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    ST-120 noise issue

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    Maintarget

    Posts : 208
    Join date : 2013-02-10

    ST-120 noise issue

    Post by Maintarget on Tue Jan 20, 2015 12:38 am

    Recently my VTA ST-120 with 800 hours of trouble free listening has developed a hum/crackling noise issue that is best described in the sticky note "quick trouble shooting guide" as a combination of problems #5 & #6

    5. Problem - Amp has a hum that doesn't change in level as you advance the volume control of your preamp
    Areas to check - bad quad cap, bad ground inside the amp, bad tube filament that is leaking AC into the audio

    6. Problem - Hiss, noise, crackling sounds on idle
    Areas to check - Noisy driver tubes, noisy resistor on driver board (especially older carbon composition resistors), bad solder joint or bad solder trace on the driver board.

    I have isolated the hum/noise to the amp by shorting the input leads from the preamp to the amp, my amp has a stepped attenuator so I'm able to use the amp without my DAC and preamp and have noted that hum is consistent and doesn't seem to vary however the hiss/crackling noise seems to come and go and is not consistent.
    Both channels have the hum, crackle noise so i started by changing out the center driver tube with no improvement then I tried swapping side to side then different set's of driver tubes with no improvement.
    I have pushed firmly on the PT while the amp is in operation and did not detect any notable difference in the hum I also tried tightening loosening the mounting screws with no positive results.
    just for the heck of it i also changed from my KT-120s to Kt-88s and from GZ34 to 5AR4 to a WZ68 copper cap, no improvement or change.
    I have noted that when I shut the amp down the hum stops but the crackling noise is still present for a few seconds (I'm guessing caps bleeding off?)
    I have pulled the cover and visually inspected for an obvious bad solder joints using a magnifying glass at this point I have not re flowed any solder joints
    Would the next logical step be to check quad cap and resistors on the driver board?
    Keeping in mind that I'm a novice what would be the next trouble shooting steps should i take any tips or advise would be greatly appreciated
    Thank You:confused:

    kaner

    Posts : 88
    Join date : 2011-09-20

    Re: ST-120 noise issue

    Post by kaner on Tue Jan 20, 2015 10:00 am

    Your noises would typically be two different issues, so it's a bit surprising that they both appear at the same time. I'm not a big believer in conicidenses when it comes to tube amps. I would focus on the crackle first, as that's a bit easier to diagnose. Use a wooden chopstick or plastic probe (not a pencil or pen!) and with the amp open and running see if you can poke around to create the crackle. BE VERY CAREFUL AND KEEP ONE HAND IN YOUR BACK POCKET. THERE"S SEVERAL HUNDRED VOLTS RUNNING THROUGH YOUR AMP. Make sure that you are NOT touching the chassis or anything other than the opposite end of the chopstick. Start around the middle drive tube socket and work from there.

    It's probably a solder joint that's opened up or a bad resistor. Find and fix. I guess I'd be surprised if it fixed your hum and am curious to hear what happens. Hum sounds like AC getting into the signal. Maybe a bad solder joint on your middle driver filament leads??

    Good luck!

    Kaner

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-120 noise issue

    Post by Bob Latino on Tue Jan 20, 2015 10:47 am

    I agree with Kaner ... If you have eliminated tube problems by swapping out tubes, then what is happening is not a tube issue. It is more likely a bad solder connection somewhere on the main power supply IF > the hum is of equal volume in both channels and it does not change with volume levels.

    I have had this same issue come up before. The person rationalizes to themselves that >"it played fine for weeks, months etc. so why NOW do I have a problem and why are you saying it's a bad solder connection?" What happens is that after many hot (expansion) and cold (contraction) cycles a joint becomes "partial" .. the connection is not 100% connected ... Sometimes just resoldering every connection in the main power supply will do it. (quad cap, SCM, ESL, triode/pentode switches, power caps on the driver board, wiring to pins 3 and 4 on each output tube)

    Another thing to try is to see if the noise/hum happens in BOTH triode and pentode ultralinear modes ? If the hum/noise is only in one mode then that might be a clue as to where the problem lies ?

    Kaner's mention of tapping parts of the (inside wiring of the) amp with an INSULATED piece of plastic or a dry piece of wood is also a good idea. You might get lucky and one tap might cause the hum/noise to go away? If so - resolder that connection and some of the surrounding connections (after you turn the amp OFF). Remember also that after you turn the amp off, the quad cap will still hold a charge for a few minutes. You can short the sections of the quad cap to the chassis with an insulated screwdriver after a minute's time just to be sure that the quad cap is completely discharged.

    Bob

    GP49

    Posts : 717
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: ST-120 noise issue

    Post by GP49 on Tue Jan 20, 2015 6:23 pm

    Bob Latino wrote: You might get lucky and one tap might cause the hum/noise to go away?

    Or you might get UNLUCKY and the mere vibration of picking up and moving your amplifier might
    "fix" the bad connection temporarily.

    Don't ask how I know. Evil or Very Mad

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-120 noise issue

    Post by Bob Latino on Tue Jan 20, 2015 7:18 pm

    GP49 wrote:
    Bob Latino wrote: You might get lucky and one tap might cause the hum/noise to go away?

    Or you might get UNLUCKY and the mere vibration of picking up and moving your amplifier might
    "fix" the bad connection temporarily.

    Don't ask how I know.  Evil or Very Mad

    GP49,

    You are so correct .. I once had someone tell me that their amp had hum and noise - BUT - when he opened it up and turned it UPSIDE DOWN to work on the amp - the hum/noise went away! I guess gravity ever so slightly "remade" the connection. He eventually resoldered all the connections in the power supply and solved the issue with his amp.

    Bob

    sKiZo

    Posts : 1285
    Join date : 2013-04-01
    Location : Michigan USA

    Re: ST-120 noise issue

    Post by sKiZo on Tue Jan 20, 2015 10:10 pm

    No bench should be without one of these for getting up close and personal with your gear ...



    Don't forget to look for long tails on capacitors and such, especially around the driver board. Those can cause all sorts of grief. Trim everything tight to the solder. Even if they're not the original source of the problem, poking around inside could move one to where it bites ya.

    Also check for good clearance around the filter caps if you're using the Russian PIOs. Wouldn't be unheard of to have some intermittent arcing from the exposed metal cases, even if only static.

    Which brings to mind humidity levels. Winter heating, lots of static in the air, metal chassis ... good potential for seasonal noise issues. First step is to use a humidifier - I keep the house at around 30% here. Might also help to add a common ground between the amp and the rest of your gear (the phono ground on most receivers is a good point as long as that's also connected to earth). I've also got a flat washer glued to the rack and connected to earth so I can discharge any static buildup due to sock shuffling and such - forced air from winter heating and heavy woolies, and we're talking veritable lightning bolts we are ...

    *** Hummmm ... Don't forget the easiest potential fix ... two wire plug, reverse it in the socket and see if that helps. If the blades are polarized, I don't imagine you'll have black helicopters hovering overhead if you snip the ends of the wide one ... ;-}

    Maintarget

    Posts : 208
    Join date : 2013-02-10

    Re: ST-120 noise issue

    Post by Maintarget on Sun Jan 25, 2015 7:31 pm

    Update
    Kaner & Bob

    I bought myself a handy little insulated screw driver rated to 1000V for probing around under the hood I made a cover for the exposed tip using two pieces of folded over shrink tube (Rated to 600V) and it slips off and on when needed.
    I poked and prodded all wiring and parts with no discernible changes after further diagnoses it does in fact seem that the hum is an amp only issue, So for the heck of it I decided to check voltages as recommended in the kit instructions everything seem to check out fine except for the Rectifier PIN 4 & 6 checks were 16.7 & 36.6 V AC and all V2, V3, V6 & V7 PIN 1 to ground and 8 to ground all measured the same at 0.6 V DC
    I have the sticky note diode upgrade, using a Weber WZ 68 copper cap and KT120s
    All checks with speakers input cables from preamp hooked up, the amp seems to play fine.
    I'm not sure what to do next, Help  pale

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-120 noise issue

    Post by Bob Latino on Sun Jan 25, 2015 7:40 pm

    Maintarget wrote:Update
    everything seem to check out fine except for the Rectifier PIN 4 & 6 checks were 16.7 & 36.6 V AC and all V2, V3, V6 & V7 PIN 1 to ground and 8 to ground all measured the same at 0.6 V DC

    Rectifier pins 4 and 6 to chassis ground should measure about 410 volts AC. I can't believe that you only got 16.7 and 36.6 VAC ? Do you have another meter you can check those rectifier pins 4 and 6 again ? That can't be correct ?

    Bob

    Maintarget

    Posts : 208
    Join date : 2013-02-10

    Re: ST-120 noise issue

    Post by Maintarget on Sun Jan 25, 2015 11:33 pm

    Bob,

    I have the sticky note diode upgrade and just figured out that if I take my measurements on Pins 5 & 7 then I come up with 418.9 V AC on each I'm guessing this would be correct?
    I still only get 0.6 V DC on Pin 1 & 8 to ground on V2, V3, V6 & V7

    MontanaWay

    Posts : 767
    Join date : 2014-02-06
    Age : 58
    Location : Cameron, Montana

    Re: ST-120 noise issue

    Post by MontanaWay on Sun Jan 25, 2015 11:50 pm

    Maintarget wrote:Bob,

    I have the sticky note diode upgrade and just figured out that if I take my measurements on Pins 5 & 7 then I come up with 418.9 V AC on each I'm guessing this would be correct?
    I still only get 0.6 V DC on Pin 1 & 8 to ground on V2, V3, V6 & V7

    pins 1 & 8 are shorted and your BIAS resistors are connected to those, what you are reading is your BIAS voltage of 600mV, so depending on your power tubes, yes, thats correct. KT120's are good at 600mVDC, KT88's should be about 500-550mVDC.
    As has been already advised, go through EVERY solder joint and triple check it, anything that looks even vaguely suspect, touch it up!

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-120 noise issue

    Post by Bob Latino on Mon Jan 26, 2015 8:54 am

    Maintarget wrote:Bob,

    I have the sticky note diode upgrade and just figured out that if I take my measurements on Pins 5 & 7 then I come up with 418.9 V AC on each I'm guessing this would be correct?
    I still only get 0.6 V DC on Pin 1 & 8 to ground on V2, V3, V6 & V7

    OK - With the diode mod, the red secondary wires are connected to pins 5 and 7 ..

    Bob

    Maintarget

    Posts : 208
    Join date : 2013-02-10

    Re: ST-120 noise issue

    Post by Maintarget on Mon Jan 26, 2015 11:24 am

    MW
    I'm obviously new at this and learning as you said what I'm reading is the bias voltage, I'm using KT120s biased at .600mv
    Bob is right in his earlier post stating
    "The person rationalizes to themselves that >"it played fine for weeks, months etc. so why NOW do I have a problem and why are you saying it's a bad solder connection?"
    I will be re-soldering all connection in the power supply ASAP.
    Thanks for your patients you guys are the best!

    Roscoe

    Maintarget

    Posts : 208
    Join date : 2013-02-10

    Re: ST-120 noise issue

    Post by Maintarget on Fri Jan 30, 2015 10:55 pm

    Update:

    Re flowed ALL solder connections in three stages over several days until all were redone long story short no changes until i re-soldered connections on driver board and at the same time loosened the power transformer mounting hardware, sweet success I'm hum-less again!
    Dam this thing sounds awesome!
    Still have a Whooshing crackling sound in between tracks that is UN detectable when audio is present
    Thanks Bob, Kaner & MW

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