AU-7900 Speakers cutting in and out

Discussion in 'Exclusively Sansui' started by Jmicah, Dec 28, 2018.

  1. Jmicah

    Jmicah New Member

    Messages:
    19
    Location:
    Portland, OR
    I'm running a Sansui SR-838 and a TU-7900 through an AU-7900 and having some issues with speakers cutting out. I can sometimes fix it by jiggling the RCA connectors on the back of the amp or tapping on the turntable. It happens with all sources, but of course tapping on the turntable on works when using that. I've tried Deoxit on all the RCA connections on the back of the amp, and sometimes it works as a temporary fix.

    The amp was supposedly recapped before I bought it, but that could still be an issue. However, I'm thinking it's the connectors because I can temporarily fix it.

    Anyone have any recommendations? I'm capable of replacing the RCA cable on the turntable, but not comfortable rewiring the tonearm. Should I Deoxit the connections on the headshell and tonearm? How easy is it to replace the RCA connectors on the amp? Or is there a better way to clean them?

    Thanks in advance.
     

     

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  2. bktheking

    bktheking Gitter Done! Subscriber

    Messages:
    12,977
    Location:
    Ottawa Ontario
    Sure it’s not a dirty relay? Also this unit has a switch to separate the pre from the amp, is it clean?
     
  3. Sansuiman

    Sansuiman Active Member

    Messages:
    428
    Location:
    St. Louis, Missouri
    If this affects all sources, I'd look first at the amp itself, not the turntable. Could be anything from a dirty/oxidized source or function selector switch, to a failing protect relay. Does the amp go into protection at all (based on the front panel indicator) when you have loss of signal? Do you get crackling sounds or distortion intermittently when signal is being passed? Further info would be helpful to start narrowing it down.
     
  4. Zekeman

    Zekeman Super Member

    Messages:
    2,425
    Location:
    Chicago
    Next time it happens just gently push each of the phono RCA's with your finger to see if that's the issue. The RCA's are an issue if not tended-to after 40 years..
     
  5. Jmicah

    Jmicah New Member

    Messages:
    19
    Location:
    Portland, OR
    How do I tell if it goes into protection? No distortion while it's working. Just crackling when the sound cuts out or gets quieter. I should note that sometimes one channel (not consistent which one) will cut out--i.e., it's not always both channels. I need to buy more contact cleaner and I'll clean the function switch.
     
    Last edited: Jan 1, 2019
  6. Jmicah

    Jmicah New Member

    Messages:
    19
    Location:
    Portland, OR
    I can sometimes get the sound to come back in when moving the RCAs. I've tried tightening the pins. Do they need to be replaced or just well cleaned?
     

     

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  7. Sansuiman

    Sansuiman Active Member

    Messages:
    428
    Location:
    St. Louis, Missouri
    In protect mode, there is a relay that cuts output to the speakers. You should hear a noticeable click when it goes into protection, or comes back out. There is also an indicator that should turn from green to red to indicate it is in protect mode. In normal operation the indicator is green.

    At least I'm assuming the 7900 has the indicator as the 9900 is so equipped. Some later models had a red only indicator that would blink when in protection mode, and remain solid in normal operation.

    Edit to add: after looking online at pictures of an AU-7900, I do no see any indicator present. This surprises me, as it is one model down from the 9900. In any case, it would appear the only way to know if it is protected mode or not is by listening for the relay click. Of course, now I'm also questioning whether it is even equipped with a relay type protect circuit given the lack of an indicator. Hopefully someone experienced with the 7900 can speak up here with some clarification.
     
    Last edited: Jan 2, 2019
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  8. slimecity

    slimecity Super Member

    Messages:
    3,148
    Location:
    New Zealand
    This to me sounds like it may be an input issue - either a dodgy RCA cable, dirty contacts at the RCA sockets, or contacts on your switches.
     
  9. Jmicah

    Jmicah New Member

    Messages:
    19
    Location:
    Portland, OR
    Any recommendation on how to fix the sockets? Today I opened it up and cleaned all the switches and sockets, but I still have the issue, although it's easier to fix by jiggling the cable. I'll be doing more testing tomorrow when my daughter isn't being noisy
     
  10. bolero

    bolero Well-Known Member

    Messages:
    642
    Location:
    Ontario
    if the 7900 has vertical RCA's at the back, like the 7700, they often get a cracked solder point due to the stress of the cables hanging

    I had to reflow solder on my 7700 RCA outputs, at least the 7900 looks easier to get at them?
     
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  11. Sansuiman

    Sansuiman Active Member

    Messages:
    428
    Location:
    St. Louis, Missouri
    Bolero's suggestion is my best best as well. Wasn't aware until a bit ago that the RCA sockets were oriented facing up on a rear terminal "shelf" with the 7900. My CA-3000 is similar in that regard, and yes, the sockets do get stress cracking in the solder joints due to how they are arranged.
     

     

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  12. Nuel

    Nuel New Member

    Messages:
    17
    Since you mentioned the amp was recapped, it is possible that the main amp's conductor side was not cleaned up well. I had faced very similar problems and removing all the flux residue and stray micro solder drops helped. I read somewhere Flux could create high impedance connections. Just my suggestion. Remove the amplifier board (need to unplug some wires and remove respective screws). There are many ways to clean including a flux remover. I normally use Isopropyl Alcohol (90% or above), squirt a little at a time and wipe clean with Q-Tips, ensure not to leave any lint from the buds. For tough residues, scrape gently with a sharp tool and clean again. Let it dry.
     
    Last edited: Jan 3, 2019
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  13. Zekeman

    Zekeman Super Member

    Messages:
    2,425
    Location:
    Chicago
    The RCA access (underneath) on the 7700 and 7900 series is the same.
     

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