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Vox AD120VT Problem

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  • #16
    Do you have AC on BOTH sides of R84 all the way to the anode of D11?
    Last edited by The Dude; 11-22-2017, 04:55 AM.
    "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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    • #17
      Originally posted by The Dude View Post
      Do you have AC on BOTH sides of 84 all the way to the anode of D11?
      195V on the top, .2V on the bottom of 84.

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      • #18
        Check R84- 1K/1W. I believe it's open. You should also check those rectifier diodes for shorts. Check the 216V rail for continuity to ground to make sure there isn't something shorted on your rail. There's probably a reason that resistor went open.

        Edit: Also check that F1 is the correct value. It should have gone first. Maybe someone put a larger fuse in there? (assuming I'm right about the resistor)
        "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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        • #19
          Originally posted by The Dude View Post
          Check R84- 1K/1W. I believe it's open. You should also check those rectifier diodes for shorts. Check the 216V line for continuity to ground to make sure there isn't something shorted on your rail. There's probably a reason that resistor went open.

          Edit: Also check that F1 is the correct value. It should have gone first. Maybe someone put a larger fuse in there?
          Will do ASAP tomorrow. Thanks a lot for your help The Dude.

          The tech I brought it too over 10 years ago said I dropped a pin onto the board and it shorted... so it's starting to make sense here now that you say this. He just had no intention on trying to mess with the insides of this amp, I don't blame him.

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          • #20
            Originally posted by The Dude View Post
            FYI: I don't think Nick was saying your output transformers were bad. He was just wanting you to check voltage there. That voltage comes from the rectifier circuit off the orange wires of the PT.
            Quite so. It was just a convenient test point.

            We're having a late night by the looks of things

            The tech I brought it too over 10 years ago said I dropped a pin onto the board and it shorted... so it's starting to make sense here now that you say this. He just had no intention on trying to mess with the insides of this amp, I don't blame him.
            I was very, very surprised he didn't attempt some basic trouble shooting - having no 218V is a pretty simple thing to test for.
            Experience is something you get, just after you really needed it.

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            • #21
              Originally posted by nickb View Post
              I was very, very surprised he didn't attempt some basic trouble shooting - having no 218V is a pretty simple thing to test for.
              What's up nickb... Yeah I don't know, maybe he did test it I'm not sure. He was a nice guy though, seemed like he was in his late 70's, old school electrical engineer, so I didn't argue. I figured he just didn't wanna mess with it. He showed me the pin it was one off of a dress shirt lol. Didn't charge me a thing of course.

              Anyway I tried my next feeble attempt at diagnostics in regard to what The Dude told me to check.

              Not sure how to check if R84 is open, I guess that means take it off the board and test it, but I might as well just get a few new ones if it's even suspect. There is not much AC getting by it at all, like .2Vac before the rectifiers so something definitely wrong there.

              I checked the rectifiers and they seem fine - I'm getting .523 V on diode check and 0L when I flip the meter leads around.

              And for the ground continuity check on the 216V line... (which I'm only getting 195V really, straight off that orange wire before R84) I don't get continuity to ground until I check after the D13 and D14 rectifiers on their anode side, not if I check right at TAG4 connection. Is that how it should be?

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              • #22
                Originally posted by imf View Post
                What's up nickb... Yeah I don't know, maybe he did test it I'm not sure. He was a nice guy though, seemed like he was in his late 70's, old school electrical engineer, so I didn't argue. I figured he just didn't wanna mess with it. He showed me the pin it was one off of a dress shirt lol. Didn't charge me a thing of course.

                Anyway I tried my next feeble attempt at diagnostics in regard to what The Dude told me to check.

                Not sure how to check if R84 is open, I guess that means take it off the board and test it, but I might as well just get a few new ones if it's even suspect. There is not much AC getting by it at all, like .2Vac before the rectifiers so something definitely wrong there.

                I checked the rectifiers and they seem fine - I'm getting .523 V on diode check and 0L when I flip the meter leads around.

                And for the ground continuity check on the 216V line... (which I'm only getting 195V really, straight off that orange wire before R84) I don't get continuity to ground until I check after the D13 and D14 rectifiers on their anode side, not if I check right at TAG4 connection. Is that how it should be?
                The ground is fine. To check R84 just turn off and measure it's resistance. I agree with the Dude - it's odd that should blow before the fuse.

                195VAC is fine too.
                Experience is something you get, just after you really needed it.

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                • #23
                  Oh ok cool thanks. Yep it's open, read 0L. I tried a couple other resistors and they read fine. I'll check the fuse for correct value now...

                  here's a pic, it says "carbon resistor" on material list, is that carbon film?

                  Click image for larger version

Name:	ad120vt-R84.jpg
Views:	1
Size:	2.31 MB
ID:	847776

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                  • #24
                    Originally posted by imf View Post
                    Oh ok cool thanks. Yep it's open, read 0L. I tried a couple other resistors and they read fine. I'll check the fuse for correct value now...

                    here's a pic, it says "carbon resistor" on material list, is that carbon film?

                    [ATTACH=CONFIG]45807[/ATTACH]
                    From the apparent physical size of it, I'd think metal oxide. What's the wattage? Probably a 1W CF?

                    Standing the resistor a little off the PCB will help if this has chronic heat issues. Catastrophic over-current, not so much.
                    If it still won't get loud enough, it's probably broken. - Steve Conner
                    If the thing works, stop fixing it. - Enzo
                    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

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                    • #25
                      Originally posted by eschertron View Post
                      From the apparent physical size of it, I'd think metal oxide. What's the wattage? Probably a 1W CF?

                      Standing the resistor a little off the PCB will help if this has chronic heat issues. Catastrophic over-current, not so much.
                      Hey eschertron, it's 1K/1W.

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                      • #26
                        BTW, you guys are good... The Dude was right... this is a T125mAL fuse in here... I'll be damned. Should be a T100mAL.

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                        • #27
                          Originally posted by imf View Post
                          BTW, you guys are good... The Dude was right... this is a T125mAL fuse in here... I'll be damned. Should be a T100mAL.
                          I think Dude had money on it being a 20A fuse

                          1W carbon film aught to be right for that size resistor. What's its function? I haven't looked at the schem.
                          If it still won't get loud enough, it's probably broken. - Steve Conner
                          If the thing works, stop fixing it. - Enzo
                          We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                          MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

                          Comment


                          • #28
                            Originally posted by eschertron View Post
                            I think Dude had money on it being a 20A fuse

                            1W carbon film aught to be right for that size resistor. What's its function? I haven't looked at the schem.
                            lol - i'm worse than an amateur here, sorry guys.

                            It's the resistor right after the 216V PT lead hits the board, right before the rectifier diodes.

                            I got 1k/1w Xicon carbon film and some of the same in metal oxide at close to the same size at mouser. The one I pulled out is 3.2mm x 10.5mm something like this. Is there a preference for metal oxide in the power section over carbon film, or really not much of a difference?

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                            • #29
                              The resistor composition makes little difference assuming the proper fuse is installed. Any 1K/1W would work. As eschertron suggested, I would leave the leads a little longer when you replace it and get it standing up off of the circuit board a bit to keep heat away from the board.
                              "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                              • #30
                                Originally posted by The Dude View Post
                                The resistor composition makes little difference assuming the proper fuse is installed. Any 1K/1W would work. As eschertron suggested, I would leave the leads a little longer when you replace it and get it standing up off of the circuit board a bit to keep heat away from the board.
                                Cool, will do as suggested. Thanks a lot everyone, I'll let you know how it goes after I get the components.

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