Sansui AU-D7

Discussion in 'Exclusively Sansui' started by gpounce32768, Jun 15, 2017.

  1. gpounce32768

    gpounce32768 AK Subscriber Subscriber

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    I've been wanting to try out fixing up a Sansui for some time, I finally came across one that I liked; missing volume knob, amplifier issues, dead lamps... perfect!

    I'll make a new knob in the shop, no sweat. Inside its a little dirty but the left channel has different driver transistors and some scorch marks on the board so definitely some trouble buried in there. The amp circuit is a lot fancier than other units I've worked on, and the absurdly high damping factor of this unit (200) caught my eye.. so we'll see how it goes as I dig in.

    1st step is to give the unit a bath & deox the controls. Then evaluate the amp section, check out the damage and see about getting spec power transistors for the right channel- or appropriate upgrades for both channels. As I go in, I'll check capacitors etc and if any are sketchy or have drifted particularly then I'll recap the set.


    20170615_223923.jpg


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    Last edited: Jun 16, 2017
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  2. leesonic

    leesonic Hold on, here comes the bass. Subscriber

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    That I've gotta see!

    :lurk:

    Lee.
     
  3. woofmytweets

    woofmytweets AK Subscriber Subscriber

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    looks like your power transformer sustained a direct hit. i hope it can still transform
     
  4. gpounce32768

    gpounce32768 AK Subscriber Subscriber

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    Short of the armature being crushed or broken not much to is going to hurt a transformer unless the coil is damaged.... I suppose the case got banged around a bit- whatevs I'm not into it for the looks. Seller reports the unit powers up. Knob will be aluminum, I'll try out a different # of flats on it than I did for the Kenwood I did previously, not concerned about making it look completely original. Maybe I'll try color anodizing so it sort of matches colorwise.
     
  5. LBPete

    LBPete Rolling Along Subscriber

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    Nice Amp. Please post the serial number for the database. Click on the link in my signature block.

    - Pete
     
  6. Overundr1

    Overundr1 AK Subscriber Subscriber

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  7. gpounce32768

    gpounce32768 AK Subscriber Subscriber

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    Left channel damage.. looks dramatic :) the little board with the 6 thermal compensation resistors isn't burned as far as I can see - its smoke from the burned up resistors. Going to clean all the crud off and use silpads instead, I really don't care much for micas and thermal paste. The board layout and mounting is not very convenient for servicing either.. kind of a nuisance to work on this amp.


    20170617_215014.jpg
     
  8. gpounce32768

    gpounce32768 AK Subscriber Subscriber

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    The left channel is a shambles. .33 ohm double emitter follower resistor open on one side, drifted on the other. The intermediate driver follower resistors also all burned, one of the larger ones got so hot the leads torched out the thru holes and half melted the 33pf styrene feedback cap. My guess is this is a botched repair that cooked things even worse than the initial failure. As found, a .22 ohm emitter follower was tacked onto the foil side of the board, connecting the open half of the double emitter resistor- perhaps that was the original failure.

    Replacing all the dead resistors with 1% metal films, 1% modern film cap for the feedback- and will replace the same on the right for symmetry. Replacing all 4 driver transistors, putting in FJA4310/FJA4210 instead of 2SC2581/2SA1106. We'll see if any of the other intermediate driver transistors are blown later on when I do the dim-bulb powerup.

    The case splits into front/rear halves. If the rear half is detached to make room for the speaker output board to move around, and the audio input cable unsoldered from the amp board it can be more convieniently serviced.
     
    Last edited: Jun 19, 2017
  9. gpounce32768

    gpounce32768 AK Subscriber Subscriber

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    Wow this unit is a mess.. This amp topology uses 2 intermediate transistors before the driver on both sides of ground so 6 devices post-bias per channel. On the left channel all 6 were blown, one of the intermediate devices was cracked- the amp must have gone up in an explosion lol As found it had a 7 amp fuse installed- spec is for 6- I am running a 3amp test fuse which is not blowing.

    I replaced all the 6 devices on the left side but theres still something in a more or less dead short, both channels still have rail coming out, starting about at the bias networks. I have been systematically testing additional transistors, diodes etc so far they are good. Diff amp voltages look plausible, so maybe the trouble is in between the diffamp and bias/compensation circuit. Next I'm going to decouple the amp primary and driver sections, see what happens downstream. Each channel has 2 +/- supplies, one for the 1st stage the other for the power stage, its unclear which is loading the power supply. DMM tests of the power side rails on both channels indicate continuity thru power supply RC network so I think there is no short there. I've fiddled a little with bias, right seems to respond to the adjustment something still going on with the left though at least there isn't rail coming out of the bias testpoints now lol.

    I test by briefly powering up on a 30watt dim bulb which holds intermediate bright, long enough to measure something then off before something cooks. The short is bad enough to brown out the power supply- but that side of things looks OK so far.

    The amp board is kind of annoying to test but I have a system now; to get at the parts side the power transistors have to be detached from the heatsink but then board swings up easily. I don't test it in free space, so after a mod I screw the amp board back down- very light torque on the mounting screws for fear of wearing out the silpads.
     
  10. gpounce32768

    gpounce32768 AK Subscriber Subscriber

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    I pulled the resistors that bring bring the differential signal into the power stage of both channels which cleared the dead short problem- power supply circuit checks out and the dim bulb only glows faintly now.

    I replaced the resistor on the right channel, power situation remained OK and I got the bias and 0 voltage in spec and the protection relay clicked in... progress- left is at 0v out since there is no bias. Those little variable resistors look nice but not worth much compared to 10 turn pots, so once I get things fixed I'll replace all of them as well as the intermediate drive transistors on the right so both channels match.

    Left is still out, but all the transistors check ok, as do the documented voltages so I'll hook up the sections again do some more debugging.
     
  11. gpounce32768

    gpounce32768 AK Subscriber Subscriber

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    Got both channels working and out of protection. The left had another bad coupling resistor and 2 shorted diodes in the bias circuit- appearance of dead short was due to too low voltage drop over the 4 series diodes so the power stage was driving at idle. Both amps now have plausible bias and zero voltage adjustments in, but still on the dim bulb. I had to move up to a 50watt bulb to avoid browning out the supply, 30watt bulb doesn't provide enough juice for the protection relays to click in. 50watt bulb lights dark orange- idle current on this amp is higher than on others I've done.

    Next is to put in matching intermediate transistors on the right channel to match the new ones on left. Subbing KSA916/KSC2316 for 2SA850/2SC1735 and MJE15032/MJE15033 for 2SC2581/2SA1106. The MJE's are overkill but I have a number of them handy- gains are reasonably close for both subs and no trouble getting the bias/center adjustments in. The 916/2316 sub have the reverse lead order so the orientation is flipped compared to OEM devices but at least the leads don't need to be twisted. The blown diodes were 1S2473, subbed in 1N4148.

    So the destruction on the left channel was comprehensive; post-bias, all 6 transistors, 2 diodes and 8 resistors variously open or shorted.

    I replaced VR1 and VR2 single turn pots with multiturn units on both channels. Adjustment is much easier now. I found the OEM pots were very touchy for bias and center. For VR1 center voltage pots, its helpful to orient such that the screw is closer to the heatsink, makes reaching it lots easier. OTOH I had latitude to do that since I did not have drop-in multi-turn pots, aux leads and bending etc was needed so I was free to locate them conveniently.

    While messing around with the new pots I found the protection circuit is quite precise, much over 20mv on either channel will put the system in protection. Its gratifying to have a meter on one of the channels bringing in the center, and have the protection relay click in as the voltage comes into range. Of course I'll do the adjustments once again when off the dim-bulb but at least the amp is looking sane now.
     
    Last edited: Jul 5, 2017
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  12. BLAH BLAH

    BLAH BLAH AK Subscriber Subscriber

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  13. gpounce32768

    gpounce32768 AK Subscriber Subscriber

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    Not a glamorous shot, but the amp is working now in my junky bench speakers. Playing Mott the Hoople, Yazoo, Erasure today. Not doing quality or power checks now, just glad to hear it alive. Will do a long run on the dim-bulb to see if any of my film resistors go up. Sansui put in 1 watt parts in place of the smaller ones called out by the schematic. I put in double the schematic power rating for them so they are smaller than the replaced parts.. so we'll see if any cook.

    All the controls work, sound is fine- no naughty behavior and nothing obvious is overheating. I can kind-of hear the tone controls though I don't think theres much point in any critical listening over my p.o.s. bench speakers.

    I recapped the amp board for kicks while I finished to matching up the transistors since I'd rather not unfasten the drivers from the heatsink AGAIN. The 6 subbed transistors on each channel make for a bit more bias so I think I'm going to change values on the bias adjust pots so I can get it lower- on the mains minimum bias setting leads to high readings (60-90mv).

    One lamp was blown on the power scale, and one on the input switch indicators. I made up a double white led replacement for the latter, fits well I may make up replacements for the other lamps unless I can find the right kind.


    (on edit)

    I upped the VR2 bias pots to 5k which provide plenty of adjustment to bring the bias voltage into spec with the unit warmed up on the mains. We'll see if a feedback resistor adjustment is needed later on.





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    Last edited: Jul 6, 2017
  14. gpounce32768

    gpounce32768 AK Subscriber Subscriber

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    The amp has been acting well so I did some work on the lamps (camera exaggerates brightness). Then hooked it to my quad system test speakers (disposable but somewhat nicer than my bench spkrs), then brought the noise. This time Tim Armstrong, cranked moderately;

    20170707_173528.jpg

    Input selector lamps are now all converted to dual white leds, right meter lamp is going dim for some reason- going to pull that bulb and replace it. The isopropyl bath left white reside between the clear bezel and the front panel which I've not been able to wash out, so thats kind annoying. The new volume knob is half made, going to work on it this weekend.

    Sound-wise the system is all there, has that sense of unlimited headroom that I first noticed in my Kenwood 5700, lots more available on this one I think. I need to get it on my good speakers to really test. I was listening to quiet passages yesterday, it certainly has more finesse on the small details in my jvc which starts revealing that stuff at higher volume. I think the equalization on this amp is better than my jvc too- that amp didn't like these quad test speakers; sibilance was exaggerated, this amp presents the same material with all the sound present but not exaggerated perhaps a bit more clearly.

    I think the system is perfect for me. I cranked it a bit more, got my daughter coming into the basement demanding I turn it down, I also noticed it was making the little 4watt pilot lamp on the back of the bench flicker lol... definitely puts a lot heat into the sink too.
     
    Last edited: Jul 7, 2017
  15. gpounce32768

    gpounce32768 AK Subscriber Subscriber

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    Just a quick update, I spent quite a while chasing gremlins in the lamps, but finally got a reasonable setup. I performed some provisional power tests. 1kc sine wave is good to well over 90 watts, but the square wave performance is of greater interest. At > 50 watt levels the amp goes from ~10hz well over 20kc, up into the 30's keeping a crisp output. 30's an higher it starts running out of gas- not enough slew rate to keep the output square; but no obvious ringing etc. My load resistors are only good for 75 ish watts continuous and they start cooking during these tests so I have to be in and out with measurements. I did kick the amp into protection once at high power using a fast square wave but no damage. This is with a 3amp fuse, spec is for a 6a fuse I wonder what it does with the other 3 amps lol. My guess is with this units very high damping factor more complicated audio material leads to substantial current spikes, making that 6a fuse necessary at high power levels particularly with both sets of speakers in- or low impedance speakers for that matter.

    So the good news is the major surgery looks like it was sucessful, though I don't fully trust the unit yet. The bias doesn't adjust as low as the service manual states it should, though the heatsink temp is only fairly warm at idle and hot but not to the point of scalding/burn at high output. Time will tell... I still have to make the knob, I ended up ordering some black plastic, will make the knob out of that instead of the aluminum- a better match I think.
     
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  16. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Nice update - kudos to you for making a knob, (I hope you have a good lathe :)) please give us another update when you can. ;)
     
    Last edited: Jul 26, 2017
  17. LBPete

    LBPete Rolling Along Subscriber

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    It shouldn't be hard to turn the knob on a lathe but I think it will be harder than you think to match the color. There are many, many subtle differences in the the color "black."

    - Pete
     
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  18. gpounce32768

    gpounce32768 AK Subscriber Subscriber

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    Happily I'm not concerned about a good match or perfect aesthetics as long as looking at it doesn't give me a headache. The case is banged up and scratched already. The black I got is at least close, a bit more gloss when polished than the factory knobs OTOH they are high mileage. I made the knob over the weekend, just have to drill for the stem and put in a marker line.

    I've put a number of hours on the unit over the last few weeks including some loud material into 4 speakers, nary a hiccup- so I'm inclined to put the spec 6a fuse in, close it up and take some final pics. I'm tempted to replace the jvc quad with it, considering how good it makes the bargain test speakers sound.
     
  19. gpounce32768

    gpounce32768 AK Subscriber Subscriber

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    Well first knob went badly wrong I accidentally drilled thru the face and then dimensions went out on me lol... the black acrylic plastic finishes well with #1500 sandpaper and color is a good match but getting something like a spline for the shaft to engage seems to want a 2-piece solution; small soft inner collar fit up to the shank, when a good fit is obtained it is then fixed into a larger recess in the back of the knob to complete it. Then the knob itself can be set up without any fiddly or repeated ops. Glad I got a foot of the plastic, back to the lathe... at least I know the knob dims now.
     
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  20. KingBubba

    KingBubba "Too Much Stuff" Subscriber

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    Did you happen to run into any of the dreaded Sansui Glue holding down any of the mid sized caps? That stuff from the earlier batches can apparently do some damage to anything it touches. I have some in my AU-D9 and I have been keeping my eye on it. Some of the nearby resistors look a little corroded, but not too bad. Nice project, nice work. Enjoy.
     

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