Sansui AU-777A xmas break rebuild

Discussion in 'Exclusively Sansui' started by smurfer77, Dec 23, 2014.

  1. smurfer77

    smurfer77 AK Subscriber Subscriber

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    *edit: adding this up front for future reference.
    Summary of variations, errors and omissions in the manual/schematic and other points of particular note.

    • R005/R006 (protect.): 220 Ohm 5W resistors on protector block PCB hit a temperature of over 100 C.
    • R611/R612 (phono): 280kOhm in amp (390 kOhm in schematic). Either value is okay
    • C725/C726 (preamp): manual says 47 uF, 6.3 V but 47 uF, 10 V found in amp
    • C823, C824 (main): 500 pF ceramic (measure voltage and update here), not listed in manual
    • C813/C814 (main): 100 pF, 50 V ceramic not listed in manual
    • R841 (driver): 3.3k Ohm in parts list. 2.7k Ohm installed and service schematic. Not a big deal.
    • R861 (driver): 3.3 (!) Ohm in parts list. 2.7k Ohm installed and on service schematic. A big deal.
    • Note there is a service bulletin on the bias resistors R851/R853 and some nearby caps. Not mandatory unless "the trouble is found" with TR811-TR814

    ----------------
    Hi everyone. This will be the first time I've done a rebuild 'live' (posting on AK as I go). So here we go...

    I picked up a 'no sound' 777A for ebuy. Here is what it looks like. You can see a missing speaker clip, some minor damage to the wood case and a bent switch or two.
    [​IMG] [​IMG] [​IMG] [​IMG]

    And some shots inside.
    [​IMG] [​IMG]

    Installed new fuses although old ones were okay. Biased no problem and voltages look good at the ~70 V and ~37 V points. Couldn't measure any AC getting through with my multimeter so I guess the filtering caps are okay.
    [​IMG]

    There is no sound output. Cleaned the heck out of the switches and yes the pre-out is connect to the main-in :). A slight hint of sound/music gets through when the volume knob is at a certain position.

    So then I hook-up my iphone directly to the main-in and nothing on both channels. So that is where I am at tonight. Will have a deeper look tomorrow night hopefully. Any suggestion welcome of course.

    Did pull the output transistors (although I didn't expect the problem to be there since it is on both channels, I wanted to see what needs replacing for the rebuild anyway). All output transistors test okay.
    [​IMG] [​IMG]

    I have ordered parts for a full rebuild over the xmas break, but I guess I would prefer to find the source of the problem before I go to town (easier for error checking if the amp works when doing a rebuild).

    Hopefully this account of the rebuild and the many many pictures will not be too boring for everyone.
     
    Last edited: Jan 10, 2015
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  2. wtppony

    wtppony Active Member

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    Excellent documentation :yes:
     
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  3. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Boring ? - I should say not ! - the AU-777A is not a model we see very often so a rebuild thread is always welcome - "many many pictures" - what more could anyone ask for ?

    The general consensus is that faults should be resolved before mass parts changeouts, as it makes it much easier to iron out any issues arising from the rebuild itself.

    Looking forward to more posts & pictures. :yes:
     
  4. skippy124

    skippy124 AK Subscriber Subscriber

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    Hi,

    Nice post and pictures, as John has said, threads on these don't come up very often.

    Re the lack of output from the main amplifier sections, try measuring the voltage on pin 2 of the main driver board F-1183 - should be somewhere between 25 to 35 volts. If it is zero, then check the protector lamp. Also measure the voltage on pin 7 of the same board it should be somewhere around 35v (note there are two pin 7's on the board, one for each side)

    Cheers

    John
     
    Last edited: Dec 23, 2014
  5. c.coyle

    c.coyle Fighting the Dunning-Kruger effect. Subscriber

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    I'm doing an AU-777A now. Look forward to following yours.
     
  6. vigman

    vigman SUBSCRIBER

    Messages:
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    Location:
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    Tape monitor switch bashed.. possible CLUE in the PRE section?
    Speaker select switch I'll bet is not so happy…
    (Do you have a scope in the shop? )

    As a quick N Dirty I have a little headphone amp that I monitor the REC OUT
    and use an iPod in the aux in … this tells you IF the input switch works and how "crunchy" it is then I can move the iPod down to the other inputs and test…

    In the power supply
    You have the +28 (B-3) + 25 (B-4) +36 (B-6) AND the +.06 (E-1 & 2)

    GO SMURF GO SMURF GO SMURF~
     

     

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  7. skippy124

    skippy124 AK Subscriber Subscriber

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    Hi Mike,

    Are you getting those voltages from the 777 or the 777A schematic? My 777A scat has no voltage shown for B6, or for B7/B8

    Cheers

    John
     
  8. smurfer77

    smurfer77 AK Subscriber Subscriber

    Messages:
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    Thanks for the encouragement to go on with the documentation of the the rebuild.

    Vig: thanks for chiming in on this thread too. Nice idea with the using the recording RCAs to check out the switches - will keep it in mind. I might have a problem in the front end but for now I'm going to focus on the main amp section since the no-sound problem persists if I send a signal directly into the main-input RCA jacks. I checked continuity between the main-in RCA and the driver board - no problem. This main-in skips all front switches and controls, except the speaker selector, if I am not mistaken. But let's forget the speaker selector for now... read on. (thanks for suggesting to look at the voltages you mentioned - this led directly to what is written below).

    I'm meant to be working at home now but couldn't help ducking down to the garage for a few minutes. I did a quick run around with the multimeter probe on the collectors on TR801/802 and TR803/804 on the main driver board F-1183-1 and I am not seeing the ~18 V and ~10 V prescribed in the schematic at the collector pins, but instead am getting nothing. (As suggested by Skippy124/John!) Tracing back in the schematic brings us to the protector block and for both B5 and B6 we have the protector bulb in the path.

    Measuring resistance across the top (protector) bulb showed that this bulk is out. The lower bulb is the power on bulb (6 V). The protector bulb is a 25 V, 0.09mA.
    [​IMG]

    Make sure your supply caps are depleted before putting your screw driver near this bulb. I measured 47 V at the top of the bulb.
    [​IMG]

    And here is the dead bulb up closer. I might have a replacement laying around but if someone has a relevant link for substituting this part, please link it.
    [​IMG]

    So John was pointing me directly in the right direction with his last post.

    Okay, I need to get back to paid work until the evening, but now I have some direction for when I get back to it - before replacing the bulb figuring out why the bulb was blown. This protection circuit sure is weird looking....

    Keep the feedback/suggestions coming please.

    -David
     
    Last edited: Dec 23, 2014
  9. skippy124

    skippy124 AK Subscriber Subscriber

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    Hi,

    It could be that the bulb is OK, but is not making good contact in the holder.

    I also edited my first post with a couple of other voltage inputs to check on the driver board

    Cheers

    John

    Ps nice close up photos!
     
  10. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    I liked the pictures straight away, This thread will be a good 'un ;)
     
  11. vigman

    vigman SUBSCRIBER

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    Darn it.. I missed the " A " part.. I quoted a 777 NON "A"... DO'H!
     

     

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  12. vigman

    vigman SUBSCRIBER

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    Boy that filament looks OK and a 25 volt bulb that will be a hard one to get...
    ohm it out...
    I was looking for 4 for my Honda generator.. 9 ~ 18 bucks a POP..
    as a test 2 12 volt ones in series??
     
  13. smurfer77

    smurfer77 AK Subscriber Subscriber

    Messages:
    1,510
    Location:
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    We have sound

    Before writing my last post I measured the resistance of the bulb with the bulb taken out of the holder and got infinity. But it seems I didn't push the probe tips on hard enough to get good contact (DOH!).

    Now I have 31 Ohms. I hooked up a 9V battery and tested the bulb -it's okay as you guys suggested!

    The bottom of the bulb contact is soft at the tip and an indentation caused it to not contact well with the holder. An extra half turn and all is good with the bulb in the holder. Soldered it back up and flipped the switch with a prayer and no obvious problems.

    And now 18 V at the collect of TR801/2 as there should be!
    [​IMG]

    Hooked up the ipod and we have music on both channels with no major issues like hum, weakness or distortion. Definitely didn't sound as clean as the 777 I listened to this morning, but this 777A has some non-original outputs on one side that look a bit funky, and a lot more hours under it's belt... hey.... it's now ready for it's rebuild (when the parts arrive - maybe tomorrow).

    So after all that, i had a loose bulb. Thanks for the input. It motivated me a lot.
     
  14. smurfer77

    smurfer77 AK Subscriber Subscriber

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    For future reference (note I haven't centered up the voltage properly so this could be a way (tens of %) off, but not order of magnitude, unless I have another problem unnoticed):

    On the AU-777A we have (approximately) the following voltages for future reference.

    TR801/802: E (7.8 V), C (17.8 V), B (8.5 V)
    TR803/804: E (0.9 V), C ( 9.7 V), B (1.6 V)
    TR805/806: E ( 9 V), C ( 33 V), B (9.5 V)

    And at the numbered (on the F-1183-1 board and on the schematic) test pins:
    1 (0.02 V)
    2 (33.6 V)
    3 (9.7 V)
    4 (9.0 V)
    5 (35 V)
    6 (1 V)
    7 (45 V) {that could be a bit high. John & schematic say this should be 5-10 V lower}
    8 (0.03 V)

    If there are major corrections to this, let me know and I will edit for reference. I will add/edit voltages at other points into here later.
     
    Last edited: Dec 23, 2014
  15. vigman

    vigman SUBSCRIBER

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    YEAH.....
    I wouldn't sweat the small stuff... at this point..
    Are you on a DBT or variac? Voltage 120??
    Does the amp bias?
    I ASSUME the test was from AUX in and not MAIN IN??
     
  16. smurfer77

    smurfer77 AK Subscriber Subscriber

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    Look I finally learned how to multi-quote!

    No. I have never owned a scope. I have always had access to really nice scopes at work so never wanted to buy a 'cheapy' for home. But, I've finally caved in and I have one arriving at home any day now.

    If I get time I am hoping to show some power & frequency distortion analysis before and after the rebuild. My fear is that once the parts arrive I wont be able to hold back on rebuilding...


    Thanks. I am of course following your thread too. I thought about hijacking yours but thought it might get messy.

    Glad you are enjoying it. More to come! I guess I've decided to make this fairly detailed so maybe it will be useful to others thinking about doing their 777A.
     

     

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  17. smurfer77

    smurfer77 AK Subscriber Subscriber

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    I am using a variac and I'm calibrated to 117 V (my house is 127 V!)

    I have tested via both MAIN-IN and other inputs including AUX. Everything is fine. The amp biases up okay, just haven't centered (since I will do that after rebuild). Not sure if you wanted to know but for the voltages i quoted at the test points there is no source playing (since it's going to make the voltage fluctuate).
     
    Last edited: Dec 23, 2014
  18. smurfer77

    smurfer77 AK Subscriber Subscriber

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    1,510
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    Santa (aka UPS) just dropped off a little box at my front door. Still waiting on some output transistors and a couple of ceramic caps.

    [​IMG]
     
  19. vigman

    vigman SUBSCRIBER

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    Ho,Ho,Ho.....
     
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  20. smurfer77

    smurfer77 AK Subscriber Subscriber

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    A bit tidier now!

    I like to separate out my bags of parts by type (electrolytic vs film etc), value and secondary value (like power handling or voltage). I forgot to order some of the higher power resistors ...

    [​IMG]
     

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