AU-999 weak and distorted channels

Discussion in 'Exclusively Sansui' started by DucatiDave, Jul 12, 2018.

  1. DucatiDave

    DucatiDave New Member

    Messages:
    30
    I have quite a few Sansui receivers, Eight, Seven, 1000a, and on this past Monday I picked up my first integrated amp, an AU-999. I gave it a quick test and it sounded good on the Aux with my iPhone, but it had some scratchy controls. So I took the covers off and proceeded to Deoxit all of the controls. After cleaning the controls, the 999 now has very weak and distorted channels.

    The highs and lows are almost nonexistent, while the mids come through reasonably well. Bass and treble controls have minimal effect on sound. At first I thought that the controls were at fault, so I cleaned and exercised them again with zero improvement. I can operate all of the controls and they work as they should, no crackles or anything like that. I am leaning on the 1208 filter board because both channels are exactly the same, and the filter, muting, and tone controls all have some effect on the sound, however minimal. I used a tone generator on my phone and was able to hear a faint tone at 200hz. It got steadily louder till about 1500 or 2000hz at which point it tapered off again.

    I recapped my 1000a, so I do have experience with troubleshooting and working on equipment. I know for a fact that the problem lies within the preamp section because I was able to use the preamp on my Eight with the 999 amp and the sound was excellent. Running the 999 preamp through the Eight amp resulted in the same weak and distorted sound.

    I have a good clean path from the aux inputs all the way to the 1208 board. All inputs have the same distorted sound, tuner, tape, phono, etc. I also ran jumpers directly from the aux input to the 1208 board, bypassing all of the switches, with the same distortion, which is why I am not leaning on the switches as the problem. I think it is just an incredible coincidence that this happened after using the deoxit. The only thing I find weird is that there are two signal pathways on the 1208 board, meaning that multiple components failed at once. I tested all of the resistors and everything came up good except for r712 and r725. The are supposed to be 1M ohm but they tested at 660k ohms.

    Transistors? Caps? Any help would be greatly appreciated. I do plan on doing the preamp mods, but I want to get it running well in stock configuration first. Doing mods on a broken piece of gear is not good, I want to start with a clean slate.

    Thanks
    Dave
     

     

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  2. slimecity

    slimecity Super Member

    Messages:
    2,586
    Location:
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    So this may be a dumb question but I presume when you did this you are only referring to the front-panel controls (and not including the trimpots on the power-amp boards to adjust DC offset and bias?)

    Otherwise start with the usual - work the speaker selector knob a few times - check all your fuses - the two on the back panel for the speakers and the four mounted under plastic covers on the chassis inside the amplifier. I have had one of these blow before (for the bias) which gave me distorted sound in one channel.
     
  3. DucatiDave

    DucatiDave New Member

    Messages:
    30
    I have not touched the trimpots. But I did clean the 3 pots on the backside for input gain. All fuses are good, but they are part of the amp section which I have already proven is good. Switches and selector knobs have been cleaned and exercised many many times to no avail.
     
  4. kevzep

    kevzep Its all about the Music Subscriber

    Ah right, I love a good AU999 Puzzle, I have another puzzle to solve at the moment, but I will be back...
     
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  5. Tom B

    Tom B AK Subscriber Subscriber

    Messages:
    1,307
    Location:
    Bay Ridge, Brooklyn New York
    See if you are getting the proper -25v and -27v from the F-1230 board. That voltage feeds the tone and filter boards. There are 3 wires (2 blue and 1 grey) from the transformer feeding that F-1230 board, but go to either 1a fuses, or some sort of fusable resistor or link before the board. See schematic shown as 30V 1a.
     
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  6. DucatiDave

    DucatiDave New Member

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    I will look at the 1230 board this afternoon.
     

     

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  7. DucatiDave

    DucatiDave New Member

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    Well....I feel very foolish and noobish. I found the problem. When I initially tested the 999, I unhooked the speakers and aux cable from my Eight, which is my living room system. I then took the 999 down to my workshop where I have my 1000a. After deoxing the controls I unhooked the speakers and aux cable from my 1000a and connected them to the 999.

    So, if any of you have a 1000a, you will know that the outside diameter of the ground on the rca jack is larger than normal and you really need to force the cable on. This in turn stretches out the outer ground portion of the aux cable....do you see where I am going with this? Anyway, when I used the stretched out aux cable on the 999, it did not make good ground contact, resulting in my distortion issue. I used a different aux cable and poof, problem is gone. Now I can move forward with the preamp mod by Kevzep.

    Dave
     
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  8. smurfer77

    smurfer77 Super Member

    Messages:
    1,383
    Location:
    San Francisco
    Yup, have been there! BTW, many older amps have those connections like on the 1000A, such as almost any Sansui before the AU-777. Those types of RCA connectors are referred to as 'spreaders'.
     
    Last edited: Jul 14, 2018 at 10:46 AM
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  9. kevzep

    kevzep Its all about the Music Subscriber

    Oh, I thought we were going to be on some expedition around the 999 to find the problem....

    well, good that you sorted the issue, I know those connectors too, they are on that CA303 I am working on too....
     
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  10. slimecity

    slimecity Super Member

    Messages:
    2,586
    Location:
    New Zealand
    Good stuff - its always better to hear about a quick fix than a complete disaster!
     
  11. DucatiDave

    DucatiDave New Member

    Messages:
    30
    I’ve since done the kevzep preamp and bass eq mods. It is sounding pretty sweet. I have the 999 hooked up to my OLA’s right now, but I have a set of AR-3a’s that I am rebuilding right now, and I can’t wait to pair them up with the 999.
     
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