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    ST120 Bias pots, cleaning.

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    ArlanB

    Posts : 52
    Join date : 2011-01-23
    Age : 69
    Location : Santa Cruz, California

    ST120 Bias pots, cleaning.

    Post by ArlanB on Sun Jun 05, 2011 10:02 pm

    My ST120 is now one month short of 3 years in age and I am having an increasingly difficult time setting the bias and having a steady reading on the meter. It appears that the pots are dirty and need cleaning. Will they withstand use of the Deoxit products discussed earlier? I would rather look the fool and ask this simple question rather than have to change these pots because I charged right in unenlightened.

    Has anyone suggestions regarding what would be best in this situation?


    ArlanB


    j beede

    Posts : 317
    Join date : 2011-02-07
    Location : California

    Re: ST120 Bias pots, cleaning.

    Post by j beede on Mon Jun 06, 2011 1:35 am

    My Mark III and ST-70 still have the original 40-50 year old bias pots in place... Before I set the bias I first sweep the pots from fully CW to fully CCW several times before leaving them roughly centered. I usually apply some downward axial force while I sweep the pots this way. I have seen this improve occasional noise in these pots.
    ...j

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST120 Bias pots, cleaning.

    Post by Bob Latino on Mon Jun 06, 2011 6:20 am

    Hi Arlan,

    After 3 years it is possible that the output tubes are starting to age. Older output tubes sometimes > won't bias, are hard to bias or develop a "cycling bias". Cycling bias is when the bias goes up and then down and then up and down etc. > on one or more tubes. Also > try a new rectifier tube in there. A weak rectifier sometimes makes it it hard to bias output tubes. Although it may be possible, I don't think the bias pots are at fault.

    Bob

    ArlanB

    Posts : 52
    Join date : 2011-01-23
    Age : 69
    Location : Santa Cruz, California

    Re: ST120 Bias pots, cleaning.

    Post by ArlanB on Mon Jun 06, 2011 9:03 am

    jbeede & Bob, Thanks for the information. I will try both suggestions. The output tubes are just coming up on a year old with about 750 hrs on them. Hopefully they are still serviceable. The rectifier was replaced at the same time as the power tubes. However I will also swap in a new one and see what happens. The reason that I am suspicious of the pots is that when I touch the center slot to make adjustments, the readings begin to jump around i.e. .548 to .569 and back to .560 and back and fourth and so on until I remove the screwdriver. Then it will settle down to only jumping back and fourth from say .551 to .549 and so on eventually settling down. Then if I touch the pot again the same thing occurs. This makes it difficult to be sure where the bias is set. Hopefully these suggestions will help.

    Again thank.

    ArlanB

    GP49

    Posts : 718
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: ST120 Bias pots, cleaning.

    Post by GP49 on Mon Jun 06, 2011 10:56 am

    One more thing: check the connections where the pot is soldered to the board. You may have a hairline crack, or perhaps the solder connection did not "wet" the surfaces at original assembly, so that the very minor physical vibration of touching it with the screwdriver is causing an intermittent connection.

    I don't know what kind of pots were used on that board. I would be surprised if the DeOxit® products would do them any harm. However if you wish to be 100% safe, do as recommended by "j beede". With the amplifier turned OFF, sweep the bias adjust pots from one end to the other a couple of dozen times and see if that makes any difference when you try to bias the amp again. This is likely to clean up any deposits on the resistance element or corrosion on the wiper.

    Do not worry about "wearing out" the pot. If that's enough to wear it out, you needed a new one anyway and the manufacturer needs to take a careful look at the materials and workmanship they used when making it.

    j beede

    Posts : 317
    Join date : 2011-02-07
    Location : California

    Re: ST120 Bias pots, cleaning.

    Post by j beede on Mon Jun 06, 2011 6:48 pm

    Embarassed I should have mentioned that the pots are best swept fully CW and CCW before the turning the amp(s) on.

    Does the ST-120 have forward diodes ahead of pins #4 and #6 on the rectifier? If not you might add these and reset the bias divider as advised in numerous prior posts. This way you will likely be using a new (and ostensibly more reliable) portion of the bias pots Smile

    ArlanB

    Posts : 52
    Join date : 2011-01-23
    Age : 69
    Location : Santa Cruz, California

    ST-120 Bias pots, cleaning.

    Post by ArlanB on Mon Jun 06, 2011 9:56 pm

    Thank you all for your input. I took the path of least resistance and tried sweeping the pots from lock to lock several times in succession and this seems to have produced acceptable results. I let the amp warm up for about 15 minutes and then began the adjustment procedure. The tendency for skip when initially touching the pots is now gone and the wild jumping around has significantly diminished. Seems that the pots are dirty. I will now include sweeping the pots several times before powering up when making bias adjustments. Very Happy


    ArlanB

    wolverine

    Posts : 60
    Join date : 2010-02-20

    Re: ST120 Bias pots, cleaning.

    Post by wolverine on Sat Jun 11, 2011 8:19 am

    Something that happened to me a while back...the bias on one side of my amp was jumping up and down. I figured it had to be the output tubes so I changed them to tubes from another amp that was running great. Same problem occurred. Finally, after digging around in the amp I found the 15.6 bias resistor was the problem. I could see just a tiny wiggle in the wire when I moved it. The solder connection was not good so after touching a soldering iron on it, the problem was fixed. I have taken to using a tiny dab of flux on the metal before soldering these days and that has helped my amature soldering problems. Freakin soldering connections right LOL.

    mantha3

    Posts : 298
    Join date : 2010-11-10

    Re: ST120 Bias pots, cleaning.

    Post by mantha3 on Sat Jun 11, 2011 8:33 am

    Nice! Good feeling when ya find the issue and fix it. Back to sweet sound

    ArlanB

    Posts : 52
    Join date : 2011-01-23
    Age : 69
    Location : Santa Cruz, California

    ST-120 Bias pots, cleaning.

    Post by ArlanB on Sat Jun 11, 2011 9:56 pm

    As Jackie Gleson used to say "How sweet it is". Bob Latino has a real gem in his ST-120 kit. Tomorrow makes 3 years since I bought the kit and have not regretted one moment of it. My kit is a stock kit with none of the extra improvements that are offered and it sounds great. Thanks Bob.

    ArlanB Very Happy

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