Help. Immortal beloved in trouble!

Discussion in 'DIY' started by birchoak, Jul 6, 2018.

  1. birchoak

    birchoak Hi-Fi Nut Subscriber

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    683
    Location:
    Amesbury, MA
    Hello folks. Well, after ten years of no trouble whatsoever, my all time bestest buddy, a Scott R74S SS receiver, has suddenly lost its right channel in stereo mode. Switching to mono cures the problem. I cleaned the pots two weeks ago so I don't think it's dirty wipers, and I'm pretty sure it doesn't have a relay but I will check later. Solder joints are impeccable, too.

    Do you guys think it could be a cap? My gut tells me it's something minor. I replaced the main filter caps about two weeks ago (preventative maintenance; no symptoms to urge me to do this) but it worked fine afterward. Is it letting more power into the circuits now, sending those caps with one foot in the grave over the edge? Just wanted an informed opinion before I opened it up. It is an outstanding receiver--the most reliable I've ever owned and the best-sounding--so I may send it out for surgery if it's not something simple.

    Thank you for any and all insights your powerful minds can shed on this--
     
    Last edited: Jul 7, 2018

     

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  2. OMGCat!

    OMGCat! AK Subscriber Subscriber

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    Gotta be a dirty switch (or possibly bad solder joint on the switch) if you're getting signal through to your speakers on mono. No separate caps or relays between stereo and mono, check the schematic for places to poke/deoxit.
     
  3. birchoak

    birchoak Hi-Fi Nut Subscriber

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    Location:
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    Oh sweet--this is the info I need to guide me. Yeah, I was thinking clean the switches again--it can't hurt and I've heard that sometimes multiple cleanings are needed, no matter how well you think you did it the first time. I will do that now. Thanks, OMG!
     
  4. elnaldo

    elnaldo Addicted Member

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    Try also some "aux out", or tape out connections to check if the signal is present there. All that , with a schematic, helps to narrow the point where the signal is lost. You already know is before the stereo-mono switch.
     
  5. birchoak

    birchoak Hi-Fi Nut Subscriber

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    Location:
    Amesbury, MA
    Both channels back in stereo! I cleaned the switches. Then I cleaned them again. Then again. Then, switches were, once again, cleaned. I started off with CRC as it seems far more effective than DeOxit, then switched over to DeOxit for the final sprays (every press of the valve I'm thinking, There goes 50 cents). Then I resoldered anything even remotely dull-looking on the switches' PCB, as well as both output boards. I looked for bulging caps, burnt components, etc. but could find none. I try to do everything I can think of while I've got the covers off, but I'm guessing it was the cleaning that did the trick.

    Thanks for the help, people! I guess old switches need a far more aggressive cleaning than previously imagined.
     
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  6. Watthour

    Watthour Electron Rancher - JS3600

    It took almost 50 years to build up all that oxidation. Ten minutes of cleaning doesn't seem inordinate.
     
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  7. birchoak

    birchoak Hi-Fi Nut Subscriber

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    Point taken. It's hard to believe that a spray alone can remove it, in fact. You really have to flood the pot and work the wipers like, 100x, doncha? A few bursts ain't gonna do it.
     
  8. savatage1973

    savatage1973 Addicted Member

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    I know you probably got it all back together, and don't want to pop the hood again, but you might want to invest in some DeOxit Shield (S5) to treat the pots/switches to prevent re-oxidation of your now clean devices.
     
  9. birchoak

    birchoak Hi-Fi Nut Subscriber

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    I have no problem popping the hood again for this loyal receiver and will invest in S5! Thank you for the advice.
     
  10. OMGCat!

    OMGCat! AK Subscriber Subscriber

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    Location:
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    I'm not sure what they use in these cleaner sprays but it really is good at removing corrosion. If you spray a bit on a penny and polish a bit you'll end up with a shiny penny. Seems like some switches completely resist the cleaner and have to be taken apart but most respond well.
    I use a cheaper version called Peavey Silent Slider to clean baked heat sink grease off of sinks, transistors and micas. You spray it on, let it sit a minute and the cement like paste wipes right off. A little rubbing and the face of the heat sink shines up and looks nice, ready for reapplication of grease.
     
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  11. Scott75s1974

    Scott75s1974 Active Member

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    I'm not an expert. As I read the schematic for the switch board, the Mono button controls the MPX board for FM. Does it help when you are using phono, tape, or some other input?

    You can test each amp circuit independently by removing the jumpers on the back of the unit. That gives you direct access to the final amp from the tone board through the drivers. Do one side at a time. If each of those channels work, then you know it's upstream in multiplex board (if FM only problem), selector switches or preamp board (if all input sources, although Tape 1 does not use the preamp). If not, you know it's downstream in tone board or drivers. Since both speakers currently work on mono, it's not the output fuses right?
    mono button copy.jpg
     
    Last edited: Jul 9, 2018

     

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  12. Scott75s1974

    Scott75s1974 Active Member

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    Sorry, I see that also ties the L and R channels together going into the tone board. But I still don't see how that switch would cause one channel to fail in stereo mode.
     
    Last edited: Jul 9, 2018
  13. birchoak

    birchoak Hi-Fi Nut Subscriber

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    You have a Scott 75S! At last, someone else with one of the Scott Professional Line. The problem went away after an extended cleaning of the controls and some semi-random re-soldering. Did you know that these receivers were warranted for ten years when they were originally sold? Are you as happy with your Scott as I have been with mine?
     
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  14. Scott75s1974

    Scott75s1974 Active Member

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    Yes. It was my grandfather's stereo. I had no idea that it had a 10yr warrantee. (I had to break the warrantee sticker to get into the bottom of chassis ;-) But the build quality is impressive in just about every way.
    I'm new to audio, but I'm really astounded by how well it plays after the recap. The R75 is 60 Wpc. For low complexity pieces (classical sonatas, 70's acoustic rock) it has really breathtaking soundstage. A violin or even a cowbell can just really stand out from attack to decay, like hearing 3 dimensions.

    This thread contains my attempts to learn how this receiver works, and to recap with some moderate upgrades. A lot of helpful advice from people who know much more than I do. It also contains links to French and German pages that discuss servicing the receiver. The French site notes an important typo in the SM regarding bias settings.
     
    Last edited: Aug 1, 2018
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  15. birchoak

    birchoak Hi-Fi Nut Subscriber

    Messages:
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    Location:
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    Wow, you really went for it. I'm impressed by your thoroughness. Mine has not shown any signs of muddiness, static, etc. but I did put new filter caps in it. I suppose I should go ahead and recap the whole thing as it is a tremendous unit. Thank you for sharing your labors.
     
  16. Nemo bit

    Nemo bit Active Member

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    Main ingredient

    Soylent Green...its not on the label however..Cheers!
     

     

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  17. birchoak

    birchoak Hi-Fi Nut Subscriber

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    Ha ha! Well, I usually leave blood somewhere, so you're partially correct.
     

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