powe fuse issue on 771

Discussion in 'Exclusively Sansui' started by Campbell, Jan 22, 2018.

  1. Campbell

    Campbell New Member

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    I have read all I can find to no avail and am a newbee at this. My 771 was going along great, no issue that I am aware of and it stopped dead in it's tracks with no warning. No power at all. No smells, no buzz, no sound issues , no nothing. Am driving a pair of Pioneer CS99A's. I suspected a fuse and finally found a blown 5A fuse wired in between the normal 5 amp fuse in the back and the power transformer. It was encased in a clear plastic tube. ( not on schematic that I can see?). All other 8 power board fuses and the main power fuse look ok. I replaced this "wired up" unit with a normal fuse holder, replaced the std. 5A fuse and turned it on. It blew immediately. So much for simply an aging fuse. I now suspect a bad power transformer or a direct short somewhere but in that there were no issues up front to begin with this doesn't sound right. Switch seems to be ok as the switched main output works fine. No "obvious" issues on the power board or driver board or elsewhere. Main transformer? Output transistor? I need some ideas & guidance asap please.
     
    Last edited: Jan 22, 2018

     

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  2. LBPete

    LBPete Rolling Along Subscriber

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    Can you post a picture? It's hard to tell what the fuse is protecting if it's not on schematic. Need to know exactly the two points it connects to.

    - Pete
     
  3. Campbell

    Campbell New Member

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    The pic titled "fuse" reflects the newly inserted fuse holder. See the removed fuse assembly perched on the transformer. It looks original to me. This end was attached to the regular fuse holder in the back of the unit where I have attached the new ( read easier to service) one. The other end was actually attached to the 115v mains ( red lead under the orange capacitor ) as seen in the pic titled cap. As far as I can ascertain all the other components, especially caps, look ok. (no marks, no swelling or discharge). This is my first issue with this unit but.... it was a gift from a friend.. Any help is appreciated. Otherwise it goes to my bud who is a retired radio technician.;) So in the meantime the Realistic STA-78 does back-up duty.
     

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    Last edited: Jan 23, 2018
  4. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    PIONEER did the same thing in a lot of their receivers. Put a fuse in line with the mains btwn the "shown fuse" and the line cord. Supposition is that it's more of a safety factor against Ham fisted owners who don't know to replace the back panel fuse with the same amperage or installing a fuse with a Reynolds Wrap Condom.

    Pull ALL of the FUSES, leaving the New fuse and the "Main" fuse installed. Turn on the unit. If either of the fuses blow, you have a shorted Main transformer. If they don't blow, replace each fuse in turn, test, and remove. Repeat for the next fuse until the new fuse blows. You've identified the circuit that is shorted, and you can move to repair that circuit. Bets are one or more of the output transistors shorted. Don't just replace the output transistor. Test (remove and do 6 way test http://audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/ ) each transistor on the board. Replace any that are bad and replace the mirror transistor on the other channel. Also check and or replace resistors that test bad or out of spec. Emitter resistors on the finals are suspect too.
     
    Last edited: Jan 23, 2018
  5. LBPete

    LBPete Rolling Along Subscriber

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    Haven't seen that before. It looks original but it's not on my 8010 (Black trim version of the 771). Can you post the serial number? I'm curious where it fits in the production run.

    - Pete
     
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  6. Campbell

    Campbell New Member

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    Hi Pete, the s/n is 814073998. I have handed the unit over to my best bud with the Audiokarma feedback above. He is an old radio technician. If he can't do anything it comes home again. It may end up at a repair shop eventually but I hope we can affect a repair ourselves. I will keep you all informed. :thumbsup:
     

     

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  7. LBPete

    LBPete Rolling Along Subscriber

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    Based on the 44 serial numbers on file, 771 production started around March of '74 and continued on and off until May of '77. Your 771 was built in July of '74 making it a fairly early example. My 8010 was built in September of '74 and doesn't have that fuse. So, it looks like the very earliest production has it and it was deleted before September '74.

    - Pete
     
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  8. skippy124

    skippy124 Super Member

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    Also, don't discount the possible product safety requirements for different markets - there may have been a requirement for an additional mains fuse inside the product.
     
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  9. LBPete

    LBPete Rolling Along Subscriber

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    Yup, that's a valid point. Does it have a 220v sticker on it?

    - Pete
     
  10. Campbell

    Campbell New Member

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    Hi all. No 220v sticker. Alas, my tech bud Robert has made progress. Transformer seems ok. He did find one of the famous IR 10DC-1 rectifiers as dead shorted. Based on above we will be replacing with qty 2 of 1N4007, times 4; for qty 2 + center tap and qty 2 - center tap (red) to get all 4 of these babies at the same time. Meanwhile still evaluating balance of board. I have informed him of the notes above regarding the transistors and emitter resistors of course. Is there anything else that history of these units dictates we should look at also or replace while we have this boat anchor apart? lol. Will keep all informed in due course.
    :D
     
    Last edited: Jan 31, 2018
  11. Campbell

    Campbell New Member

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    Can I replace the power caps with any voltage higher than 50v and any cap higher than 4700uf? Was thinking of 50v, 10,000 uf as a safety measure? Nichicon? Advise.
     
    Last edited: Jan 31, 2018

     

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  12. LBPete

    LBPete Rolling Along Subscriber

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    50 volts is fine. I would look at 8,200mfd. 10k may stress the power switch and diodes.

    - Pete
     
  13. Campbell

    Campbell New Member

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    Bob, my tech bud, just texted that the double 1n4007 fix seems to work and no fuses blew. He is delivering Sunday. Next we try with speakers..lol :) I found 2 of 6800uf by 50 v Caps in a Sony amp PS I have received from another bud so they will likely be donners sooner or later for the 771. The Sony is only good for powering horn speakers at a skating rink in my opinion. ( quick listen was all I needed.... No bass!). More to follow.
     
  14. LBPete

    LBPete Rolling Along Subscriber

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    Don't reuse caps. New ones are not that expensive.

    - Pete
     
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  15. Campbell

    Campbell New Member

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    Hi Pete. My bud obviously did not test for more than a few seconds. The replacement diodes for D03 & D04 (10DC-1R's) were incorrect ( based on my comparison the the original DS10-1R current flow direction ) and consequently one of the 4700uf 50v power filter caps exploded after about 20 seconds with great stink and smoke. :yikes: Then the 5 amp fuse also eventually went. I was not impressed with our work so far. :( What a stink! Time to try to do this myself. First I removed both of the 4700uf 50v filter caps and then the perceived to be offending "common cathode" 1N4007 diodes in D03 and D04 (where the red dual 10DC-1R's were). Then I also removed D01 & D02 (original 10DC-1's) for testing. The 2 "common cathode" replacement diode sets still tested ok despite the ordeal. :eek: Then I tested the regular black 10DC-1's I was suspicious of. One of these 10DC-1's was also shorted both ways and the other tested quite fine. I replaced both with the just tested common cathode 1N4007's pairs. So now I have the "common anode" pairs of 1N4007's replacing the 2 red DS10-1R's ( as should have been originally) and the common cathode pairs replacing the DS10-1's. I believe this set up to be correct as tested flow duplicates the original dual diode set up. Obviously the filter caps will be replaced but do I need to do more specific testing before submitting to a dim bulb test? Can I do a quick test without the filter caps in?
     
    Last edited: Feb 6, 2018
  16. Campbell

    Campbell New Member

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    See my "current set-up" in the pic. Correct? ( I really shouldn't be trusted with anything more complicated than a crayon :) )
     

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    Last edited: Feb 6, 2018

     

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  17. Campbell

    Campbell New Member

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    Hi Pete, now my bud tells me he "may" have put one of the filter caps in backwards. :whip:That may explain a lot. He had both negatives going to # (15) and one positive to F02 and the other positive to F03/F04. I see the schematic with one cap reversed however. ( blue indicating negative on caps, yellow positive) Advise please!
     

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  18. Campbell

    Campbell New Member

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    No responses eh? Oh well. Dim Bulb Tester finished. Next to check output transistors and emitter resisters.
     
    Last edited: Feb 8, 2018
  19. LBPete

    LBPete Rolling Along Subscriber

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    I took the cover off mine and the filter caps mount differently. They are not on the power supply board. Here's a snip from the schematic showing the wiring of the filter caps. The wiring is fairly straight forward. The negative of one and the positive of the other go to ground, and the other sides connect back to the diodes indicated.

    - Pete
     

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  20. Campbell

    Campbell New Member

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    Thanks a lot Pete. I am not good at reading circuits so I test and duplicate. This however confirms that my bud reattached one 4700uf backwards. My caps were clamped beside the transformer of course but the board is indeed marked up for C16 & C17 {but not populated} vs C106 & C107 in the clamps. Are the emitter resistors the 4 big ceramic .330K ohm ones on the power board close to the output transistors?
     

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