AU-999 weak and distorted channels

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

  1. DucatiDave

    DucatiDave New Member

    Messages:
    37
    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

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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:
    37
    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

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    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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    37
    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 AK Subscriber Subscriber

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    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
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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

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    2,779
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    Good stuff - its always better to hear about a quick fix than a complete disaster!
     
  11. DucatiDave

    DucatiDave New Member

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    37
    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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  12. DucatiDave

    DucatiDave New Member

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    I’ve set up a bit of a comparison, my 999 versus Eight versus Marantz 2245. I enjoy my bass and while the bass mods have made a considerable improvement to the 999 I wanted to see how it stacked up versus my other receivers of similar wattage. So after playing the same songs through all 3 rigs, the 999 comes in last in the bass department. Not surprisingly the Marantz is the bass champion with the eight a VERY close second, but ultimately I prefer the Eight over the 2245 due to the superior clarity. The 2245 might have too much bass which overpowers other elements.

    I know that this has been said before but the 999 really suffers from a lack of a loudness feature. I don’t have a dedicated sound proof listening space so many times I have to listen at lower levels to ensure family harmony. It’s at these low levels where the loudness feature really comes in handy on the Eight.

    Which brings me to my next question, what parts of the tone boards actually sets how much bass the unit puts out? Transistors, resistors, caps? The sequence in which they are installed? Would it be possible to rework the tone section of the 999 to resemble the Eight? I’ve also come to the realization that the 999 might not be the amp for me, no big deal, until you try something out you won’t know if you like it or not.

    Dave
     
  13. kevzep

    kevzep Its all about the Music Subscriber

    No, completely different. Its not as simple as you suggest, quite the opposite. You can't very easily make one amp sound like another, if you wan't the amp to sound like another amp, then buy the other amp.

    My work on the 999 was correcting what appeared to be design misgivings, not trying to make it sound like something else.

    Its a passive filter network on the AU999 for the EQ, we can shape the filters, and what I have done in that bass tone stack is as much as you want to go. After the mods the AU999 has a flat frequency response within 0.5db from 20hz all the way up to just above 8khz where it slowly rolls off, at 20khz its -2db.
    That is excellent for an amplifier of that era. The distortion is also very low, well below what Sansui specs it at full power.
    The one I did this morning was 0.06%THD at full power which was 63 watts per channel into 6.7Ω.

    For me personally, this is what works for me in an amplifier, other people want different things and thats all good...

    Oh and loudness controls? I never use them.....
     
  14. slimecity

    slimecity Super Member

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    2,779
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    I think if the 999 had "more" bass, it would sound weird. I like the sound of the 999 as being balanced and neutral.

    Do ensure that you have also done the stereofun mods to the output boards - ie: matching the hfe of the input transistors & dropping in some metal film resistors - as I found this also made a noticeable improvement on my unit.
     
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  15. DucatiDave

    DucatiDave New Member

    Messages:
    37
    Like I said before I am totally ok with admitting that this is not the amp for me, just curious if it could be tweaked to suit my needs before I sent it packing. What I listen to along with my listening style just does not jive well with the strong suits of this amp.
     
  16. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    That isn't unusual, and what you say - along with the mentioned lack of the loudness switch - really does suggest it isn't the amplifier for you. ;)
     

     

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  17. stereofun

    stereofun Super Member

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    No sin in not liking an amp, or rather, not have it fit with your rig and personal taste. You've done everything possible to give it a fair shot. Kudos.

    But....there might be one more thing. It border lines herecy, and it will mess with the core of the 999's soul. But because it is so easy - just one resistor -, and the results so noticeable.....it might be worth a try before you give up on an amp of such high build quality in which you have put in spuch effort.

    A few month back Smurfer77 did an extensive 999 overhaul, he noticed that the value of the feedback resistor changed as compared to the near identical 888. So in typical Smurfer77 style he rigged up a full battery of test with lap equipment and came to the conclusion that the value of the feedback resistor had significant effect on the sound signature. He tested 3 values, one that made the 999 sound like it's cap coupled siblings, warm and bassy, the stock 999 value, and then finally the 888 value that gave the 999 a tighter bass.

    I would normally provide a link, but on a smart phone now headed to work, so either PM Smurfer77 or look through some of his recent threads. The mod is documented to a very high standard. And since so easy might be worth a shot.
     
  18. DucatiDave

    DucatiDave New Member

    Messages:
    37
    I know the thread you are talking about, I will look it up and give it a try. I’ve already missed it this far, may as well go one step further, plus changing a resistor is no big deal and easily reversed.

    Thanks
     
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  19. bolero

    bolero Well-Known Member

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    I'll have to try those options out too, whenever I get around to rebuilding...thx for the info
     
  20. slimecity

    slimecity Super Member

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    2,779
    Location:
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    Hmm - this sounds worth trying for sure.

    I do find that with a lot of content with my 999, I need to tweak the bass up one notch.
     

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