installation, and how to set them when they are installed. If you simply guess, or blindly follow the manufacturer's instructions which often contain errors or are not appropriate for the newer trimmers, you will run into difficulties.
     
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  6. sajjad amer

    sajjad amer Active Member

    Messages:
    255
    Location:
    Lincolnshire, United Kingdom
    Thank you John, I am glad you replied as instructions I posted above in blue are actually yours when you were fixing your 701. I have followed your instructions to the letter as copied above. but amp is still stuck in protection. Kindly show me the path.
    Regards
    Sajjad
     
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  7. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
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    You must have excessive DC voltage between hot (red speaker terminal) and cold (black speaker terminal) do not concern yourself with Hot/Gnd or Cold/Gnd measurements at the moment, concentrate on Hot-Cold measurements. Note that either channel having excess DC voltage here will cause the protection circuit to activate. You can set bias when in protection mode but if there is a fault causing protection activation it may prevent correct setting of bias - AND the two settings can interact with each other and may require repeated (2-3 times) re-adjustment before the correct setting is achieved.

    You should be able to easily adjust Bias and/or DC offset while on DBT, if you can't and the voltages are stuck and won't move then you have a fault which must be rectified before proceeding.

    Remember to TURN DOWN the bias after having set it while on DBT, as when you give the amplifier full mains obviously bias will be higher - maybe too high, and possibly disastrously so.

    Coming out of protection mode while on DBT will depend on the wattage of bulb in the DBT and the current drawn by the unit (which includes bias current). Some units are reluctant to come out of protection mode with normal DBT bulb wattage - for this amplifier a 60W bulb should be fine. Think carefully before using a higher wattage bulb in the DBT attached to any amplifier with a fault, the duration of brightness of the bulb during the power on sequence should be your guide.

    I hope this helps.
     
    Last edited: May 18, 2018
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  8. sajjad amer

    sajjad amer Active Member

    Messages:
    255
    Location:
    Lincolnshire, United Kingdom
    I have noticed that on right channel voltage on 5 pin jumper is 34v DC and on left side it's 9v
    2sa103g gets extremely hot and I'm unable to reduce voltage on out,out pins below 1v DC
    The transistor in question checks fine on meter.
    On closer inspection the two resistors 560ohm R64 and 120ohm R66 are both burnt. I am waiting for parts but could anyone with experience tell me
    • if these burnt resistors could have fried any other component?
    • or if the resistors were fried due to failure of another component?
    Many Thanks
     

    Attached Files:

    Last edited: May 19, 2018
  9. sajjad amer

    sajjad amer Active Member

    Messages:
    255
    Location:
    Lincolnshire, United Kingdom
    Also on right channel protection board Q32 is snapped in half. I cannot find any data on this part, see attached images. Can any one please recommend a substitute. Thanks
     

    Attached Files:

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  10. bluesky

    bluesky Addicted Member

    Messages:
    7,608
    Fine amp!
     
  11. sajjad amer

    sajjad amer Active Member

    Messages:
    255
    Location:
    Lincolnshire, United Kingdom

     

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  12. sajjad amer

    sajjad amer Active Member

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    Anyone, please show me the light , never had this pin drop silence before, not that I am complaining :naughty:
     
  13. rcs16

    rcs16 Super Member

    Messages:
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    Location:
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    I always look for a datasheet first then try to match for a sub. 2SA115 is a very old number.
    I see it is a Ge bjt, something not right here, a AU-X901 using a Ge bjt, how can this be so?
     
  14. sajjad amer

    sajjad amer Active Member

    Messages:
    255
    Location:
    Lincolnshire, United Kingdom
    Do you mean that some one previously may have replaced these parts? The amp looked unmolested when I opened it.
     
  15. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    I am not in a position to help Sajjad at present as I am on holiday - Sajjad you have incorrectly identified the semiconductors you may need to replace. As rcs16 says 2SA115 is not a semiconductor that was ever used in your model of amplifier, and would not work if it was fitted by anyone.
     
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  16. Goldie99

    Goldie99 Well-Known Member

    Messages:
    792
    Location:
    The original Boston, UK
    Last edited: May 22, 2018
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  17. rcs16

    rcs16 Super Member

    Messages:
    4,076
    Location:
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    So it is a 2sa1115 and 2sc2603, as written in the service manual. You only need complementary pairs in push-pull type ckts.

    ksa1015/ksc1815 are close enough to be used as subs.
     
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  18. sajjad amer

    sajjad amer Active Member

    Messages:
    255
    Location:
    Lincolnshire, United Kingdom
    Thank you so much @Hyperion @Goldie99 @rcs16 for your help, I will go ahead and order these parts and report with outcome.

    to avoid any confusion I am talking about the single transistor picture in post no.9. as it says 115 on it I thought it is 2SA115, instead of 2SA1115.
     
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  19. sajjad amer

    sajjad amer Active Member

    Messages:
    255
    Location:
    Lincolnshire, United Kingdom
    Just to update, I have replaced the transistors as per above advice, but amplifier is still in protection mode. The new resistors at R64, R66 that I had replaced have also burnt again. I have removed the output transistors and 2 transistors out of 4 on right channel are short, so awaiting delivery of new transistors from CPC. Found original output transistors for just 3£, cannot believe it. I will keep updating you all with my progress.
     
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  20. Goldie99

    Goldie99 Well-Known Member

    Messages:
    792
    Location:
    The original Boston, UK
    Sajeed, I don't know this amp / topology at all, but if R64 & R66 are burning, and assuming the shorted outputs are Q61 & Q63, then you should probably be checking Q53, 55, 57, 59, and possibly even further 'back' if necessary - you have excess current through R64 & R66, so you need to find where it's coming from / going to, and fix that.
     
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