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SX-780 sound question

Discussion in 'Pioneer Audio' started by KeithD, Jun 7, 2017.

  1. KeithD

    KeithD New Member

    Messages:
    32
    Location:
    La Crosse, WI
    Picked up a SX-780 about 6 months ago. Since then I discovered it has a couple of issues.

    Issue 1: left channel louder than right channel. If I flip down the Mono switch, the left channel meter deflects more than the right channel.

    Issue 2: left channel has no sound at low volume. This is a strange one to me. After I turn on the receiver, and turn the volume up a bit, there is sound on the right channel, but not on the left. When I get the volume to about 10 o'clock the left channel has some static and then comes in fine. I can then turn the volume back down and both channels have sound. After about 1 - 2 minutes, the left channel fades out. If I turn the volume up again, the process repeats. This happens for about the first 15 minutes of operation. After that the left channel volume is fine at all volume levels.

    What I have done so far:
    Use Deoxit D5 on all pots and switches from inside (volume, balance, etc.), working each pot/switch multiple times. Followed this with tuner cleaner to remove the Deoxit residue. Checked that the "loudness" switch was not the fault, but channel imbalance did not change with switching this on or off.

    The balance issue is better after doing the cleaning. I now notice only a slight imbalance if any. However, the left channel volume issue remains. I'm guessing this is a dirty volume pot on the left channel? How easy is it to take this apart for cleaning, or should I be taking this to a tech?

    Thanks!
     
    Last edited: Jun 7, 2017
  2. bberkom

    bberkom AK Subscriber Subscriber

    Messages:
    709
    Location:
    Nashville, TN
    I would guess a dirty speaker protection relay with the symptoms you're describing. I had the same issue in one of these. You can try removing the relay and cover to clean it, but it's just as easy to replace it.
     
    Last edited: Jun 7, 2017
    Imanoldee likes this.
  3. KeithD

    KeithD New Member

    Messages:
    32
    Location:
    La Crosse, WI
    Thanks for the insight on that. Forgot to mention, I don't know if it makes any difference, but the same thing happens with the volume using headphones or speakers.
     
  4. Imanoldee

    Imanoldee AK Subscriber Subscriber

    Messages:
    146
    Location:
    Livermore CA
    I agree with bberkom.
    Its a very good chance it is the relay.
    They are easy to replace too.
     
  5. KeithD

    KeithD New Member

    Messages:
    32
    Location:
    La Crosse, WI
    OK, would the relay be the source of both problems (R/L channel imbalance and weak low volume on L channel)? I'd be interested in understanding why the low volume on the L channel goes away after a while. Is something in the relay (or elsewhere) "warming up" to the point where low volume on the L channel works?

    Any suggestions for a source for a relay?

    Thanks,
    Keith
     
  6. KeithD

    KeithD New Member

    Messages:
    32
    Location:
    La Crosse, WI
    I assume that means just un-soldering the relay from under the board and then soldering in a new one?
     
  7. bberkom

    bberkom AK Subscriber Subscriber

    Messages:
    709
    Location:
    Nashville, TN
    Yep that's it. I bought mouser part 653-MY4-02-DC48 to replace mine and just had to break off the unneeded tabs on it. Worked great. What is basically happening is when you increase the volume, you are increasing the strength of the signal which breaks through the corrosion on the relay contacts. Once that contact is made, it'll work for a bit, but if you decrease the signal by turning down the volume, eventually contact will be lost as it cools down. Hope that helps.

    RL imbalance could be caused by that or by something else, but I'd try the relay first and see if that helps.
     
    Imanoldee likes this.
  8. KeithD

    KeithD New Member

    Messages:
    32
    Location:
    La Crosse, WI
    Yes, that makes a lot of sense. I also get it that since you have to take the relay out to clean it, I might as well just put a new one in instead of re-soldering the old one that might or might not work after cleaning. Thanks for the help and a part number!
     
  9. bberkom

    bberkom AK Subscriber Subscriber

    Messages:
    709
    Location:
    Nashville, TN
    No problem. There's a pioneer relay replacement thread somewhere in the pioneer forum if you ever need to reference one. Just did this recently so was easy to look up. Hope you get it back up and running.
     
  10. KeithD

    KeithD New Member

    Messages:
    32
    Location:
    La Crosse, WI
    Just ordered the part. Probably won't get to this for a week, but wish me luck on my first repair!
     
  11. rcs16

    rcs16 Super Member

    Messages:
    2,724
    Location:
    Baldwin, Ontario, Canada
    You could place a temporary short across the relay contacts in order to verify that is indeed the speaker relay.
     
  12. KeithD

    KeithD New Member

    Messages:
    32
    Location:
    La Crosse, WI
    That's probably a good idea. How do I do that, or is there a reference for that somewhere here on AK?
     
  13. KeithD

    KeithD New Member

    Messages:
    32
    Location:
    La Crosse, WI
    OK, looked at the service manual, and found the relay contacts to check, which I did, and when shorted, the sound comes out loud and clear on the left channel. Tried the same on the right channel, and the sound is louder when the relay contacts are shorted, so this does seem to point to the relay.

    Received the replacement relay from Mouser. Compared the pins on the new relay to the old setup. I have not UN-soldered the old relay yet. The four pins across line up perfect, however, the second row of pins do not line up with the second row on the old relay. The second set on the new relay are closer to the first set of pins, than they are on the old relay. Difference appears to be 2 to 3 millimeters. Same with the third set. How do you solve this? Do I have to bend the pins on the new relay to line up with the holes in the board, or do I need some type of adapter?

    Need to pick up a solder wick, so I won't get to this until tomorrow.

    Keith
     
  14. markthefixer

    markthefixer On Hiatus, dealing with Dad's estate full time Subscriber

    Messages:
    20,497
    Location:
    Bensenville,Illinois
    No bending, there will be cutting of superfluous pins.

    I think if you look closer, you will find that you have an extra row of four pins.

    Starting at the first row, the coil row, there are just two pins , for the relay coil connection
    then a second row of four "common" pins ,
    then there is the third row of the "normally open" pins that are OPEN when the relay coil is unpowered, and closed when the coil pulls in the relay
    then there is the fourth superfluous row, these are the "normally closed" pins that are closed when the relay coil is unpowered, and OPEN when the coil pulls in the relay,

    each channel of the amplifier outputs are connected to two pins each in the (second) "common" row of four pins
    each of the speaker side connections are connected to two pins each in the (third) "normally open" row of four

    There are NO connections on the board for the fourth, superfluous row, and the pins should be clipped as close to the relay base as possible.
     
  15. KeithD

    KeithD New Member

    Messages:
    32
    Location:
    La Crosse, WI
    OK, thanks Mark. I was looking at the board and relay wrong. What you are calling the "fourth row" of pins is numbered 1 through 4 on the MY4-02, so I thought that would line up with the speaker side pins on the board. But, now I get it. If I clip off the fourth row on the MY4-02 (pins 1 - 4), then everything does line up perfectly!

    Now to get a solder wick this morning. Any particular type of solder I should use for soldering in the new pins? I do have 40 Tin/60 Lead Rosin Core Oatey solder I use when repairing connections on my Lionel trains. Is that OK?
     
  16. KeithD

    KeithD New Member

    Messages:
    32
    Location:
    La Crosse, WI
    OK, old relay out. Took some time. Did get a used solder "sucker" and some solder wick. A bit of a job to get the solder out from around the pins, but that's done and new relay in. Sound works great now, even at low volumes. There is still a very slight imbalance between L and R, but I can live with it. Thanks for everyone's help!
     

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