Announcement

Collapse
No announcement yet.

Deluxe Reverb strangeness

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • #31
    "Have you stabbed the red probe into the board here and there to see if it's conducting? And there's still the possibility that the bottom board might be doing it around HV contacts even if the component board isn't. "

    Yes. I have done that pretty early into this, on the top board. How do we check the bottom board?
    It's weird, because it WAS working fine.....

    Comment


    • #32
      Originally posted by Randall View Post
      "Have you stabbed the red probe into the board here and there to see if it's conducting? And there's still the possibility that the bottom board might be doing it around HV contacts even if the component board isn't. "

      Yes. I have done that pretty early into this, on the top board. How do we check the bottom board?
      I've never checked a bottom board. I was just observing that it's a possibility. I suppose you could try to wedge the two boards apart and jam some spacers in between. If the problem changes or goes away then that could be evidence, or circumstantial.

      Super great that you're not getting voltage reading on the component board. That would be a really big "Phew" (wipe forehead) for me because I've had to replace a few conductive boards and have learned to expect it as often as not. So good news there.

      I think it could still be something like a cold solder joint. I've had a couple of amps do really weird things that couldn't be entirely explained (by me) by cold joints I eventually found and fixed. You could try flexing the board a bit while the amp is operating. I know you've already poked at the components.

      Have you taken DC measurements when inducing the problem? Just to see if it's something happening in the power supply rather than the signal chain?

      Since you suspected that one tube as being the cause, but then the problem returned, did you reflow the joints on that tube socket? I would.

      You mentioned having a guitar on the bench when inducing the problem. You've also mentioned witnessing an anomalous waveform. I assume these are the same issue? One test audible with the guitar and the other visible on the scope?

      And you didn't mention if there's been any modification to the amp like blackfacing, etc.

      Just a thought... If touching a certain point in the circuit with your meter probe always fixes the problem you could just solder a 10M resistor parallel to a 10p cap from one of those points to ground and then see if you can't re instigate the problem. If you can't then the amp is fixed. Well, sort of. It's a band aid for an oscillation issue, but it might be fine and it would get the amp off your bench.
      "Take two placebos, works twice as well." Enzo

      "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

      "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

      Comment


      • #33
        All good points. I have probed and prodded, and flexed the board. I have resoldered around V2, I have replaced the two V2 plate resistors. When the problem is happening, if I measure DC at either plate or red B+ point, it resets to normal. I don't see any mods except that the 120pF power tube caps are gone. Well, actually some of the tone caps have been changed.

        I know this has gotten confusing, but when Hemholtz suggested an oscillation issue, I dialed in what I thought was a 45K wave that was only there when the amp was normal. When the fault happened, the wave largely collapsed to about 1/5 of original size. This was pretty consistant for a while. Then the opposite happened. Since then I have had the fault happen with and without the wave present, and I have had it sound normal with and without the wave present. So at this time I think there is no correlation between the two. Also, I replaced the tube I suspected, with no change in results.

        I think the next thing I will do is just hit every solder joint. Can't hurt. This is such a beast because of the randomness of the fault condition. I have tried all day and some of last night to get it to do it again, but it hasn't done it once.
        It's weird, because it WAS working fine.....

        Comment


        • #34
          NO WAVE on the scope!
          No visible wave at the output doesn't mean the amp is not oscillating as explained above. The OT would hardly transfer something like 200kHz. So you need to look for oscillation before the OT.

          Freeze spray may help to identify a bad contact or component.

          I am sure the fault mode changes some cathode voltage. No need to touch a plate.
          Last edited by Helmholtz; 04-06-2020, 12:50 AM.
          - Own Opinions Only -

          Comment


          • #35
            Originally posted by Randall View Post
            When the problem is happening, if I measure DC at either plate or red B+ point, it resets to normal.
            I'm just registering this detail now. That is weird. The impedance at that node should be virtually zero, so it's isolated from the signal path. That amp should have a grounded chassis, correct? So what sort of shift could your meter be making to the operating conditions when touching a B+ node???
            "Take two placebos, works twice as well." Enzo

            "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

            "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

            Comment


            • #36
              So what sort of shift could your meter be making to the operating conditions when touching a B+ node???
              It's exactly this weird behaviour that made me think of oscillation from the start.
              At very high frequencies a small inductance becomes a considerable impedance.
              - Own Opinions Only -

              Comment


              • #37
                Originally posted by Helmholtz View Post
                It's exactly this weird behaviour that made me think of oscillation from the start.
                At very high frequencies a small inductance becomes a considerable impedance.
                So the "L" of the old electrolytics in the power supply was suspect?

                Ok then... The capacitors have been replaced. How "new" the capacitors are I cannot say because Randall keeps them as inventory and I don't know how "fresh" they may have been. I've received stale electrolytics from suppliers before that performed so poorly they needed to be replaced. So.?. The filter grounds are said to measure below 1 ohm. Though I don't know at what points they were tested.

                Are we still suspecting higher than normal inductance in the power supply then? I have some considerations.

                Is it possible, Randall, that the caps you installed were not entirely fresh (be honest now )

                And Helmholtz, is it possible for that to make a difference of increased inductance? Considering the physical/mechanical structure I wouldn't think so, but I don't know. And...

                Is it possible then that if the caps were a little stale that they are now charging and breaking in and that would lower the inductance? This might explain why Randall hasn't been able to recreate the problem in a while.

                Just thinking out loud.
                "Take two placebos, works twice as well." Enzo

                "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

                Comment


                • #38
                  I thought maybe Helmholtz was referring to the inductance of the meter stopping the oscillation when probing certain points.
                  "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


                  • #39
                    I was primarily referring to the "touch sensitivity" of the amp when probing high voltage points. This is something I have seen with self-oscillating amps. It is likely that the circuit point being contacted carries high frequency. That should be verified if possible.
                    The meter leads mainly cause capacitive effects.

                    Old style ecaps typically had higher series inductance (ESL) than the better modern construction types which use multiple foil contacting. ESL does not change with age.

                    While the ESL + wiring inductance may add only a few ohms of impedance at high frequency, things change when the inductance resonates with a capacitive partner. Impedance at the resonant frequency can be quite high.
                    Connecting meter leads is likely to change the resonant frequency.

                    Self-oscillation often means that resonance is involved.

                    THat said I have no clue what might have caused the amp to become unstable.


                    In any case I would clean and verify all mechanical contacts. If the amp uses a multisection cap, good can to chassis contact is essential.
                    Last edited by Helmholtz; 04-06-2020, 05:26 PM.
                    - Own Opinions Only -

                    Comment


                    • #40
                      Thanks for sticking with on this one.

                      The caps I put in (F+T) were not that old. Several months maybe. ESR of the old caps I took out are 1 - 1.5 ohms. I didn't measure the new ones. I brought them up slowly on a variac the first time, as I usually do. Grounds were measured from the cap negs to chassis, all look good. Today I hit every solder joint on the amp, and verified the five chassis grounds coming off the board are solid. I pulled the jacks and cleaned those grounds as well. One thing I did find is the Normal jacks had three toothed washers each, and the Vibrato channel had three and four washers. I put them all down to two each, The internal speaker jack had four washers as well, I put both speaker jacks down to two each and gave everything a nice firm twist after a bit of pencil eraser and a squirt of DeOxit. So one speaker jack sticks out farther than the other, so what? I didn't like those stacked washers.

                      After that, if it doesn't go into fault again by the end of the day, I am sending it out. If it comes back, it comes back.
                      Last edited by Randall; 04-06-2020, 11:04 PM.
                      It's weird, because it WAS working fine.....

                      Comment


                      • #41
                        So I power it up in fault mode after sitting for a couple of hours, first time in two days. I touch my scope probe to the first grid on V2, and with a pop! it's back to normal.

                        Seriously, how am I going to ever fix this?
                        It's weird, because it WAS working fine.....

                        Comment


                        • #42
                          OK, you turn it on, it acts up, THEN you touch a probe to it, and it gives up. Does it do this with a meter instead of a scope?

                          In either case, connect the scope when the amp is off, NOW turn it on and let it go. Will it go into fault then? That eliminates the pop signal that seems to trigger things.
                          Education is what you're left with after you have forgotten what you have learned.

                          Comment


                          • #43
                            Yeah meter or scope. And I have it powered down with both grids with scope probes on them right now. But given how random this is, how will I know what effect this will have unless it goes into fault this way. If it doesn't, what have I learned?
                            It's weird, because it WAS working fine.....

                            Comment


                            • #44
                              We already know the problem mode is off and on - no pun intended. So we just use the amp as you have been, leaving the scope connected. If it never screws up, I am not sure what to conclude, but if it still screws up, at least we get the chance to see what is going on on the scope.
                              Education is what you're left with after you have forgotten what you have learned.

                              Comment


                              • #45
                                Originally posted by Enzo View Post
                                We already know the problem mode is off and on - no pun intended. So we just use the amp as you have been, leaving the scope connected. If it never screws up, I am not sure what to conclude, but if it still screws up, at least we get the chance to see what is going on on the scope.
                                From post #32:

                                Originally posted by Chuck H View Post
                                Just a thought... If touching a certain point in the circuit with your meter probe always fixes the problem you could just solder a 10M resistor parallel to a 10p cap from one of those points to ground and then see if you can't re instigate the problem. If you can't then the amp is fixed. Well, sort of. It's a band aid for an oscillation issue, but it might be fine and it would get the amp off your bench.
                                Hmmmm?
                                "Take two placebos, works twice as well." Enzo

                                "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                                "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

                                Comment

                                Working...
                                X