Sansui AU-111 rebuild

Discussion in 'Exclusively Sansui' started by smurfer77, Jan 24, 2018.

  1. smurfer77

    smurfer77 Super Member

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    1,331
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    Well breaking with my tradition of chronological threads, here is a sneak peak at the final result. The sides are very much hammer finish silver-grey in person. The hand holds were not something I wanted to attempt to make myself. I fold that aluminium myself and it took some planning to get thing right in terms of fit. I toyed with more holes, but there is method to the position and airflow so I didn't play with this. i will post more details later, when I get through the electronics in this thread.

    [​IMG]
     
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  2. craggd

    craggd AK Subscriber Subscriber

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    400
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    NorCal
    Very nicely done. If I didn't own one I would assume it was stock. Particularly impressive that you folded it yourself!
     
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  3. smurfer77

    smurfer77 Super Member

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    1,331
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    Thanks. i was really trying to keep it original as possible since it is such an iconic amp which is becoming rarer. I thought about attempting to recreate the hand holds but it was out of the question without investing in a more serious metal sheet break (folding device), as the thickness of the metal I used is not really sufficient to pick the amp up by... that unit is heavy.
     
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  4. _mano

    _mano Active Member

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    427
    Location:
    London, UK
    Excellent, brochure quality, well done.
     
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  5. timmatt

    timmatt New Member

    Messages:
    2
    A thing of beauty. I've stared at mine for hours. DSC01929.JPG

    With the bottom cover removed, what initially looks like a rats nest, becomes a thing of beauty if you stare at it long enough.
    This is a 'before' picture, taken over a year ago. I've hauled this thing around for 35+ years -- time to fix it -- especially after I've read so many good things about it. I had never heard its full glory.
    Before repair it had very noisy pots and switches and no bass on the right channel. There's no evidence of any prior service -- all of the original soldering inspectors marks (red or blue ink) on the solder joints are intact -- they disappear if the joint is resoldered.
    I have given the inspectors names -- Inspector Red and Inspector Blue.
    (Note: A very long time ago, during breaks from school, I worked in a factory on Long Island, building power supplies. Solderers dressed their connections out and placed components so that inspectors did not have to move stuff around in order to check them. -- Explains the way this chassis is wired -- all of the components and connections are visible without moving stuff around. Also, in our factory, Inspectors had X-ray vision and could see through lead/tin solder and reject a connection for broken strands in a wire even though it was not visible to us mortals.)
    1-DSC01389.JPG

    When I had rescued the amp from the curb I was with a co-worker, who insisted that I stop and for this "big black box with silver knobs all over it". I was an 'on the road' service tech at the time & if I stopped for every pile of junk I passed I'd never get any work done. So, heavy -- it even has handles and wheels...
    Too heavy for me to carry it like it was a case of Bud long necks. (How do you spell sciatica?)
    Where there's a will, there's a way... Here's an extra handle -- I only used one on the variac-isolation transformer-current limiter thing that I built. I probably should have it welded...
    DSC02039.JPG

    Ok -- all kidding aside(maybe) I've replaced all of the oil filled caps in the power supply and bottom of the amp - except the 2 ceramic ones that protect the power switch contacts(wired to the outlets on the back panel) and the one on the voltage selector that , I think, keeps the fuse from blowing due to transformer remanence during power up. ( or maybe it keeps my curtains from catching fire when I yank the power cord out of the wall socket.) I think there were 49 caps on the bottom, four big ones on top, 10 more to go on the front panel.
    DSC02017.JPG

    After the power supply caps, I wasn't sure whether I should troubleshoot and find the problem, or just go for broke and start replacing caps -- I can't find my scope, haven't seen it since before my last move. I knew the tubes were good. I had isolated the problem to the pre-amp. I changed the 4 power tube coupling caps first, they were all no good - One was double the value it should have been, one was open, and two measured at a couple hundred picofarads. Surprised that a had good noise from the left and anything at all from the right.
    So, from there I started with the last 12ax7 and worked backwards replacing caps one at a time, then power up and test, and then replace its pair and test again. Each time verifying that it did not get worse. Changing C56 resolved the issue with the right channel, initially sounding edgy and raspy -- I played this -- -- and the miracles began -- sounded horrible at first, but after a few minutes I was in heaven.
    Sitting curbside for this music should punch you in the chest, but for enjoying it on youtube - hey, the clowns with the basses are in the fifth or sixth row - need the loudness and presence switches turned on, the bass filter off, bass tone switch set @ 500 c/s, treble @ 2.5k c/s and tone controls at 1 o'clock. I'm driving a set of Wharfedale W70E's.
    I think my bass filter switch(Sw6) is wired upside down -- no marks from Inspector Blue or Inspector Red on the front panel. Found this vid on youtube -- -- He goes through the switch functions. At 1:36 he flips the bass filter switch on, but the filter goes off, then at 1:41 he turns the switch off, but the filter is on. Mine was like that, seemed wrong, so I stared at the schematic for too long and then changed it, so it is now consistent with the action of the treble filter switch.
    I continued to work backwards through the pre-amp, and then the pre/middle amp and bias sections. Powering up after each pair of caps to make sure I didn't screw it up, each time playing the same music. Lots of improvement in volume and quality as I went forward. I've let some of the caps age in for several hours --same music, and this one -- -- with a bit more bass reverberating off the buildings - do my caps need a workout?? Not sure if they'll continue to mellow(gradually??)
    I'd probably not want to use the loudness or presence features at all for properly recorded music -- what those switch settings do, seems excessive unless you are playing your 78rpm blues records and need to pump 'em up.
    I'm done with the bottom for a while, so i put the bottom cover on. Who knew there were holes to adjust the hum pots? DSC02014.JPG

    I powered it up -- all is well -- remarkable that I can hear no 'on' noise -- you know that presence from the speakers that lets you know the tubes have warmed up -- this amp is absolutely quiet - if there is no input, there is absolutely no noise at all.
    Same music, same switch positions & things started to mess up -- the 2 watt resistors in the middle section moved when they got hot and touched bottom, intermittently. A few scary moments, but easily resolved -- and let that be a lesson to me. (Wires get longer when heated)
    There are some clear and present dangers in this amp - it clearly would not have received UL approval, even by the standards of its day. The 2 watt resistors in the power section move when'd they get hot and can rub against each other -- potential smoke release? My resistors test ok but the glaze has deteriorated to the point where the outside surface is conductive and if they touch, bad things could happen.
    Also, insulation cutting, abrasion & breakdown is another clear danger. Wiring in the amp is routed through holes and around corners in the sheet metal without grommets or anything to prevent the sharp metal from abrading through the insulation.
    The wiring should be laced into a wiring harness they way Johan P. did it or some other means of preventing strain, abrasion or cutting of the insulation.
    1-DSC01389-001.JPG AU111_005.jpg

    OK -- enough for now -- I have a thumb thing from screwdrivering too many screws -- must rest now... more thoughts later.
     
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  6. smurfer77

    smurfer77 Super Member

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    Location:
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    Thanks for the interesting story. Those coupling caps like C56 are common issues. What is lucky is that your failing output coupling caps didn't go short circuit :).

    I agree there are some safety issues here, and I did a fair amount of rerouting as I did my recap. I spent a lot of time thinking about what was too close to the chassis or could move to the chassis... it's a bit scary in there. I repositioned quite a few things and also added some insulator in a couple of places. I also had the issue with the slightly cooked resistors in power supply with resistors losing there coating. I also found several drifted resistors in other sections....

    In terms of dead caps the main issue I had was a leaky mica in preamp that was easy to find as I had a 'pop' when turning tone defeat on and off. I found those brown rectangular mica caps to be problematic in other amps from this era.

    Will do a full write-up when i get time.
     
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