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1000a weak in one channel

Discussion in 'Exclusively Sansui' started by FireGuruMN, Jul 5, 2018.

  1. FireGuruMN

    FireGuruMN New Member

    Messages:
    34
    I have a Sansui 1000a and some caps and new tubes that were patiently waiting on my shelf for me to provide them some attention. Today was their lucky day because I was feeling in a "tube" mood. Since the previous owner said he had already powered it up I figured I would verify that it still had some life in it before I dug into it. It made music but one channel was pretty awful sounding. I expected that given the appearance of the tubes.
    I started by replacing the big grey caps on the amplifier section (see pictures). I messed up the order so some of them were too large to fit. But I plowed ahead anyway.
    After replacing the caps I powered it back up and it sounded very good even on the old tubes. I put in the new tubes and it sounded a bit better.
    I did check the values of all the old caps and they were surprisingly still in spec except for one that was reading double the expected value. But I will sleep better knowing they oil filled caps are in a museum and not my amplifier section.

    So back in for round 2: I installed 10ohm bias measurement resistors off pin 5 for each tube to ground. Basically replaced the black wire with a resistor (see pictures). I powered it back up and the bias readings were all over the place. After decoding the strange associations between the tubes and the bias pots I was able to get everything dialed in around 330mv for each tube.
    I settled in for some musical enjoyment and one channel got quiet and distorted. That channel will occasionally make some "grinding" noises when there is nothing being played. I have swapped power tubes around and the problems stays on the same channel. I have also swapped driver tubes. No change. Wiggling tubes does not seem to change anything. When I first got this receiver I sprayed Deoxit on all the switches and pots. So that is not likely the problem. Although I did have to cycle the tape monitor switch a couple times to get it going today.
    The phase switch on the back seems to make some "crispies" when I wiggle it. Should I just bypass that?

    Am I going to have to trace a oscilloscope signal all through this spaghetti of can someone come up with another test/solution I should try?

    And while I am at it: Two of the screws sockets on the speakers terminals are stripped. I put a nut on the back side and managed to get some banana sockets to screw in but it is not classy. Any idea where to dig up new sockets for those screws? Maybe socket is not the right word? But I am talking about the screw hole where the screw drives into the back of the amplifier. see the picture. I had to grind away half of the nut to keep it from shorting on the chassis.
     

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  2. FireGuruMN

    FireGuruMN New Member

    Messages:
    34
    Update:
    I pulled the two screws for the AM radio antenna and used them for the speakers. It looks original and I will never use AM radio anyway.
    The weak channel has magically cured itsself after cleaning some tube sockets and re-tensioning the pin holders.
    But I have noise on one channel when I swapped tube V14 and V18. When I swap them back the noise goes away. Those looked like they were symmetric in the circuit so that has me a bit confused.
    I think I need to run all the little tubes through the tube tester.
    I also have scratchy noises when adjusting the tone controls even though the pots have been cleaned.
    But I am encouraged enough to go ahead and replace the rest of the grey capacitors and see if that cleans things up a little more.
    Anyone want to offer an opinion on running this with 4 ohm Boston Acoustic A400's? The switch on the back just has settings for 16 and 8 ohms. The internet seems undecided on the subject. My thought is that it will be ok for the limited time I intend to run this.

    Next up: Pioneer SX-800. I just finished getting 50 years of garage goo cleaned out of it. Lots of grey caps in there also. Some green ones on the output tubes even. Anyone got an extra selector knob? Mine did not have one when it showed up.
     

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