SCA-35 pot noise problem

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randroid42

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SCA-35 pot noise problem
« on: 27 Aug 2010, 05:31 am »
Hi folks,

I'm working on a factory-wired SCA-35 that I've been hauling around without use for more than 15 years.  After a careful power-up with the variac and re-heating a few cold solder joints, it is sounding pretty good through my old Wharfedale W60-E speakers.

HOWEVER, when I touch Balance, Bass or Treble knobs, I often hear a loud hum, and when I move these controls there are loud transients.  Wallering the shafts a bit, or grounding the shafts with a jumper actually addresses the problem, but this is not a practical fix.

Although my circuit theory is pretty weak, I can't see how a component or wiring problem could give me this behavior in all three pots.  The volume control pot does not behave this way.

I guess the shafts are just not always making good electrical contact with the sleeves they pass through, but I don't see why the shaft of the pot should be part of a circuit anyway.  After poking around the network for a few days I've found only one report of a similar problem, and without a solution.

So:  is this a known issue with this amp, or something off the wall?  Am I right in thinking I just have to replace these pots or maybe find a way to lubricate the shafts with something conductive (which sounds dangerous)?

Thanks a bunch.

Randy

Bill Thomas

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Re: SCA-35 pot noise problem
« Reply #1 on: 29 Aug 2010, 06:37 am »
Hi, Randy!

     I hope this will help you with your problem potentiometers.  It sounds like these are exhibiting the symptoms of "dirty pots."  If this were *my* SCA-35, I would remove the potentiometers from the amplifier and do a thorough cleaning regimen on them.  IF YOU ARE CAREFUL, you can clean them "in-place," but be prepared to clean up a mess.

     You will hear all sorts of stories about the BEST cleaners and lubricants to use.  I can only give you the method that *appears* to work well.  If your potentiometers are still noisy and intermittent after trying this method, you will *probably* need to replace them.  (Not necessarily an easy thing to do nowadays.)

     OK, before you clean 'em, you need *something* suitable.  I suggest you purchase a can of common lighter fluid (which is naphtha) and then go to your local Radio Shack and purchase their deoxIT kit.  The kit will contain deoxIT DN5 and deoxIT GN5.  First, SOAK the control with the Lighter Fluid.  Work the control back and forth through its rotation.  Douse it again with Lighter Fluid and work it back and forth again.  Shake the excess Lighter Fluid from the openings in the control.  Allow it to dry.

     Next, get the little spray can of deoxIT DN5 and spray it inside the control.  You don't need to douse the control, just a good spritz.  Work the control back and forth a little and allow the control to "sit" for a few minutes (approximately 2 to 5 minutes works well).  Then another small "spritz" of the DN5 along with more "working" of the control - back and forth.  Allow to dry.

     Now, douse the control with Lighter Fluid again.  REALLY soak it and work the control back and forth again.  Shake excess Lighter fluid from the openings and allow to dry thoroughly.

     Finally, spray an EXTREMELY fine mist of deoxIT GN5 "contact conditioner" into the control and rotate it back and forth a few more times.  The key here is to apply a REALLY fine mist.  Less is more here.  Ideally it would be good to get a coating that is only a few molecules thick, but you'll just have to "wing it" to get close.  If you slip and douse it, clean it with Lighter Fluid again, allow to dry and follow it up with that teensy mist of GN5.  Repeat with the rest of the controls.  That's it!

     Do NOT buy anything else.  Don't try to "go cheap" with Radio Shack's own products.  (Nothing against them, but I've never had much luck with them on potentiometers.)

     Chances are, this will improve things, or perhaps *eliminate* the problems completely.  Make SURE you let us know how this works out for you!

Sincerely,

Bill Thomas

randroid42

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Re: SCA-35 pot noise problem
« Reply #2 on: 16 Sep 2010, 12:55 am »
Hi Bill.  I enjoyed your SCA-35 restoration story.

I have succeeded in removing the easy pot (the treble control) and, as soon as I can acquire the materials, I will try your cleaning regimen.

Thanks and regards,

Randy