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Peavey VT Classic 212 Broken Phase Help

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  • Peavey VT Classic 212 Broken Phase Help

    Hey everyone,

    I have a Peavey VT Classic 212 amp. Completely stock (with Sylvania tubes) except for the replacement Celestion speakers I installed. I bought the amp about 10 years ago with the phaser feature broken. Just curious as to what it would take to fix it?

    Here are the symptoms:
    The amp will switch to and from the phase channel. But when the knob is switched on, the phase wont sweep, like its stuck on one position/frequency along the sweep. However, if I turn the phase knob from OFF quickly and directly up to 10, once it hits 8, it will make one cycle of the phase and then stops. (I'm working on getting a video and putting it up here).

    Because of this, I'm thinking its the knob itself. Any suggestions?

  • #2
    Peavey Classic VT phaser

    O/k, I posted the schematic below.
    The phase goes through IC U7, U8, U9 & U10.
    IC U13 is the switch.
    IC U11 & U12 are the rate.
    Your going to have to do some snooping to see where it is failing.
    Attached Files

    Comment


    • #3
      Where to start?

      Thanks Jazz, I guess my next question is where to start and how to do it? I have multimeters and stuff like that, but im not sure what to test on each individual component in the schematic. I'm pretty good at digging into technical stuff like this and taking stuff apart, i just need a little electrical engineering wisdom, haha.

      Comment


      • #4
        Peavet Phase

        Try this.
        Set your DVM to measure Vac.
        Black lead to chassis ground.
        Hook up a sound source into the input of the amp, Amp on, volume not too loud.
        The first thing you want to check, with Red lead, is IC U13. It is a switch. Pin 7 is AC signal in. Measure the Vac signal at Pin7. (if it not there, we will have to back up)
        With Phaser switch on, you should see the same AC amplitude on pin 3.
        If you do not, the IC may be bad.
        Check it out, we'll take it from there

        Comment


        • #5
          U11,12 are the phase clock, so there is either a slow oscillation at U12 pin6 - the rate pot - or there is not. Start there.
          Education is what you're left with after you have forgotten what you have learned.

          Comment


          • #6
            getting started

            Ok fellas, before I tear into this thig (tomorrow), I wanna make sure I'm gonna be testing the right components. I looked over the 2nd page of the .pdf you sent over, Jazz, and couldn't find the component labeled "IC U13" (what's "IC" and "U" stand for?). I do see a U13 component with 8 black dots around it, which I am assuming which is the one youre talking about? Im also not sure which pin is #7. And Enzo, which pin is #6 on U12? Sorry for all the questions guys, I'm an engineer and a nerd so I just wanna be sure.
            Attached Files

            Comment


            • #7
              What kind of engineer? I am guessing not electronic?

              IC - integrated circuit. A "chip."

              U - designates an IC on a schematic. Some engineers use IC, some use U. SO in some drawings IC1, IC2, IC3, etc, but in others U1, U2, U3. Just like R means resistor, C means capacitor, L means inductor, D (or CR) means diode.

              The integrated circuit chips have 8 legs mostly in our amps. So IC number U13 will indeed show 8 legs. Or in your picture, 8 holes where the legs would fit.

              All ICs count pins the same way. Each IC has either a notch at one end or a round dot in one corner. Either indicates the pin 1 end. Note on your drawing a little 1 near a corner of each 8-leg IC place.

              From the pin 1 corner - upper left when the pin 1 end is up - we count down the left side and then up the right side. SO the top end has pins 1 on the upper left and 8 on the upper right. Lat leaves lower left pin 4 and lower right pin 5.

              Remember this is always counted looking at each IC with its #1 end up. It doesn't matter which way the board has them facing. Kinda like the steering wheel of your car is always on the left, regardless of which way it is going. AT least in the USA.
              Education is what you're left with after you have forgotten what you have learned.

              Comment


              • #8
                i got nothin...

                Enzo, perfect description, thanks again. I'm a civil engineer, so that would explain my lack of electronics knowledge. But i have a nack for details and follow directions really well.

                That being said, I tested pin #7 on U13 and it had very little (I would say negligable) voltage. Also checked pin #6 on U12. Again, nothing. But when I was testing #6 U12, I accidentally touched the lead to #5 and #6 and I heard the phasing sound, but only for one cycle, just like i mentioned in my first post.

                Whats the next step?

                Comment


                • #9
                  Peavey Phaser

                  I believe Enzo described U11 & U12 as an oscillator.
                  DVM on Vac should measure a variable rate change at U12 pin 6 when the rate pot is turned.
                  Did you have your DVM set to Vac?
                  There is a engineering note at U12 indicating a Vdc change when the rate pot is rotated. Can you verify this test point?

                  Comment


                  • #10
                    DVM is set to AC voltage. When I turn the rate pot all the way from 0 to 10 quickly, the voltage spikes from almost 0 to around 2.6 to 2.9, right when the phase completes one cycle.

                    Comment


                    • #11
                      U12 DC

                      Can you verify the Vdc of pin 6. This tells whether or not the rate switch is working.
                      It sounds like you do not have a functioning oscillator circuit.
                      Pin 6 should be a free running oscillator vaveform.
                      It may be bad capsacitors on U11 & 12.
                      I do not understand the "low leakage" term on the capacitors of U11 & U12.
                      You may have to contact Peavey for replacements.

                      Comment


                      • #12
                        Other than the phasor, right now the amp works, right? That means U14 must be working. it is a "478." U11,U12 are your phase oscillator - its clock - and they are both 478s. SO powering down for each moove, substitute U11, and then U12 into the U14 socket. Note which direction EACH of them faces at home so you can get them back. Does the amp still work with either one of them in U14 socket?

                        We may also wind up having to test U7-10 that way.

                        For future refernce, does the reverb work?
                        Education is what you're left with after you have forgotten what you have learned.

                        Comment


                        • #13
                          Here we go:

                          JPB: Vdc of pin #6 is a steady 14.43V with the phase on. When the rate knob is turned up, voltage dips to anywhere from 9V to 11V.

                          Enzo: The amp works and functions properly, the reverb too. All functions on footswitch work as well. When I put U11 into the U14 socket, the amp powers on, but no sound. When I put U12 in the socket, there's a weird hum.

                          Comment


                          • #14
                            OK, you have three 478 ICs. One works good in U14, and the others do not. I am led to believe the other two ICs are bad then.
                            Education is what you're left with after you have forgotten what you have learned.

                            Comment


                            • #15
                              So I should get 2 new 478 IC's?

                              Comment

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