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Same Prosonic, Different Problem - ocillation/scratching noise on loud low note decay

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  • Same Prosonic, Different Problem - ocillation/scratching noise on loud low note decay

    Working on a Prosonic combo that somewhat intermittantly seems to breakup into ocillation/microphonics coupled with scratchyness at the loudest part of the note and trails off during the decay. Occurs when the amp is turned up and you hit a low note of a certain frequency (like the A string at the 5th fret) or sometimes a chord hard and let it resonate. Most evident on drive channel, not as noticable on the normal channel but still there.

    I have isolated it to the preamp by using the effects loop out to the power amp section of an Orange amp, same noise out of the Orange. Using the Orange preamp back to the Prosonic power stage is clean hitting the same notes.

    Also notice that using an external cab seems to cut back on the intensity of the noise, but its still there. Noise is at its worst when playing notes or chords that find the resonate frequency of amp itself.

    Tried swaping preamp tubes with another Fender (non Prosonic), all voltages look good. The amp is biased hot, but I know its not in the power circuit anyway. Coupling caps have no DC on them at idle. No mods that I've found. Cleaned input jacks and swapped guitars, Circuit board looks clean, no bent leads or baked flux and resistor values around tubes look ok, no apparent problems with tube sockets. Basically at lower volumes it sounds fine, doesn't like to be cranked.

    I have not tried a complete new set of preamp tubes, wondering if I should at least eliminate this.

    Any thoughts?

  • #2
    Bad preamp plate resistors...or bad connections in the preamp, re-solder...
    Bad channel switching relay contacts. OR just dirty contacts in the jacks, clean all jacks with deoxit D100.


    • #3
      If it seems mechanically sensitive, then ball up your fist and whack the amp. Does it respond to that? it ought to ignore that. Tubes are most likely to be microphonic, but other parts can get that way. If you have swapped out preamp tubes, then get a wooden chopstick and start poking components on the board while signal runs through it. Looking for anything that reacts.
      Education is what you're left with after you have forgotten what you have learned.


      • #4
        What I have found on some recent model Fenders is that the resistors are bunk.
        A bad plate resistor causes microphoincs / distortion / ringing, just like a bad tube.
        We did not see this problem until the past few years, apparently the resistors have cracks or develop cracks over time.

        The funny thing is that about half the amps we fix---are diagnosed by banging a fist on the cabinet. Or taping the board with a plastic handle nut driver. This reveals (as Enzo pointed out) bad solder connections, or bad relay and jack contacts.
        Many problems like this amp are just bad connections, in fact about half of all amps repaired, are just a bad solder joint or dirty controls and jacks. (the contacts in the jack are dirty).
        Then again there are a few that actually have component failures. But those are the vast minority.
        You would be surprised how often you can fix an amp with some cleaner spray and a soldering iron. (that's most of them, right there!)
        And now you see why most of the amps that come here are fixed for $50 or less.
        Last edited by soundguruman; 01-13-2012, 01:01 PM.


        • #5
          Check the PI plate loads first. I've seen these fail quite a bit. Replace them with 1w MF types if this is found to be the problem.
          The farmer takes a wife, the barber takes a pole....


          • #6
            Appreciate everyone's help so far, it's not fixed but maybe getting closer....

            Using a paint stick (heavy artillery, now a permanent addition to the tool box,) with the gain and volume turned up at idle I found two cold solder joints while taping in the preamp section. One of them being the plate load resistor at v2 which screamed like a kid with a tooth ache when I tapped it. I reflowed the solder and it quieted right down, but I went ahead and replaced it anyway. Also the coupling cap C2 between v1 and v2 was making noise but quieted down when I reflowed it.

            Now I can pound on the board and it won't make a peep, but hit the right note and it sends the preamp right into ocillation/microphonics. Same issue.

            After the above I did a wholesale swap of V1..V3 tubes, no change. I also reflowed V1..V3 wire connections at the board, not at the socket yet.

            Unless anyone has an idea, tomorrow I plan to clean/adjust/reflow the sockets and pull the circuit board to reflow the entire preamp section.


            • #7
              Yes re-flow the preamp section, sometimes it's hard to find the one connection that is bad. Just re-solder them all in that area.
              Sometimes I have to freeze the parts one at a time, with a can of freeze spray, to find the defective one.
              A cracked part, or connection, will often let out a scream when you freeze it.
              A noisy part, like a resistor with an internal crack, can be a challenge to find, but keep at it. Intermittents are the hardest problems to solve, so take a deep breath and have patience. If you have replaced the plate resistor, and that still did not do it, it could be the grid or cathode resistor as well, although it is less common a problem, or any resistor in the signal path.
              Pull out V1, does the noise go away? V1 has 2 plate resistors.

              bad contacts, in the mechanical channel switch relays, can also be at fault.

              sounds like you are on it, so keep at it.


              • #8
                Thanks again for the help.

                Everything is resoldered, pretty sure the relay is good as it defaults to the drive channel so I measured resistance while beating it with a stick. Never wavered from a dead short. Froze everthing with component cooler pretty well, at idle nothing unusual.

                I may be describing this sound incorrectly. When a low note is hit, it rings but almost immediately I get a light underlayer of a static like sound for a few seconds which drops off before the note finishes the decay. Ocasionally I hear a light amount of it on the normal channel, but most evident on drive. The particular settings (for troubleshooting) are no mid or base, treble is maxed and drive 1 and drive 2 are set around 3, master volume around 3. Hitting a low note like the 5th string on the 5th fret is the only time I get the static noise. The amp is on the bench and out of its box, I'm running it through a Marshall 4x12 cab at 16 ohms.

                This is my first prosonic so I don't have anything to compare it to, but a player familiar with prosonics tells me this isn't right. If I can figure out how to post a recording of the noise I will. Again, I only hear the noise when hitting the low note.

                I do plan to shotgun all the plate resistors, but wondering if a coupling cap could be leaking DC at certain frequencies?


                • #9
                  Yes a coupling cap could do that, sometimes they leak or distort when the music is passing but will not do that without audio.
                  But does it do that with V1 removed? Try to narrow down the stage that's distorting before changing more and more parts.
                  Seems like you could narrow it down with a scope.
                  Try eliminating possibilities before changing parts.
                  I really don't mean to encourage random parts replacement, but some amps just need the signal path changed out, or at least some of it. If there is a bunk cracked resistor or a leaky cap, it might not show up in a conventional measurement / test.
                  But when the power is applied, it fails. Or just when audio passes it fails.
                  And just for fun take a peak at the power supply filter cap for the preamp, sometimes the "Illinois Capacitor" will open up or fail in strange ways, try removing it temporarily and see. There is a 470K resistor across that cap in the preamp power supply.


                  • #10
                    Good suggestions, noise present only when audio passes through. This amp had P2 and P7 cross wired which took me a while to find (previous post). I'm wondering if it had some effect on the surrounding components of those pots. I've ordered a few parts today to target these areas. Will post an update in a couple weeks. Thanks a bunch for the help on this.


                    • #11
                      Update, It's offical, this Prosonic amp is kicking my butt.

                      All the plate and a few of the screen resistors have been replaced, no change. The rest of the resistors in the pre-amp have been checked by lifting a leg if necessary.

                      I haven't done a good job explaining what I'm hearing. For those of you that have heard tube rattle in the Peavy Classic 30 when a chord or note is struck, (prior to installing a tube tamer) the noise sounds similar to that and goes away immediately by muting the guitar strings or eventually before full decay. The noise is not there (amp plays fine) at lower volumes on either channel.

                      Does not seem to be mechanical, I retensioned the sockets, and stretched the retaining springs. The preamp tubes are in there tight. Banged on the relays, rest of board, power supply, etc.

                      Verifying with the Send/Return jacks it is definitely in the preamp. All power supply voltages look good (scope and meter,). I've reflowed the solder on all preamp sockets and PCB wire connection and pretty much the entire preamp section.

                      It show up more in the gain channel, but will also show up in the normal channel when volume is cranked to ~6 or 7. Also, for testing I have treble on 10, base and mid on zero, fender strat with tex mex pickups, but it will also do it with a standard tele.

                      My current thinking is its either in the first stage V1b, or final stage V3b. Schmatic attached. I have not replaced R27 (1W 680ohm) tests okay, but ordering a replacement today to be sure.

                      What am I missing, any thoughts?
                      Attached Files


                      • #12
                        Have you tested the filter caps? I don't see many Prosonics, but all Fender can show these sorts of symptoms due to bad filters in the preamp stages. Not enough decoupling between stages.


                        • #13
                          I think what you keep referring to is that the bias is set too cold.
                          Random replacement of parts is a trap to fall into, when you don't have test equipment to verify where the noise is.
                          You should really try to decrease the bias voltage and heat the output tubes up, before going any further.


                          • #14
                            OK whats the voltage on pin 5 of the power tubes, when the amp is on, with no signal?


                            • #15
                              Originally posted by Jecarroll7 View Post
                              Everything is resoldered,
                              Everything? Filter caps? Have you resoldered and cleaned all the pots and jacks?
                              "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


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