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Another Problem with another Peavey Classic VTX 212 65 Watts

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  • Another Problem with another Peavey Classic VTX 212 65 Watts

    Hello people! I am new here, and on forums in general, so apologize if I do something incorrectly. (Any and all comments are welcome). Same goes for questions.

    A short preamble:

    I just bought an old Peavey Classic VTX combo built, I suppose, in the early '80s. (Am trying to find that out from Peavey, but they didn't reply to my e-mail yet in 4 days). The combo is in pretty good shape, all working, the lamps were not changed in about a dozen years (according to the previous owner), if ever (they are original Peavey). Allegedly never gigged, originally used at a guitar playing school, and then at home. Was just sitting collecting dust the past number of years.

    I've searched and read about others on this forum having problems with the same gear, and my problem seems to be different.

    The problem:

    The Combo hums (like AC humming) quite strongly (loudly) in certain modes.

    With guitar disconnected, and if all the effects are off, the humming is quite quiet, not disturbing at all. I'd say even natural, the kind one'd expect. In fact, if you turn up the Post Lead Gain, which, as I understand, acts as sort of Master volume on the High Gain channel, beyond 50%, the humming decreases significantly, almost to none.

    However, if you switch on the Reverb (which itself works quite nicely), the humming increases significantly even on low reverb settings, (and low Pre and Post and Saturation) and increases drastically as you increase the reverb level, to levels which become very unpleasant. Increasing the EQ also adds to it -- i.e. increases the loudness of the humming. Increasing Phase also adds, but not as much.

    With the guitar (early '80s Korean modified Squier Strat with 2 S & 1 HB) connected, all is the same, maybe a bit louder, as the guitar itself adds a bit, depending on how close it is to the amp.

    Possible EMI:

    I live in the country. The house is the last in a row of sort of townhouses, 5 in total, a converted cow barn. )) No more houses around within 100 yeads/meters, just grass and trees. The nearest high voltage line is at least 200 yards/meters away. When checking, I unplugged all the appliances, the TV, stereo, PCs, etc.

    Attempts to fix:

    I've did some googling and reading also, and decided to start with changing the caps in the power supply circuitry. Found and downloaded the schematic, thanks to Enzo, and this web site.

    This I did, with no effect -- the humming is the same with the new caps, as it was with old ones.

    Question:

    Can anyone help with fixing this problem? I have some basic, amateur skills in radio electronics, which are quite rusty, can more or less read the schematics. But, this is not enough. Do have a multimeter, but no access to an oscilloscope.

    Looking forward to your kind advices.

    Igor
    Last edited by Casastr; 11-06-2009, 09:19 AM. Reason: Correct mistakes

  • #2
    You have a Peavey CLassic VTX.

    Peavey has a data base problem with the serial number records and cannot look up dates of manufacture for old amps any more. What is your serial number? The first number is probably the last digit of the year it was made, so if it looks like 3A-12345, then it was from 1983. The Classic VTX amps were from the early 1980s. You can also get an idea of its age by looking at date codes on the controls, transformers, and other larger parts.

    This amp has only the two large tubes - your lamps. The rest of the amp is solid state.

    If you do not plug anything into the input jacks, then all you will hear is the amp's own noise. If the amp is reasonably quiet that way, then the hum you get with a guitar connected is probably coming from the guitar or the input jack. Turn the volume control on the guitar to zero. Does that stop the hum?

    You mention that having the guitar closer to the amp increases the hum. That sounds to me like your guitar pickups are picking up the magnetic field from the amp power transformer. Try this. STand near the amp and turn the guitar up so you get some of that hum. Now turn to one side. Turn 90 degrees. Your guitar is like a radio antenna, it is directional, and the direction you aim it determines the directions it receives from. So if turning to the side (but getting no farther away) reduces or increases the hum, then you know it is the guitar pickup.

    That was with the effects all at zero. You mention having additional hum when the reverb is turned up. Is the hum with the reverb the ordinary sort of hum - the same sort of sound you get if you touch the tip of a guitar cord? And is it there the moment you turn the control up? Or does it take a couple seconds and build up to a low "WOOOOO" sort of sound? The WOOOO is the reverb feeding back acoustically, while the regular hum is electrical.

    Inside the amp, check the power supplies. All those integrated circuits run on +15v and -15v. Those two voltages are made by two zener diodes in the power supply, CR42 and CR43. Are both those voltages about 15v? And whatever they are, are both about the same? Also, scope the two power supply voltages. Are they completely free of ripple? If you have no scope, set your voltmeter to AC volts and measure those DC supplies. If you read zero AC volts, that is good. Any AC volts read that way is ripple. No AC means no ripple.

    The zeners themselves are on the main board close behind the reverb control.


    The reverb? Look at pin 1 of U6. Is there DC voltage there? SHould be none. If there is, U6 is probably bad.

    I think the ICs are in sockets, yes? If so, you can exchange them within the amp to test for function.
    Education is what you're left with after you have forgotten what you have learned.

    Comment


    • #3
      Hello Enzo,

      Thank you very much for your very prompt reply. I was not even expecting a reply so fast. I really do appreciate it.

      As mentioned before, the hum is audible without the guitar connected. I.e. nothing connected to any of the inputs. The hum is just regular AC hum, as if you had a speaker connected to the AC outlet. I would not call it "wooooo" sound. I could probably record it, if it would help.

      It appears when switching the reverb on, even at the lowest Master Reverb setting (the knob turned counterclockwise full, to "0"). When increasing the reverb level, the hum becomes louder, and seems to be the loudest at the middle -- 5-6 -- setting, and then slightly decreases as you turn the knob towards 10, but does not disappear.

      Once again, the reverb effect itself works fine.

      I've checked the voltages, as per your instructions. They are (measured on the Zener diodes', against the chassis):

      On the plus side: +14.61 DC
      On the minus side: -15.47 DC

      There seems to be ripple, but weirdly, when I sort of "reverse" the polarity of the probes, the multimeter shows "0". By "reversing" polarity I mean that I put the red (+) probe to the DC minus side, and then the black (-) probe to the DC plus side. Even though I am measuring AC. When measured with "correct" polarity, I get the following readings:

      On the plus side: 31.5 AC
      On the minus side: 33.5 AC

      You are correct, the chips are all in sockets.

      There is no DC voltage -- shows 0.00 -- on pin 1 of U6 (again, against the chassis).

      The serial number is 1A-976350. The label also says "81C" on the bottom right, so I guess, this is year 1981.

      Thanks again,

      Igor
      Last edited by Casastr; 11-06-2009, 08:51 PM.

      Comment


      • #4
        Any more suggestions on what to do next?

        Comment


        • #5
          The reverb pan is down in th bottom of the amp cabinet. Two cables connect it to the amp chassis above.

          The two cables should meet in one plastic Molex connector on the amp chassis. Unplug that connector and see if the amp no longer hums the same when the reverb control is turned up.

          Also, pull the reverb pan from its black vinyl bag and check the connections there.
          Education is what you're left with after you have forgotten what you have learned.

          Comment


          • #6
            Tried disconnecting the reverb assy. Same thing -- no change in the humming. All contacts are clean, and OK.

            BTW, are the voltages OK?

            Comment


            • #7
              Your DC readings look OK, but those 30v AC readings look to me like your meter is too basic. Try this: put your meter on AC volts and then measure a 9v battery. Measure both polarities. You should get zero volts of AC after a brief pulse at first. If you get 10-20 volts on the AC scale, your meter cannot be used for this sort of measurement.

              If you pulled the molex connector off the bottom of the chassis for the reverb cables and the hum on the reverb doesn;t change, there is a limited area the problem can hide in. Inspect the male pins for that Molex and make sure the ground pin has continuity top ground. No, I don't know which pin is ground, look in the schematic. Since the ICs are in sockets, you casn exchange the reverb IC for one of hte other ones and see if the symptom changes at all.
              Education is what you're left with after you have forgotten what you have learned.

              Comment


              • #8
                As always, your are right, Enzo. the meter is no good. It showed 19 volts on a 9 volt battery when set to AC.

                BUT, I've found and fixed the problem with humming. The foot switch cable shield was not connected to the chassis ground in the DIN connector. I've soldered it to the ground, and the humming reduced drastically.

                However, getting rid of this hum revealed a bunch of other noises, which I hope I will be able to get rid of by changing all the rest of electrolytic capacitors.

                Do you think that regular (non-electrolites) caps should also be changed? They are, after all, 28 years old...

                Comment


                • #9
                  Change the other caps? QUite possibly it needs that. Only way to tell is further work.
                  Education is what you're left with after you have forgotten what you have learned.

                  Comment


                  • #10
                    HEY guys. I have a Classic VXT 212 and I can't get very much volume. I can hear it slightly but not much. I don't really know what to do besides change tubes and see if that helps. Any suggestions? Everything else seems to work fine. Just not much volume. I've checked everything as far as volume settings. Don't really know what else to do

                    Comment


                    • #11
                      Jumper the preamp out to the power amp in and see if that makes it work. It could be a bad switching jack.

                      A thread about it
                      Last edited by The Dude; 12-22-2016, 10:51 PM.
                      "Yeah, well, you know, that's just, like, your opinion, man."

                      Comment


                      • #12
                        Thanks "The Dude". I tried to jump the loop but that didn't seem to help. I'm sort of at a loss with it. May just trash it

                        Comment


                        • #13
                          How about if you plug your guitar straight into the 'power amp in' ? This tests the power amp section.
                          Can you run the preamp out into another power amp to test whether the preamp is ok?
                          "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

                          Comment


                          • #14
                            Did you ever get this worked out? I have a similar issue.

                            Comment


                            • #15
                              Hi Dave
                              I suggest just start a new thread for your amp.

                              Our procedural advice probably still holds, but problems can be due to SOOO many things.
                              Education is what you're left with after you have forgotten what you have learned.

                              Comment

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