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Thread: Loud hum when turning the standby off on my 1990 Fender the twin "red knob" for 20sec

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    Loud hum when turning the standby off on my 1990 Fender the twin "red knob" for 20sec

    Im experiencing a loud hum whenever i turn my 1990 Fender red knob twin standby switch off. This only happens when the amp is cold and has not been used for last couple of hours? Just to mention all of the filter caps were replaced in both the power section and the preamp section including the bias filter cap. Should i call my tech to solve this or can i fix it myself? The loud hum just lasts 20 secs. After that the noise starts to decrease and goes back to normal operation.

    Any clues???

  2. #2
    Supporting Member Jazz P Bass's Avatar
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    It may be that the main power supply capacitors are at the end of there life.

    If one of them is really bad, the amp will hum at turn off.
    When the caps are bad you will see an abnormal amount of Volts ac ripple on them, which can be measured with a decent volt meter set to read Vac.
    If you have anything with a close value (voltage value must be equal or greater) you could run them parallel to the old ones to see if that helps.

  3. #3
    Senior Member potatofarmer's Avatar
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    Have you tried turning the amp on in standby mode, letting it sit for maybe five minutes, and then flipping the standby switch into "play" mode?

    My guess is one of the power tubes (or a pair on one side) takes a little longer to warm up, but I guess that shouldn't matter much on this model.

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    Quote Originally Posted by potatofarmer View Post
    Have you tried turning the amp on in standby mode, letting it sit for maybe five minutes, and then flipping the standby switch into "play" mode?

    My guess is one of the power tubes (or a pair on one side) takes a little longer to warm up, but I guess that shouldn't matter much on this model.
    I guess not. I am just letting them warm for 30 secs and then fire it up. It wasnt producing this loud hum last week. the 4 power tubes are Brand new (Groove Tubes 6L6 GE matched quad) made in china and bought at guitar center in Orlando. Paid US$155 for the set. Only V7 is Brand new (Groove tuve 12ax7-C).

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    sounds like very slow charging filter caps,if the amp is original i guess its time to replace them,you can try parallel a good 22/47uF one at a time on the preamp/PI ones and hear if it gets better.

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    Its weird since i replaced the three preamp 22uf filter caps with brand new ones.

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    Old Timer J M Fahey's Avatar
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    Unplug the PI tube and repeat the test: results?

    As a side note on:
    Im experiencing a loud hum whenever i turn my 1990 Fender red knob twin standby switch off. This only happens when the amp is cold and has not been used for last couple of hours?
    If an amp is cold and unused, so presumably OFF, how can turning Standby switch OFF create any sound?
    It would be switching zero Volts in any case.
    By the same token, how can any amp cold an unused produce any sound at all, no matter what you do with the Standby switch, which would be "switching" zero Volts?
    What element would be producing an acoustic/audible hum sound?

    EDEIT: please post schematic.
    Juan Manuel Fahey

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    Supporting Member eschertron's Avatar
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    Quote Originally Posted by J M Fahey View Post
    What?
    I presume turning standby 'off' is the same as putting into 'play' mode. Power on, standby off.

    Quote Originally Posted by jalexquijano View Post
    I am just letting them warm for 30 secs and then fire it up.
    If 30 sec isn't long enough, but 20 sec more is, try letting it warm up with standby 'on' for a whole minute. If it still hums after that, there's something screwy with what ever follows the standby switch.

    Quote Originally Posted by J M Fahey View Post
    please post schematic.
    +1 so we can see what's before and what's after the standby switch.
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

  9. #9
    Senior Member Enzo's Avatar
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    STandby SWITCH must be on. Both power and standby have to be on to operate.

    Standby switch ON is PLAY mode.

    Turning that switch ON brings the amp out of standby.

    Turn the switch OFF to put it in standby mode.
    Education is what you're left with after you have forgotten what you have learned.

  10. #10
    Old Timer J M Fahey's Avatar
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    Thanks but we so far are having a communications problem with our Friend jalexquijano, we must first sync with him for a successful solution to his problem.

    In any case and to save some time, I already suggested a test to be performed
    Juan Manuel Fahey

  11. #11
    Senior Member Enzo's Avatar
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    I was responding to eschertron.
    Education is what you're left with after you have forgotten what you have learned.

  12. #12
    Supporting Member eschertron's Avatar
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    Quote Originally Posted by Enzo View Post
    STandby SWITCH must be on. Both power and standby have to be on to operate.

    Standby switch ON is PLAY mode.

    Turning that switch ON brings the amp out of standby.

    Turn the switch OFF to put it in standby mode.
    Why of course, you are right
    My personal confusion results from odd labeling. While some amps label their switches as "standby/on", others do not. Case in point: the standby rocker switch on my Fender Super 60 has an "off" position and an "on" position. So of course for the amp to be in standby, the switch has to be in the "off" position. Clever? Confusing enough that I never look at the labels, I read them with my ears.
    Justin Thomas and g1 like this.
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

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    Ok. Once again. I turn turn the amp on, wait for 20 secs and flip the stanby switch on. I start hearing a loud hum for 20 secs which suddenly dissapear. I already swapped some 12ax7 tubes and the proble continues. Should i consider changing the plastic input jacks on the effects loop or else. I reas somewhere that these jacks must be replaced to avoid noises.

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    Supporting Member Jazz P Bass's Avatar
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    Quote Originally Posted by jalexquijano View Post
    Should i consider changing the plastic input jacks on the effects loop or else. I reas somewhere that these jacks must be replaced to avoid noises.
    If you want to 'rule out' the Fx jacks, simply run a jumper cable from one to the other.

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    Quote Originally Posted by jalexquijano View Post
    ...Should i consider changing the plastic input jacks on the effects loop or else. I reas somewhere that these jacks must be replaced to avoid noises.
    Clever, experienced people have taken the time to respond to your specific query.
    How about reading, and acting on, the 1st lines of post #3 and #7?

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    Well i left it on standby for 5 minutes and when flipping to play mode, there was no issue whatsoever. Sorry for the late reply. I thought you only had to leave it on standby for 30 secs and then flip.

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    Quote Originally Posted by eschertron View Post
    I presume turning standby 'off' is the same as putting into 'play' mode. Power on, standby off.



    If 30 sec isn't long enough, but 20 sec more is, try letting it warm up with standby 'on' for a whole minute. If it still hums after that, there's something screwy with what ever follows the standby switch.

    Schematic herein attached!!

    +1 so we can see what's before and what's after the standby switch.
    Schematic attached
    Attached Files Attached Files

  18. #18
    Supporting Member eschertron's Avatar
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    I'm glad you checked a longer warm-up time and found that it eliminates the problem.

    While all the HT components are after the standby switch, filaments and bias are in front. A potential issue with the bias supply?
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

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    Quote Originally Posted by eschertron View Post
    I'm glad you checked a longer warm-up time and found that it eliminates the problem.

    While all the HT components are after the standby switch, filaments and bias are in front. A potential issue with the bias supply?
    Its really strange. Just came home left it on standby for 1 minute, then flip it to play mode and loud hum came out for 15 secs. I start moving the volume 1 pot on the clean channel and it produced some noise during the swipe.

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    Quote Originally Posted by alexradium View Post
    sounds like very slow charging filter caps,if the amp is original i guess its time to replace them,you can try parallel a good 22/47uF one at a time on the preamp/PI ones and hear if it gets better.
    THis morning i power the amp on and let it sit for 5 mins. I then took the standby off by flipping it to "play" mode, result = no issue. If i let it warm only for one minute you hear a big hum which starts decreasing until it gets to normal hum level. Is this a preamp or power capacitor issue?? I replaced all of these capacitors 3 months ago. Could one of them be faulty?? How do i verify without removing them? i dont see any leaking whatsover!!

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    I think that message #3 offers the most likely explanation of the hum.

    To investigate further, I suggest that you use a meter and, as per the schematic notes, monitor the bias then balance mV readings over 2 cold starts.
    Just leave the standby switch in 'operate' mode (standby is a user convenience rather than being a technical necessity or benefit).
    Note how the bias and then balance readings change and then settle down over the first few minutes after powering up from cold.

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    Quote Originally Posted by jalexquijano View Post
    Well i left it on standby for 5 minutes and when flipping to play mode, there was no issue whatsoever. Sorry for the late reply. I thought you only had to leave it on standby for 30 secs and then flip.
    one minute is ok for power tubes filaments to go and pull current.
    I stress the fact that one or more of the filter cap is slow,with this standby on AC side its normal that caps charge in a few seconds,but hearing hum is more related to PI and preamp side for me,just get a new good cap and put it in parallel with a couple of clip wires,and hear if something changes.

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    It should be simple to identify whether the hum is coming from the preamp or from the power tubes / power supplies.
    As noted in post #7; remove the LTP phase splitter tube and see what happens to the hum.

    https://www.phrases.org.uk/meanings/...-to-water.html

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    Quote Originally Posted by pdf64 View Post
    Clever, experienced people have taken the time to respond to your specific query.
    How about reading, and acting on, the 1st lines of post #3 and #7?
    Well last night i removed the chassis from the cabinet. retensioned all preamp and power tube sockets and spray some deoxit D5 on each socket. Left the amp on standby for 1 minute and then switched to play mode. Result: No cracking sound but still some 60 cycle hum present on the background. What else should i check to eliminate this 60 cycle hum. I want this amp to be as clean as posible. I could also determine the following:

    1. Volume 1 potentiometer on clean channel is a Little bit scratchy when swiped.

    2. When you raise the volume to 10 you can hear a Radio Station very low in the background.

    There must be some leak somewhere.

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    Quote Originally Posted by jalexquijano View Post
    .Left the amp on standby for 1 minute and then switched to play mode. Result: No cracking sound but still some 60 cycle hum present on the background. What else should i check to eliminate this 60 cycle hum...
    Remove the tube in the V4 LTP phase splitter socket and see what happens to the hum.

    If it still hums, use a meter and, as per the schematic notes, monitor the bias then balance mV readings over 2 cold starts.
    Just leave the standby switch in 'operate' mode (standby is a user convenience rather than being a technical necessity or benefit).
    Note how the bias and then balance readings change and then settle down over the first few minutes after powering up from cold.

    I wonder if you are viewing the forum in 'threaded' mode, thereby missing messages?
    Check your settings - settings, general settings, thread display mode, set it to linear
    Last edited by pdf64; 11-02-2017 at 08:23 PM.

  26. #26
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    Quote Originally Posted by pdf64 View Post
    Remove the tube in the V4 LTP phase splitter socket and see what happens to the hum.
    These amps have a very awkward layout where tube numbers are not called out from left to right.
    The attached schematic calls out the tube numbers on page 5 (top view of chassis).
    Fortunately, the phase splitter is positioned nearest to the power tubes as is somewhat 'normal'.
    Attached Files Attached Files
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    Quote Originally Posted by pdf64 View Post
    Remove the tube in the V4 LTP phase splitter socket and see what happens to the hum.

    If it still hums, use a meter and, as per the schematic notes, monitor the bias then balance mV readings over 2 cold starts.
    Just leave the standby switch in 'operate' mode (standby is a user convenience rather than being a technical necessity or benefit).
    Note how the bias and then balance readings change and then settle down over the first few minutes after powering up from cold.

    I wonder if you are viewing the forum in 'threaded' mode, thereby missing messages?
    Check your settings - settings, general settings, thread display mode, set it to linear
    I did this test yesterday. I removed V4 which is the phase inverter preamp tube and the hum was gone. However the amp is not playable without that tube.

  28. #28
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    Quote Originally Posted by jalexquijano View Post
    I did this test yesterday. I removed V4 which is the phase inverter preamp tube and the hum was gone. However the amp is not playable without that tube.
    Of course, this is a ***TEST*** to search where the hum comes from !!!!!!!

    Let me guess your future answers to our testing questions:

    * "Plug your amp into a known good speaker" ...
    - "I want to use my own speakers"

    * "Unplug your speakers and measure DC resistance"
    - "amplifier does not sound without speakers"

    * "bend down and listen whether hum comes from speakers or itīs transformer buzzing"
    - "I play standing, why would I bend down?"

    * "move treble pot from 10 to 0 to check whether it influences hum or not"
    - "I always set treble to 7"

    and so on and on and on .....

    Hope you enjoy Hum
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    Juan Manuel Fahey

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    Quote Originally Posted by jalexquijano View Post
    I did this test yesterday. I removed V4 which is the phase inverter preamp tube and the hum was gone. However the amp is not playable without that tube.
    That's important info which has been requested several times, why didn't you disclose it?

    How is the hum affected if a guitar (with the volume controls turned down) is plugged into the 'power amp in' socket (not the 'power amp thru' socket)?

    How about if the tube in V3 socket is removed?

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    Quote Originally Posted by pdf64 View Post
    That's important info which has been requested several times, why didn't you disclose it?

    How is the hum affected if a guitar (with the volume controls turned down) is plugged into the 'power amp in' socket (not the 'power amp thru' socket)?

    How about if the tube in V3 socket is removed?
    V4 (phase inverter) removed, no hum but no sound either

    V3 removed, same as above.

    Guess the problem is somewhere in the preamp section. Its weird as the 3 preamp filter caps were changed with Illinois Brand new caps.

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    Turned amp again today and did noticed that the volume 1 pot of the clean channel produces a scratchy noise when dialing across 0 to 10. It fades out once the amp is fully warmed. Could there be some leak somewhere that is the cause of the background hum and radio frequency signal very low in the background? Or its just the ac outlet causing this hum?

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    Quote Originally Posted by jalexquijano View Post
    Turned amp again today and did noticed that the volume 1 pot of the clean channel produces a scratchy noise when dialing across 0 to 10. It fades out once the amp is fully warmed. Could there be some leak somewhere that is the cause of the background hum and radio frequency signal very low in the background? Or its just the ac outlet causing this hum?
    Use your meter and check for DC on the volume pot, clip leads on there so you can monitor from cold to warm.
    It could be a lot of things but the thing I noticed before is people are trying to get you start with basic troubleshooting and with your replies suggesting a next step.
    nosaj
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    Should i measure dc with the amp on play mode?

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    Quote Originally Posted by jalexquijano View Post
    Should i measure dc with the amp on play mode?
    Clip leads on first turn amp on let it warm up see what measures on vol pot then flip standby switch and see what measurement you have.

    nosaj

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