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Thread: Another Blues Jr problem

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    Supporting Member Randall's Avatar
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    Another Blues Jr problem

    Same amp as previous post, different issue. I changed out the tube board and sorted out the ribbon issue, three wires on one triode needed to be switched around to accomodate this early green board version. I have confirmed by metering from pins 6, 7, and 8 to their respective resistors on the main board. I installed new EL84s and a new speaker jack.

    I thought the amp sounded good when it left, but the customer brought it back saying it wasn't as loud as iy ususally is, and that it had an "overtone". Actually, it sounds kind of like a ring modulator, or a note beating against the played note, or a spaeker issue. A slight fuzz box sound. I have changed all the tubes, clipped across the axial filter caps, and probed around to no avail. I have tried a different cab. In the process, I have isolated a microphonic preamp tube, and a bad new one right out of the box. But I must admit, I'm not sure how to approach this now.

    https://www.thetubestore.com/lib/the...-Schematic.pdf

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    It's weird, because it WAS working fine.....

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    Supporting Member The Dude's Avatar
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    Start old school and insert something into the return jack to see if it's a preamp or power amp problem.

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    Sorry, no FX loop in the Blues Jr. Randall, did you check the control grid and screen grid resistors on the power tubes?

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    Supporting Member The Dude's Avatar
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    Apologies. For some reason I was thinking of the Deluxe/Deville type amps.

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    Supporting Member Jazz P Bass's Avatar
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    "In the process, I have isolated a microphonic preamp tube, and a bad new one right out of the box"

    Well, that right there should be A problem.

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    Lifetime Member Enzo's Avatar
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    and a bad new one right out of the box"
    A lesson everyone has to learn.

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  7. #7
    Supporting Member Randall's Avatar
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    "Randall, did you check the control grid and screen grid resistors on the power tubes?"

    They look good.

    The microphonic and bad new tubes were replaced, problem still exists. This is what it sounds like:

    https://drive.google.com/open?id=1Dg...bFB5x6M5XU_XKv

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    Last edited by Randall; 02-22-2020 at 11:35 PM.
    It's weird, because it WAS working fine.....

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    Don't forget the joker g1's Avatar
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    I know you've tried paralleling good caps, but have you scoped the supply while it's happening?

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    Supporting Member Randall's Avatar
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    Sorry for the bad camera work, not my strong suit. This scoped at the 47mF cap. There is also 68vac ripple at that point. Also, the B+ is only 270vdc at that point, with the other nodes being low as well, so I guess I have a power supply problem.

    https://drive.google.com/open?id=1iw...coevXb10U3Qx2R

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    It's weird, because it WAS working fine.....

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    Lifetime Member Enzo's Avatar
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    That looks like a missing filter cap. make sure connections to same are sound.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Randall
    There is also 68vac ripple at that point. Also, the B+ is only 270vdc at that point.
    Quote Originally Posted by g1 View Post
    I know you've tried paralleling good caps
    Have you clipped another cap across the first B+ filter to see if that sorts out the huge ripple & low power supply voltage? All the evidence points straight to there.

    If that ain't it, I'll eat a second bowl of guacamole.

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    Enjoy. Every. Sandwich.

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    Lifetime Member Enzo's Avatar
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    either bad cap or broken traces to it.

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  13. #13
    Supporting Member Randall's Avatar
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    I get OL on my meter in circuit on the 47uF cap. 20+ year old IC caps, time to go!

    It is going to be hard for him to swallow that this failure came two days after some other work was done.

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    It's weird, because it WAS working fine.....

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Randall View Post
    I get OL on my meter in circuit on the 47uF cap. 20+ year old IC caps, time to go!

    It is going to be hard for him to swallow that this failure came two days after some other work was done.
    Old caps do unexpected things, like fail just when you thought it was safe to rock. At least it's a cheap fix.

    By chance, was that amp left unplayed for a long time? Caps that haven't been charged for years also have a habit of breaking down.

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    Enjoy. Every. Sandwich.

  15. #15
    Supporting Member Randall's Avatar
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    Actually, this amp gets used quite a lot, he gigs like four times a week.

    edit: I was on a hold to not do anything more until he decided. He did decide to do the filter caps, and I cut him a break to make it easy for him. I pulled the 47uF cap, and it is definitely bad.

    Funny how that can happen like that. I always feel like the customer may be suspicious of me with this type of things, so I feel the need to do it at a cut rate. Probably more more my issue than theirs I suppose.

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    Last edited by Randall; 02-23-2020 at 05:11 AM.
    It's weird, because it WAS working fine.....

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    I was on a hold to not do anything more until he decided. He did decide to do the filter caps, and I cut him a break to make it easy for him. I pulled the 47uF cap, and it is definitely bad.
    Apart from bad filtering, the scope pics looks like half-wave rectification. Do you get full B+ with the new 47µ cap? Repetition rate of ripple peaks should be 120Hz. If not check the rectifier diodes/connections.

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    Last edited by Helmholtz; 02-23-2020 at 04:35 PM.
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  17. #17
    Supporting Member Randall's Avatar
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    The 47uF was definitely the problem, it measured 6 nF out of circuit. The 22uFs measured good, but I replaced them while I was in there. Now B+ is back to normal, only a volt or two of ripple, and the scope looks normal. Amp sounds normal again. All power supply connections looked good on the board, but I hit the diodes just in case.

    Glad to be done with this!

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    It's weird, because it WAS working fine.....

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