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Thread: VOX AC15 Heritage... TB Channel bleeds into the EF86 channel.

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    VOX AC15 Heritage... TB Channel bleeds into the EF86 channel.

    Hello to everyone! This is my first post on Listen Live.

    I have a beautiful 2007 AC15 Heritage amplifier that was working fine. I replaced the JJ EZ81 with a NOS RCA EZ81 rectifier tube. The amp began emitting strange static, almost "frying egg" noises, so I shut it down, and put the JJ EZ81 back in. I also replaced the stock tubes with NOS tubes, which all test good. Here's what's happening...

    - I can hear volume when the TB channel is set to zero.
    - The TB channel is bleeding into the EF86 channel, with nothing plugged into the TB channel. If I plug into the EF86 channel only, I can turn the TB channel volume pot up and down, and hear it through the EF86 channel.
    - The tone controls do effect the bleed-over signal, so I assume that this "leak" is making it past the tone stack.
    - The EF86 channel operates normally, and does not bleed signal into the TB channel.
    - If I remove V2, there is no bleed-over, as there's no sound from the TB channel at all.
    - EF86 channel operates normally.

    I've tried swapping tubes, clipped the bright cap, retouched a few iffy solder points... so far, nothing. Issue remains. I'm no electronics expert by any stretch, but I can effects repairs with direction. Could the NOS EZ81 tube have damaged some capacitors? If the problem is a failed TB channel volume pot, I would not expect the bleed-over signal to increase through the EF86 channel, as the volume pot is turned up. Could the TB channel be "polluting" the EF86 through the B+ somewhere? If yes, would the EF86 channel not also bleed into the TB channel? I have a full set of electrolytic capacitors, if needed.

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    Master Destroyer nosaj's Avatar
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    Service manual link http://dealers.korgusa.com/svcfiles/...&L_SManual.pdf

    Did you test the RCA ez81 before you put it in? NOS can also mean old dead tube stuck in a nice box. Things like rectifier tubes I like to always test for shorts before using them just because it can be so catasstropic.....
    nosaj

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    I sure did. I checked it on a Hickok 6000A tester. Tested good.

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Cooper1962 View Post
    I sure did. I checked it on a Hickok 6000A tester. Tested good.
    And it still tests good for no shorts?
    Put all stock tubes back in please and start at the beginning. How does amp sound with all stock tubes?


    nosaj

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    I just re-checked the 6CA4 (EZ81). Tests good. I've tried several sets of preamp tubes, including the originals... Issue remains. I'm tempted to redress every solder point on the tube sockets.

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Cooper1962 View Post
    I just re-checked the 6CA4 (EZ81). Tests good. I've tried several sets of preamp tubes, including the originals... Issue remains. I'm tempted to redress every solder point on the tube sockets.
    please take some pictures of the work you've done.

    thanks,
    nosaj

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    Quote Originally Posted by Cooper1962 View Post
    Could the TB channel be "polluting" the EF86 through the B+ somewhere? If yes, would the EF86 channel not also bleed into the TB channel? I have a full set of electrolytic capacitors, if needed.
    I'm thinking along those lines. A hi voltage cap could have been damaged by a dodgy rectifier. Or just went bad anyway. Given the "quality" of components put in MI amps these days, we sometimes see electrolytic caps going bad in as little as 7 years. Try clipping-in a filter cap parallel to the others, one at a time, see if the problem doesn't resolve. If the crosstalk does diminish when you apply this trial to any of the filter caps, then that's your bad cap. Prime candidates are C1, C2, C3, C17, C18. Should take just a couple minutes. Good luck!

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    I haven't done any work, yet.

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Cooper1962 View Post
    I haven't done any work, yet.
    clipped the bright cap, retouched a few iffy solder points... so far, nothing.

    That counts as work to me.
    nosaj

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    I have a full set of Nichicon VX series and Mallory 150 caps for the amp. Should I just replace the caps?

    The wires going from the pots to the board are 3 into 1 bundled type. I've ever seen this before in a tube amplifier. Should I rewire the pots with individual wires to each connection point?

    Pics of the 3 into 1 leads from the pots...


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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Cooper1962 View Post
    I have a full set of Nichicon VX series and Mallory 150 caps for the amp. Should I just replace the caps?

    The wires going from the pots to the board are 3 into 1 bundled type. I've ever seen this before in a tube amplifier. Should I rewire the pots with individual wires to each connection point?

    Pics of the 3 into 1 leads from the pots...

    i wouldn't rewire the pots, but how about this recreate your problem then use a chopstick and move the wires from the volume pots and see if there's any change.
    nosaj

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    Double post.

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    Quote Originally Posted by nosaj View Post
    clipped the bright cap, retouched a few iffy solder points... so far, nothing.

    That counts as work to me.
    nosaj
    The bright cap leg was soldered back to its original position.

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    Quote Originally Posted by Leo_Gnardo View Post
    I'm thinking along those lines. A hi voltage cap could have been damaged by a dodgy rectifier. Or just went bad anyway. Given the "quality" of components put in MI amps these days, we sometimes see electrolytic caps going bad in as little as 7 years. Try clipping-in a filter cap parallel to the others, one at a time, see if the problem doesn't resolve. If the crosstalk does diminish when you apply this trial to any of the filter caps, then that's your bad cap. Prime candidates are C1, C2, C3, C17, C18. Should take just a couple minutes. Good luck!
    Should I use an equal value or higher cap for the testing?

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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by Cooper1962 View Post
    Should I use an equal value or higher cap for the testing?
    YES, Equal or higher voltage.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Cooper1962 View Post
    I have a full set of Nichicon VX series and Mallory 150 caps for the amp. Should I just replace the caps?
    That's called "shotgunning." We prefer to not do that around here.

    Although... you may have seen the notorious YouseTube tech "guitologist" do it. He had an infamous case a couple months ago where he shotgunned a Mesa combo amp, and never did solve the problem. He ended up buying the amp, then dismantled it into components, just for spite. Then he cussed out commenters who called him a "parts changer." Which is exactly what he was doing. NOT the way to fix things... Let's us avoid doing like that.

    You could use any of your filter caps for a test, I'd pick a 22 uF 450V. It's not necessary to match the value of the cap you're paralleling, just for a quick test. Please make sure you have the polarity right. Ask me how I know . . . yes I've wrecked a couple perfectly good caps. Oh well, live & learn. And better to learn from other people's misteaks, right?

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    Quote Originally Posted by Leo_Gnardo View Post
    That's called "shotgunning." We prefer to not do that around here.

    Although... you may have seen the notorious YouseTube tech "guitologist" do it. He had an infamous case a couple months ago where he shotgunned a Mesa combo amp, and never did solve the problem. He ended up buying the amp, then dismantled it into components, just for spite. Then he cussed out commenters who called him a "parts changer." Which is exactly what he was doing. NOT the way to fix things... Let's us avoid doing like that.

    You could use any of your filter caps for a test, I'd pick a 22 uF 450V. It's not necessary to match the value of the cap you're paralleling, just for a quick test. Please make sure you have the polarity right. Ask me how I know . . . yes I've wrecked a couple perfectly good caps. Oh well, live & learn. And better to learn from other people's misteaks, right?
    Thank you. I don't generally source YT for advice on repairs. I was speaking in general, since I already have the parts. Yes, I'd rather us track down the problem, and learn from the experience.

    I just got through redressing the solder points on the tube sockets, and the TB channel volume pot. When I tested it, there was no bleed-over. Yay! I shut it down, then thought; Wait a minute, I better check it again... After turning it back on, it's back to doing the same thing! Why would it test fine at first, then go back to having the same issue?

    I have some cables with alligator clips. Should I use these to position the caps in parallel?


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    Quote Originally Posted by Cooper1962 View Post
    I have some cables with alligator clips. Should I use these to position the caps in parallel?
    Nice clip lead kit! Should work a treat.

    Now I've had a peek at your close in photo of the circus board, interesting build they have, with turret posts for components, and those turret posts poking thru the board onto the solder land side. Imma thinkin' .... what if one of those turret posts broke free of its solder on the back of the board, you could have a perfectly good filter cap, but if its turret isn't firmly attached to the land side of the board, that is, has an intermittent connection there, that would give you the crosstalk symptom which cuts in & out. Yes a total PIA to have to access the back of the board, but that may be where your problem is.

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    Quote Originally Posted by Leo_Gnardo View Post
    Nice clip lead kit! Should work a treat.

    Now I've had a peek at your close in photo of the circus board, interesting build they have, with turret posts for components, and those turret posts poking thru the board onto the solder land side. Imma thinkin' .... what if one of those turret posts broke free of its solder on the back of the board, you could have a perfectly good filter cap, but if its turret isn't firmly attached to the land side of the board, that is, has an intermittent connection there, that would give you the crosstalk symptom which cuts in & out. Yes a total PIA to have to access the back of the board, but that may be where your problem is.
    I'll try bypassing the caps.

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    Last edited by Cooper1962; 07-03-2019 at 07:14 PM.

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    Quote Originally Posted by Cooper1962 View Post
    I'll try bypassing the caps.
    Drain your FILTER CAPS!!!
    So try this with power off, for C1 clip one lead to chassis ground, then the other lead to negative side of cap, then see if pressure or wiggling turret fluctuates the ohms seen.
    For positive side clip one lead to positive side of C1 then the other lead to v7 pin3 and see if pressure on turret fluctuates reading

    nosaj

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    After checking several capacitors, I decided to just replace all the caps. I was planning on doing it anyway, so I figured I may as well just do it, then check for defects. I fired the amp up, and the issue remains. The bleed-over doesn't seem as bad as it was, but it's still there. So, I'm back to square-one. I'm out of Deoxit until Friday, so I can't clean the sockets until then. I'll retouch the rest of the pots tomorrow with 63/37 Kester solder.

    Here's the amp, post capacitor replacement...




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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Cooper1962 View Post
    After checking several capacitors, I decided to just replace all the caps. I was planning on doing it anyway, so I figured I may as well just do it, then check for defects. I fired the amp up, and the issue remains. The bleed-over doesn't seem as bad as it was, but it's still there. So, I'm back to square-one. I'm out of Deoxit until Friday, so I can't clean the sockets until then. I'll retouch the rest of the pots tomorrow, with some 63/37 Kester solder. I'm beginning to think this issue may not a circuitry problem, but possibly one of inductance.

    Here's the amp, post capacitor replacement...



    Have you chop sticked the wires an board to see if it affects noise? Making all these changes without a good reason can introduce other issues. I would turn amp on do what ever you need to do to make the noise then move the grey wires. we are trying to see if Lead Dress has any effect on noise.
    nosaj

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    I fired it up to do just that, but there's popping through the speaker due too lightening.

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    Last edited by Cooper1962; 07-04-2019 at 01:40 AM.

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    Quote Originally Posted by nosaj View Post
    Have you chop sticked the wires an board to see if it affects noise? Making all these changes without a good reason can introduce other issues. I would turn amp on do what ever you need to do to make the noise then move the grey wires. we are trying to see if Lead Dress has any effect on noise.
    nosaj
    Okay, I just got through probing the circuit. All seems good, so I'm thinking there's something going on with the volume pot. Sure, it tests good via DMM, but there does seem to be a delay in the way the volume rolls-on. I'm beginning to think this issue may be related to signal induction? Should I commit to wiring the pots with an independent lead at each terminal, vs the 3 into 1 bundled wire? I suppose I will also need to get some shielded wire as well. Maybe use a shielded wire to the V2 (TB channel) grid?

    I'm also thinking about nixing those PCB inputs in favor of CLIFF jacks with metal panel nuts. What do you think?

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    Quote Originally Posted by Cooper1962 View Post
    Okay, I just got through probing the circuit. All seems good, so I'm thinking there's something going on with the volume pot. Sure, it tests good via DMM, but there does seem to be a delay in the way the volume rolls-on. I'm beginning to think this issue may be related to signal induction? Should I commit to wiring the pots with an independent lead at each terminal, vs the 3 into 1 bundled wire? I suppose I will also need to get some shielded wire as well. Maybe use a shielded wire to the V2 (TB channel) grid?
    I'm just gonna go ahead an say it..

    All this from plugging in a NOS rectifier tube? If it was truly working before you put the EZ81 in , I fail to see how changing the volume wires will help this.
    In this case I think a scope is warranted.

    nosaj

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    I see the ORIGINAL problem as possibly being caused by 2 things.. microponic tubes physically coupling or the power supply being modulated. It may be normal. If you are having a volume control issue now.. check for DC on the pot and suspect a coupling cap. It seems like you are throwing parts at a problem.. not analyzing it first.

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    okay, does the volume on the EF86 channel have any effect on the signal that's bleeding through?

    Try grounding the grid to V3B. Does the signal/noise bleeding go away?

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    Quote Originally Posted by olddawg View Post
    I see the ORIGINAL problem as possibly being caused by 2 things.. microponic tubes physically coupling or the power supply being modulated. It may be normal. If you are having a volume control issue now.. check for DC on the pot and suspect a coupling cap. It seems like you are throwing parts at a problem.. not analyzing it first.
    Thank you, I will do as you recommend. How do I check for DC on the pot?

    I was planning recapping it anyway, so nothing has been wasted. It actually enjoy doing it. I love the look of blue Nichicon + Mallory 150 mustard caps!

    I forgot to mention... I had also replaced the OEM EF86 with a =C= EF86 when I installed the RCA EZ81. Sounds fabulous! The OEM EF86 tube comes heavily shielded in shrink wrap. It has been said this was done to lessen vibration, though I don't believe this is entirely accurate. The VOX AC15 (AC30) Heritage combo version chassis' come equipped with a "floating" EF86 socket. The head version chassis' do not. The cushioned EF86 socket minimizes vibration, and wrapping EF86 helps minimize microphonic effects.

    Could an unshielded EF86 valve pick up signal from the TB channel V2? They sit less than 3" apart, so...

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    Last edited by Cooper1962; 07-04-2019 at 03:43 PM.

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    Quote Originally Posted by SoulFetish View Post
    okay, does the volume on the EF86 channel have any effect on the signal that's bleeding through?

    Try grounding the grid to V3B. Does the signal/noise bleeding go away?
    This was something that I checked on day 1 or day 2... I'm going to say, no. It's difficult to tell, really, as there is limited equalization on that channel. 1 x 3 way voicing select + 1 x bass shift switch.

    I'm a bit nervous about grounding things, as I've never done this before. As long as a terminal or lead references to ground, is it okay to bypass these points directly to the chassis?

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    Last edited by Cooper1962; 07-04-2019 at 04:40 PM.

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    Quote Originally Posted by Cooper1962 View Post
    Thank you, I will do as you recommend. How do I check for DC on the pot?

    I was planning recapping it anyway, so nothing has been wasted. It actually enjoy doing it. I love the look of blue Nichicon + Mallory 150 mustard caps!

    I forgot to mention... I had also replaced the OEM EF86 with a =C= EF86 when I installed the RCA EZ81. Sounds fabulous! The OEM EF86 tube comes heavily shielded in shrink wrap. It has been said this was done to lessen vibration, though I don't believe this is entirely accurate. The VOX AC15 (AC30) Heritage combo version chassis' come equipped with a "floating" EF86 socket. The head version chassis' do not. The cushioned EF86 socket minimizes vibration, and wrapping EF86 helps minimize microphonic effects.

    Could an unshielded EF86 valve pick up signal from the TB channel V2? They sit less than 3" apart, so...
    Do you have a pic of the oem tube shielded in shrink wrap?

    nosaj

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    Quote Originally Posted by Cooper1962 View Post
    This was something that I checked on day 1 or day 2... I'm going to say, no. It's difficult to tell, really, as there is limited equalization on that channel. 1 x 3 way voicing select + 1 x bass shift switch.

    I'm a bit nervous about grounding things, as I've never done this before. As long as a terminal or lead references to ground, is it okay to bypass these points directly to the chassis?
    In this instance, it is perfectly fine to directly ground that grid. But, it some circuits, it isn't okay.
    But, since we're troubleshooting noise issues here, we can AC ground different points of the circuit to help us locate where the source of the problem is and eliminate others. Here is how we can do that:
    Take a fairly high value/high voltage film capacitor, lets say .47F/630V (or as close as you can get to that), and connect one lead to ground via an alligator clip and VERY CAREFULLY use the other lead as a probe. You can use this to AC shunt different points of the circuit (ie grids, plates, pots, whatever) to ground. This won't upset any of the DC operating points for the most part and will not harm your amp. But keep in mind, please, that when probing high voltages, the cap will charge up and can shock the shit out of you. So, it's good practice to discharge it after each time you probe the a part of the live circuit.
    I want to ensure you're comfortable and understand what I'm describing so you can do this safely, so let me know if you're unsure of anything.
    The two most common ways that this problem can happen is that you have a compromised ground in the channel that is bleeding through. In this case, I'd be suspicious of the volume pot. The other common cause is a failing filter capacitor that is not decoupling as it should. This could be from the electrolyte degrading, or from one of the solder joints failing (probably on the grounded lead). But, there are always all of those really uncommon ways things can go wrong as well.
    This will help us look for the common ways first.

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    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  32. #32
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    Quote Originally Posted by nosaj View Post
    Do you have a pic of the oem tube shielded in shrink wrap?

    nosaj
    Sure! Here's a OEM EF86 in a combo version. Notice the floating socket mount... The head version (which is what I have) doesn't have this.

    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  33. #33
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    Quote Originally Posted by SoulFetish View Post
    In this instance, it is perfectly fine to directly ground that grid. But, it some circuits, it isn't okay.
    But, since we're troubleshooting noise issues here, we can AC ground different points of the circuit to help us locate where the source of the problem is and eliminate others. Here is how we can do that:
    Take a fairly high value/high voltage film capacitor, lets say .47F/630V (or as close as you can get to that), and connect one lead to ground via an alligator clip and VERY CAREFULLY use the other lead as a probe. You can use this to AC shunt different points of the circuit (ie grids, plates, pots, whatever) to ground. This won't upset any of the DC operating points for the most part and will not harm your amp. But keep in mind, please, that when probing high voltages, the cap will charge up and can shock the shit out of you. So, it's good practice to discharge it after each time you probe the a part of the live circuit.
    I want to ensure you're comfortable and understand what I'm describing so you can do this safely, so let me know if you're unsure of anything.
    The two most common ways that this problem can happen is that you have a compromised ground in the channel that is bleeding through. In this case, I'd be suspicious of the volume pot. The other common cause is a failing filter capacitor that is not decoupling as it should. This could be from the electrolyte degrading, or from one of the solder joints failing (probably on the grounded lead). But, there are always all of those really uncommon ways things can go wrong as well.
    This will help us look for the common ways first.
    Should I first attach one end of the cable to the middle terminal of the TB volume pot, and the other end to the chassis, then turn on the amp?

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    I'm curious... What does this diode + capacitor + resistor circuit assembly do?

    Click image for larger version. 

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  35. #35
    Supporting Member Jazz P Bass's Avatar
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    I have seen that on Marshall amps.

    Some kind of reverse protection chassis reference.

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    Last edited by Jazz P Bass; 07-05-2019 at 03:27 PM.

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