Sansui AU-X11 Restoration

Discussion in 'Exclusively Sansui' started by SanthoshA, Sep 21, 2017.

  1. SanthoshA

    SanthoshA Active Member

    Messages:
    138
    Yes the X11, is ready and i had it running for couple of hours yesterday !.
    All Bias and DC offset calibrations were re-done for the restored driver board.
    Adjustments were a breeze with the multi turn trim pots, and i am so glad i replaced all of them.

    The offset differences in AC voltage across the left and right channels, is now beyond my equipment's capacity to measure ;) .
    I have exactly the same AC voltage across the left channel and the right channel measured with a 1Khz sine wave. I am really happy as this issue got fixed during the restoration.

    I am going to give it some time to break in post which i will post my listening impressions.

    Overall , the differences are so huge that it cannot be compared to how it was sounding earlier before the restore. I would rather focus on how good the On Semi's sound compared to the original output transistors and probably do a RTA measurement of the speakers response apart from just providing my listening experiences/feedback.

    I am planning to do two measurements, one with On Semi and the other with the NMA1718 and NMC1718 output transistors, i do not have a good set of speakers, i have some very old vintage speakers, and any signature change induced by transistors can or cannot be the same for each person.

    Since i am using speakers as a reference, i believe we can only make out any peaks/dips in the frequency response graphs induced by changing transistors. To be frank i am also not sure if we will really see a difference in the frequency response with output transistor changes but i am just going to give this a try.

    I am now building a dummy load to test the amplifiers full potential, i have the ARCOL Non-inductive resistors , but i lack a proper heat sink to do the tests, the resistor can hardly hold till 60watts without a proper heat sink.

    A To-Do List:
    1. Check For Oscillation in output (Bias and DC Offset Stabilization also)
    Inspect sine wave across speaker terminals in the scope, and watch for distortion or clipping
    2. Mount original output transistors
    3. Fix PCB trace on one channel driver board
    4. There is still a minor restore pending on of the smaller PCB's F-3454
    5. Clean RCA sockets

    I might do these points after doing the load tests(apart from point 1 as it is mandate for any load testing), i am trying to localise a heat sink if possible in my area, but it is quite challenging to source one for the load testing.

    And the Metal Film upgrade is real worth the effort !
     
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  2. SanthoshA

    SanthoshA Active Member

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    I had a good technical discussion with one of my friends GKannan yesterday night, this was regarding the preamp sections and i was discussing on couple of load tests i had done on my ADCOM GFA535MKII amplifier yesterday night, the absence of a proper heat sink to dissipate the heat on load resistor when testing the X11 is still not ready.

    I finally ventured to dig deeper in the X11 to understand how its preamp/flat amp and how the jump switch mechanism works.
    Many might have already been aware of the discussed technicalities below, but i am just posting, as this was something i learnt while i worked on the restoration of the amplifier.

    Here are some points and significance's of the Jump switch that has been used on the X11/X1, based on my understand and i am open to inputs/corrections from fellow members.

    The X11 can be operated in various modes, with preamp enabled/bypassed by setting to external inputs 1/2 to use it as a dedicated power amp.
    There is also another way using which it can act as a simplistic passive preamp.

    The Jump Switch:
    This switch has more significance to it, than the text imprinted on the front panel, and there are more further reasons to clean the main big volume potentiomter, and it is extremely important to have it in good shape.
    The X11 has a simplistic basic passive preamp stage followed by the active preamp/flat amp section.

    1. With the Jump Switch set to on, the analog inputs fed to inputs of the amplifier are attenuated by the Dual Gang big volume knob.
    Typically acting as a simplistic passive preamp, whose output are fed to the power amp for amplification.
    Note: The flat amp/preamp board is completely skipped during this process.

    2. With the Jump Switch set to off, the dual gang front volume potentiometer, still controls the input signal acting as a simplistic passive preamp as discussed in point no.1 , but feeds the output to the flat amp stage for active preamp amplification, the output of the flat amp is then fed to the power amp/driver boards.

    With sources with higher preout voltages like mordern dac's or cd players one can set the jump switch to on, and use the X11 as a passive preamp(flat amp/preamp bypassed), if one feels that the preamp of the X11 adds its own signature or would like to avoid it in the chain, and still retain volume control on the amplifier.

    Just iterating there are different ways you can get the sound you like !. In case you have a good enough source that you dont want the X11's flat amp signature to add in or colour the sound signature you could always set the Jump switch on to bypass the active preamp/flat sections used in the X1/X11.

    Since input signal attenuation/controlling is controlled by the big dual gang main volume knob, it is best to keep in good shape or atleast cleaned during a restore.

    Meanwhile I did hear also the Sansui 9900A with a Bel Canto Power Amp, speakers were the Harbeth SHL 5, I am thankful to my friends who let me audition the combo .
    The preamp of the 9900A has a very very good analog sound signature with no digital foot print present . I could listen to the entire setup for couple of days with no fatigue at all even pretty moderate to high volumes . The 9900A is a series contender with the only down side of having a lower power output than the X11.

    I wish Sansui had included tone and bass controls to the X11 though I am a prefer of direct mode.
    Not all music is recorded in high definition for some old songs from the 80’s and 75’s that I listen to, have screechy highs and I wish these controls were present .
    When it comes down to music and listening preferences to each his own .

    Penned down my thoughts on the same, and i hope it is useful .
     
    Last edited: Apr 17, 2018
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  3. Ronito6

    Ronito6 Super Member

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    [​IMG]
     
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  4. ghamilton

    ghamilton Super Member

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    Location:
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    Now lets take a listen...

    Now lets compare it to a Luxman L-590AX
     
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  5. SanthoshA

    SanthoshA Active Member

    Messages:
    138
    Adding more points to the to-do list:
    1. The phono servo adjustments fail to reduce till 0mv, the max it reduces is till 30mv which falls in the range of 0 to 50mv, but this is something i need to look at to get the amp in perfect working condition.
    I was able to identify this problem thanks to Hyperion for confirming the issue.
     
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  6. Ronito6

    Ronito6 Super Member

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    1,153
    Just for the sake of clarity (and for anyone interested following this thread), what caused the issue?
     

     

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  7. SanthoshA

    SanthoshA Active Member

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    To be frank its still work in progress, i haven't ruled out the issue, but i will for sure update once i have a resolution.
    I will have an update in couple of days time .

    Also i faced the same problem(dc not falling to 0mv) in the other board F-3450, luckily it was a dry solder(no solder literally) and i have a picture of the same.
    F-3450.jpg

    I have boxed the specific areas in red if you zoom in the picture you can see it clearly, actually there is literally no solder left on that specific joint, i wasnt too careful with the phono boards as i don't have a turntable to listen too, else i would have caught this issue easily before mounting the board back. Pretty sad i missed this !.

    If you have a problem in adjusting voltages for board F-3450 its one the resistors lined up, or solder issues mostly,as for the other board F-3449 it is pretty challenging i haven't had much time to look at it yet.

    For board F-3449 you should easily be able to dial in to 0mv, it falls in place so easily.
     
    Last edited: Apr 18, 2018
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  8. SanthoshA

    SanthoshA Active Member

    Messages:
    138
    Power Amp Level Gain Potentiometers:

    I recently encountered a stray resistance in the signal path from RCA jacks to the driver amp board with power amp level knobs set to full, ideally this should not be the case, with the knobs turned fully what goes in should come out exactly.

    In order to diagnose this, the following approach was taken:
    The amp was set to external inputs mode, and a rca cable was connected to the rear power amp inputs(the jacks are self shorting) a cable must be plugged in.
    I then set the power amp level knobs to full, and measure the resistance between the signal pin of the rca jack and corresponding input to the respective driver board pre amp inputs.
    This yields a stray resistance of 62ohms and 72ohms for left and right channels respectively.

    This approach was taken to rule out the bigger preamp volume knob and negate influence of the flat amp board, in aux connections.
    Hence i preffered to use the external amp inputs.

    Ok so now the resistance was there, i diagnosed this back to the gain pots, and desoldered the pots to verify the same.

    The resistance measurements in the images shown below are for the stock pots when turned fully clockwise across pins 2 and pins 3 of both the potentiometers.
    IMG-7707.JPG
    IMG-7708.JPG
    Ideally this should be a short when the pots are turned fully clock wise.

    This how the readings on a modern pot from tocos/cosmos looks like.
    IMG-7705.JPG

    pots cleaned.jpg

    Pot disassembly was done, and as you can see there a small crack in the shape of a lightning bolt in the white plastic tab (pot on the right most).
    I have cleaned in throughly and this stray resistance does not go away.

    What is noticed is that when turned counter clockwise ( attentuate the power amp signal completely) the pins 1 and the wiper 2 yield a proper short.
    Unfortunately the taper is designed in such a way that this stray resistance gets induced when turned fully clock wise.

    The last 5 to 10 percent of the pot is pretty much out of sync and not exactly the way i wanted it to be, this will yiled a uneven sound stage to a certain extent when turned fully clock wise, unless or until you have the exact stray resistance for both the potentiometers.

    If the stray resistance is 62 ohms for both channels i prefer not to touch the pots and leave it as it is, in my case the difference is marginally but not perfect which i wanted .

    Cons:
    1. If the resistive difference is too much, there will be a imbalance in sound stage (when turned fully clockwise).
    Some times it happens due to resonance in your hall or room, or due to acoustical imperfections.

    In order to avoid issues with these pots, and their resistances and such, the following methods can be chosen to negate the imperfections.
    Feed a sine wave and gain match both channels using the power amp level knobs, and make sure not to fiddle with them as it changes the adjustments done with the sine wave.
    Or one can just set/gain match the power amp levels knobs just by using ears to get a even balanced sound stage and not tinker with them.

    The other more complicated apporach is to use REW or a DB meter and set the gains accordingly , i prefer doing it by ears through.

    The only hassle in adjusting the knobs is that they are extremely sensitive even a small 1mm change makes a big difference in the 75 to 100 percent clock wise rotation section.

    There are multiple methods that one can do and adjust them accordingly .

    The above are just the findings and could be useful, sometimes it is better to just enjoy the music and forget the imperfections.
     
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  9. SanthoshA

    SanthoshA Active Member

    Messages:
    138
    Thanks to John, i have also made some progress on the phono boards, two of the boards are now reading a better value for the servo adjustments.
    I had to replace two zeners, DZ1 and DZ2 on the servo adjustment board, these zeners are 18v.

    The heat sink builds for the load resistors are in progress and i have Mr.Amrinder from Audiocrafts helping me out, the heat sinks will mostly be ready in couple of days time .
    Heat sink 1.jpeg
    Heat Sink 2.jpeg
     
    Last edited: Apr 30, 2018
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  10. Ronito6

    Ronito6 Super Member

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    1,153
    Any chance for a picture of this kind of checking. (Visual learner.)

    ALSO
    Thanks for this great detective work!

    You are showing that just because a pot is clean does not mean that it is functioning 100% as it could or should.

    It am coming to an appreciation that their measuring correctly is the best way to judge a potentiometer's ability to remain in service.
     
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  11. SanthoshA

    SanthoshA Active Member

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    138
    I was pretty busy for couple of months, and i managed to work on the amp only yesterday night.

    Couple of parts have been replaced.
    1. Speaker Relays (2 Numbers)
    Speaker Relays.jpg
    Left Old Vs Right New Omron Relays

    While performing this work i also cleaned the speaker PCB board, rear which again had lots of oxidation and dirt.
    Speaker PCB Rear.jpg
    I re-did couple of solder joints and wiped the board clean with IPA, picture was taken after all the work was done.
    Speaker PCB.jpg
    Newly mounted relays need no mods, they just fit in to the already existing holes in the PCB.

    2. Binding posts ( i have used ones from Superior Electric which i had already )
    The inner circle of the binding posts were cut into a square shape that could be added into the rear panel grill.
    Binding post.jpg

    A close friend of mine Mr.Bhuvanesh in Chennai helped me in fabricating the binding posts to fit into the rear panel.
    Thanks to him i got the job done !.

    The whole rear panel has been mounted back in the amp, and i fired it up for a test run, it was running all good and fine.
    Rear Panel mounted.jpg

    I have cleaned only the phono rca sockets for now, i will continue with the work once i have some free time again.
     

     

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  12. Ronito6

    Ronito6 Super Member

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    Well past the mid-point. Looking good there.

    [​IMG]
     
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  13. SanthoshA

    SanthoshA Active Member

    Messages:
    138
    Guys just a quick question, i was trying to re calibrate the phono boards last weekend, and i noticed the following.
    1. With source set to aux(no inputs connected) , i finished the calibration for Equalizer Circuit adjustment and also the MC head adjustments
    2. I then changed the source to Phono1 or Phono2 and when i recheck the calibrations are no longer in place, and the values are extremely different compared to what was set earlier.

    Is it deal to set the MC and EQ Circuit calibrations with the inputs set to Phono 1/Phono 2(no inputs connected) and then adjust them, or should i be setting it to aux and then perform the adjustments accordingly.
     
  14. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Yes, do select the appropriate inputs when setting up MC & MM head amps. ;)
     
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  15. SanthoshA

    SanthoshA Active Member

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    138
    I have now removed the sacrificial outputs and put back the original Sansui transistors .
    There is though a difference in sound signature compared to the on semis .
    The on semis seem to have a bit of an edgy high frequency response retaining the warmth of the Sansui.

    Where the original Sansui NMA/NMC are more musical sounding and literally sound very very clean at higher volume levels . The edginess in the high frequency is no longer present and the high frequency response is more smoother and softer .

    Overall the original output are clearly better than the on semis .
     
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  16. slimecity

    slimecity Super Member

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    I would hope that is the case, given how much we hype these high-speed devices!
     

     

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