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Vox AC30 CC1 repair

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  • #1

    Vox AC30 CC1 repair

    Hi I'm trying to repair an AC30 reissue from a friend. It's the CC1 from the 2000's. At first sight it looks good: Power Led is getting current, Standby Led is not. All tubes are glowing (4x EL84 good, preamp 3x 12AX7 slightly glowing, GZ34 glowing). No aparent burnt stuff on circuit boards. Here's what I've done so far:

    1º I did continuity test on the standby switch. it switches well.

    2º I checked AC input fuse, standby fuse, 2 fuses on the secondary of the power transformer and a resistor fuse. All OK.

    3º Plugged off all tubes. I did continuity tests on them based on their datasheets, they all seem good.

    4º Checked voltages on secondary: I get 356Vac (pin6 rectifier tube input to trafo sec central tap), 293Vac (pin4 rec tube input to trafo sec central tap), 5.9Vac, 6.45Vac, 24.9Vac.

    5º Plugged on the GZ34. Checked voltage on output (pins 2 to 8). Getting no DC rectified voltage and 5Vac.

    Could it be the GZ34 the one that it's broken, even glowing and with the continuity test OK? Appearance looks good, it glows bright. Although If I shake it I get a small clinging. I'd rather not spend money stupidly since I have no idea how to fix amps. I've got basic electronic knowledge but I'm a total newbie on this stuff. I appreciate every help, because I have no idea how to keep going from here :P

    Thanks in advance,
    Pablo
    Tags: ac30, cc1, gz34, rectifier tube, vox
  • #2
    Correction, I'm seeing it's not a GZ34, it's a Sovtek 5AR4 SOV. Couldn't see well the print because the ink went away. But I'm seeing in the schematics pdf that it has to be that one.

    Comment

    • #3
      Have you checked the standby switch?
      Experience is something you get, just after you really needed it.

      Comment

      • #4
        Originally posted by nickb View Post
        Have you checked the standby switch?
        Yes, I think it's working properly. I did a continuity test with a multimeter and I get a short with standby OFF and open circuit with standby ON. Also I can see it drains voltage from one of the inputs of the rectifier tube, instead of 356Vac I get 120Vac with standby ON, which I guess it's supposed to do that, not sure though.

        Thanks for the answer Nick,
        Pablo

        Comment

        • #5
          If you get 120VAC out of the transformer instead of around 350VAC then that is a problem right there. Check the secondary DC resistance and all connections form the secondary.
          Experience is something you get, just after you really needed it.

          Comment

          • #6
            Originally posted by nickb View Post
            If you get 120VAC out of the transformer instead of around 350VAC then that is a problem right there. Check the secondary DC resistance and all connections form the secondary.
            I'm not sure if I'm understanding correctly the schematics then. Isn't standby circuit supposed to pull back the input voltage to the rectifier tube, so you don't demand too much power before getting the rectifier tube proper hot and not damage it?

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            • #7
              Ok here's what I get:

              pin 6 rectifier to trafo central tap:
              standby OFF: 377Vac, standby ON: 300Vac

              pin 4 rectifier to trafo central tap:
              standby OFF: 120Vac, standby ON: 300Vac

              You're right there must be something wrong somewhere if I'm getting the 120Vac with standby OFF. I'll keep looking
              Thanks!

              Comment

              • #8
                Even those numbers numbers don't make much sense. Are you sure you are on the PT secondary center tap - hint - it goes to the HT fuse ( which you have checked?).
                Experience is something you get, just after you really needed it.

                Comment

                • #9
                  Originally posted by nickb View Post
                  Even those numbers numbers don't make much sense. Are you sure you are on the PT secondary center tap - hint - it goes to the HT fuse ( which you have checked?).
                  .
                  Yes I was connecting to TT2 which should be the HT fuse output. I get the same results both ways

                  Comment

                  • #10
                    I don't have a CC1 schematic so I'm looking at the CC2 so the TT2 numbers may not mean much. That said, TT2 on the CC2 is ground - not the same thing as the sec CT if the fuse is blown and/or standby is open.


                    Would be mind using 'standby closed' or 'standby open'? 'On' or 'off' can be ambiguous. Thanks.
                    Experience is something you get, just after you really needed it.

                    Comment

                    • #11
                      Originally posted by nickb View Post
                      I don't have a CC1 schematic so I'm looking at the CC2 so the TT2 numbers may not mean much. That said, TT2 on the CC2 is ground - not the same thing as the sec CT if the fuse is blown and/or standby is open.


                      Would be mind using 'standby closed' or 'standby open'? 'On' or 'off' can be ambiguous. Thanks.
                      Yes, sorry. standby on would be closed:

                      pin 6 rectifier to trafo central tap:
                      standby open: 377Vac, standby closed: 300Vac

                      pin 4 rectifier to trafo central tap:
                      standby open: 120Vac, standby closed: 300Vac

                      I tried to polarize the standby led with a power supply with 2V and 20mA. It didn't light up, then I removed the power supply from circuit, switched on again the trafo and somehow now is the standby led the one actually lighting and the power led not, all voltages are the same at secondary. Now I really have no clue what's going on, does that mean anything to you?
                      BTW I'm also looking at a CC2 schematic

                      Thanks a lot for the help, I really appreciate the interest you're taking

                      Comment

                      • #12
                        Power supply schematic attached for those of you that want to play along at home.

                        AC30CC.pdf

                        Vox_AC30CC2_AC30CC2X (2005).pdf

                        Comment

                        • #13
                          I'm going to ignore the reading with the standby open. I'm 100% convinced you are not on the CT, but ground. So you have 300VAC on both rectifier pins that is good. The fuse and standby are OK. That only leaves a bad rectifier.

                          You typically need at least 2.4V for a red led to light, more for other colors. 20mA is a little on the high side. I can't seem to find those indicators on the schematics so there not much more I can say about them for the moment
                          Experience is something you get, just after you really needed it.

                          Comment

                          • #14
                            Originally posted by nickb View Post
                            I'm going to ignore the reading with the standby open. I'm 100% convinced you are not on the CT, but ground. So you have 300VAC on both rectifier pins that is good. The fuse and standby are OK. That only leaves a bad rectifier.

                            You typically need at least 2.4V for a red led to light, more for other colors. 20mA is a little on the high side. I can't seem to find those indicators on the schematics so there not much more I can say about them for the moment
                            Yes you're right, I wasn't really measuring on CT. I get 598Vac on pins 4 to 6 so this must be right. I'm measuring DC voltage from TT27 to TT26 with the rectifier tube plugged and definitely not getting any output DC voltage (2mVdc). It also hums a lot. I really have no explanation to having now lighted up standby led instead of power led

                            Comment

                            • #15
                              How do I measure properly the rectifier tube output? I'm not sure if I'm doing it well

                              Comment

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