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    Attenuator/Knobs/Pots

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    Digdug

    Posts : 18
    Join date : 2012-10-05

    Attenuator/Knobs/Pots

    Post by Digdug on Thu Mar 07, 2013 12:07 am

    Hello,

    I have a ST-120 power amplifier and a SP12 + PH12 preamp. A while back I had a problem that this forum diagnosed as dirty pots. I think Bob told me to buy some anti-corrusive lubricant spray, use it to clean the pot, then twist the knob back and forth again and again. I did this, and it worked. The only problem is, I've had to do it more and more frequently until I have to clean it every four days or so. My question is, what would be a more permanent fix? Do I need to buy a new knob, a new attenuator? Both? Can I get an upgraded part that won't need cleaning as often? Any recommendations on a specific part to buy? Can I change these parts without having to solder anything? If need be I can have my friend solder for me. As you can tell, I don't know anything about electronics.

    Thanks for any help you can provide,
    Doug

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: Attenuator/Knobs/Pots

    Post by arledgsc on Thu Mar 07, 2013 11:55 am

    I would recommend replacing the potentiometer(s). It appears they are just worn out and on their last leg. If you can explain further which pot is given you issue (.i.e volume, tone, etc. on XX equipment) would help in giving direction on how to proceed. Unless too old and brittle the knob can usually be saved and only the part that was sprayed to clean is replaced. And normally the parts are soldered so you have desolder the old one, redress the wire leads, and solder a new one in place. Perhaps someone can also recommend a competent shop or forum member in your area to do this for you.

    Digdug

    Posts : 18
    Join date : 2012-10-05

    Re: Attenuator/Knobs/Pots

    Post by Digdug on Thu Mar 07, 2013 12:32 pm

    Hi,

    Thanks for the response. This is the volume knob on the SP12 preamp. The preamp is only about a year old. I'd like to fix it so that I don't have to replace this every year, so if better parts would help, I'd be willing to shell out extra dough.

    Thanks again,
    Doug

    Digdug

    Posts : 18
    Join date : 2012-10-05

    Re: Attenuator/Knobs/Pots

    Post by Digdug on Thu Mar 07, 2013 12:33 pm

    Oh, and I live in Houston.

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: Attenuator/Knobs/Pots

    Post by arledgsc on Thu Mar 07, 2013 3:38 pm

    A year old pot should not be doing this under normal circumstances. But it may have been defective initially. So once fixed you should have years of trouble free performance. Usually a scratchy pot is a sign that the device is worn out.

    Do you live near an oil refinery? The reason for asking I used to live in West Texas and the sulphur gas emitted from refineries would corrode relay contacts very rapidly. Anyway, hopefully someone will be along in the Houston area to lend a hand or recommend a place for service.

    turbotoy

    Posts : 47
    Join date : 2012-04-15

    Re: Attenuator/Knobs/Pots

    Post by turbotoy on Thu Mar 07, 2013 10:38 pm

    I also have a SP-12/PH-12 that is now about 1.5 years old. I built this from kit using the stepped attenuators and the regular (import) stepped volume attenuator. I'm guessing you actually have the same and not potentiometers. My volume attenuator was noisy right from the start. I talked to Roy and he suggested touching up all the solder joints. That helped a little, but not much. Like you describe, if I sprayed cleaner and rotated the knob about 20 times it would stay quiet for a few days then get noisy again. I then noticed that one of the balance attenuators was making poor contact - wiggle the knob and you would get noise or a channel would actually cut out. On closer inspection, all of the balance attenuator solder lugs were visually corroded. Everything else in the preamp looked mint.

    So what did I do? Spent the $150 for a Goldpoint attenuator and bypassed the balance attenuators (they are "retired in place" for now). I never needed to adjust the balance attenuators anyway. The Goldpoint attenuator has been dead quiet for the last 4 months and feels incredible; well worth the money. You will need to solder a few wires, but it's about as simple as any solder job gets.

    By the way, an autopsy of my original noisy volume attenuator didn't reveal any "smoking guns". The contacts actually all looked to be in OK condition, but I didn't put them under a microscope either. All the symptoms suggest low quality contacts forming an oxide layer over time.

    tubes4hifi
    Admin

    Posts : 1286
    Join date : 2008-11-30

    Re: Attenuator/Knobs/Pots

    Post by tubes4hifi on Thu Mar 07, 2013 11:04 pm

    yes, I'd also assume that's the 24 step volume attenuator, as the ALPs controls are sealed and I'd expect them to live 20-30 years.
    The attenuators are low cost and audible superior, but chinese quality construction, you definitely get what you pay for.
    The Goldpoints are amazing parts but unfortunately not many people are willing to pay $150 for a volume control.
    On an SP12 or SP14 they are definitely worth the extra cost.
    Also, I don't recommend getting the volume control AND the dual mono balance controls from Goldpoint, they are ALL volume controls.
    Get the stereo volume, or get dual mono controls if you'll ever need balance, but not both.
    You loose about half the gain of the preamp when they are combined, due to the way they function. The ALPs type balance control is OUT of circuit at midpoint.

    Digdug

    Posts : 18
    Join date : 2012-10-05

    Re: Attenuator/Knobs/Pots

    Post by Digdug on Tue Mar 12, 2013 12:45 pm

    Well,

    Just thought I'd give you guys an update. I took the SP12/PH14 into the shop to get it worked on. I'll let you know what they say... Thanks for everybody's help so far!

    -Doug

    Digdug

    Posts : 18
    Join date : 2012-10-05

    Re: Attenuator/Knobs/Pots

    Post by Digdug on Wed May 01, 2013 12:27 pm

    I'm a little late in posting a follow-up, but I did get my preamp back from the shop a while ago. They bypassed the left/right gain knobs and added a goldpoint volume attenuator. Everything is working well and back to normal, so I'm a happy camper. Meanwhile I ordered a little beginner headphone amp kit so I can get some soldering under my belt.

    Thanks everyone, for the assistance with this!

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