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Vox NT15 various problems

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  • #16
    Had a Vox before that ran the heater trace under one of the filter caps and close to it's high voltage terminal. Couldn't find the arcing until I removed the cap. A bit of electrolyte from the leaky cap led to the arcing.
    Since C14 is already suspect, and you can test the amp without it, I highly recommend you remove it. Then proceed as per post #14. Replace any resistors that are bad and then proceed to test voltages with C14 removed.
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


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    • #17
      Without C14 the amp might oscillate/motorboat.
      - Own Opinions Only -

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      • #18
        It might. But with no signal I thought it would probably be ok.
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


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        • #19
          Oscillation can be avoided by pulling V3.
          Oscillation doesn't necessarily require an input signal.
          When there's positive feedback at some frequency a turn-on transient or even thermal noise may suffice to start oscillation.
          Last edited by Helmholtz; 12-26-2023, 10:22 PM.
          - Own Opinions Only -

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          • #20
            Thanks. This amp is my personal amp but 2 DRRI's came on my desk that needs attention. Will keep you posted.

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            • #21
              Damn, I finally got back to this. I found a pack of new 22uf, 450V caps so I replaced C14. I forgot to take measurements before putting in the new cap. R1 is no longer smoking but we are kind of back to square one. Should I remove the new cap and take measurements?

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              • #22
                Ok, I removed C14 and took new measurements.

                120vac in, mode is pentode/bright.

                Q1-R1 340vdc
                R1-R18 321v
                R18-R16 175V
                R16-R13 106V
                R13-R26 40v
                V1p7 342 v
                V1 p9 318v
                V2p7 335 V vV
                V2p9 321v
                V3p1 28V
                V3p6 39V
                V4p1 143v
                V4p6 82v

                OT ct 342vv
                tag 11 322v
                tag 12 322v

                I can see there is a little.more voltage, but it looks like there is still too much current draw. Other than that, I don't know how to interpret this.

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                • #23
                  Disconnect C17, remove V3. Recheck voltage at both sides of R13.
                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


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                  • #24
                    Will do, on Tuesday.

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                    • #25
                      Ok, I had to remove some hard goop (a ceramic type substance as opposed soft silicon gel). But I was careful and got it done. From r16 to r13, I read 103dcv; from r13 to r26 I get 39v. Pretty close to what I got Thursday. Here's the rest.

                      Q1-R1 331vdc
                      R1-R18 310V
                      R18-R16 169V
                      R16-R13 103V
                      R13-R26 39V
                      OT ct 331V
                      V1P7 327V
                      V1P9 310V
                      V2P7 327V
                      V2P9 311V
                      V4P1 138V
                      V4P6 78V​​​​​

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                      • #26
                        Originally posted by rockman627 View Post
                        R16-R13 103V
                        R13-R26 39V
                        Lift the end of R13 that goes to R26. Check that the other side of R13 now has proper voltage, or higher.
                        Then with R13 still lifted, measure resistance across C14 or C17.

                        Originally posted by Enzo
                        I have a sign in my shop that says, "Never think up reasons not to check something."


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                        • #27
                          Ok those readings were with C14 and C17 and V3 disconnected. I hope that was alright. I will lift R13 on the R26 side, reconnect C14, leave C17 out and take the reading.

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                          • #28
                            I reconnected C14,C17 and V3. I broke C17 taking it out so I have a .015 uf chicklet cap instead. I lifted R13 and I get 279vdc on the other side. Turned the amp off and measured 5.7k ohms resistance on C17. Tell me what you think. I'm going to study the schematic and see what I come up with.

                            Just remembered something. Do you want me to measure esr of the cap? I don't have anything to do that.
                            Last edited by rockman627; 03-14-2024, 03:42 PM.

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                            • #29
                              Hey I removed C17 and measured it. I get OL. There is 5.7k ohms resistance between the solder pads.
                              I also removed C14 and measured it. OL also. And also 5.7k ohms between the pads.
                              That can't be right. But the board looks clean.
                              Last edited by rockman627; 03-14-2024, 04:31 PM.

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                              • #30
                                No signs of arcing anywhere?
                                Originally posted by Enzo
                                I have a sign in my shop that says, "Never think up reasons not to check something."


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