Page 2 of 2 FirstFirst 12
Results 36 to 53 of 53

Thread: Deluxe Reverb strangeness

  1. #36
    Supporting Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    3,216
    Thumbs Up/Down
    Received: 2,349/1
    Given: 1,337/2
    Rep Power
    8
    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.

    0 Not allowed! Not allowed!
    - Own Opinions Only -

  2. #37
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,458
    Thumbs Up/Down
    Received: 3,077/5
    Given: 3,719/0
    Rep Power
    32
    Quote 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.

    0 Not allowed! Not allowed!
    "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

  3. #38
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,638
    Thumbs Up/Down
    Received: 2,185/24
    Given: 5,505/11
    Rep Power
    24
    I thought maybe Helmholtz was referring to the inductance of the meter stopping the oscillation when probing certain points.

    0 Not allowed! Not allowed!
    "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

  4. #39
    Supporting Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    3,216
    Thumbs Up/Down
    Received: 2,349/1
    Given: 1,337/2
    Rep Power
    8
    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.

    1 Not allowed! Not allowed!
    Last edited by Helmholtz; 04-06-2020 at 05:26 PM.
    - Own Opinions Only -

  5. #40
    Supporting Member Randall's Avatar
    Join Date
    Jan 2013
    Location
    Cape Coral, FL
    Posts
    2,659
    Thumbs Up/Down
    Received: 640/0
    Given: 114/0
    Rep Power
    10
    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.

    0 Not allowed! Not allowed!
    Last edited by Randall; 04-06-2020 at 11:04 PM.
    It's weird, because it WAS working fine.....

  6. #41
    Supporting Member Randall's Avatar
    Join Date
    Jan 2013
    Location
    Cape Coral, FL
    Posts
    2,659
    Thumbs Up/Down
    Received: 640/0
    Given: 114/0
    Rep Power
    10
    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?

    0 Not allowed! Not allowed!
    It's weird, because it WAS working fine.....

  7. #42
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    33,459
    Thumbs Up/Down
    Received: 2,528/7
    Given: 0/0
    Rep Power
    57
    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.

    1 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  8. #43
    Supporting Member Randall's Avatar
    Join Date
    Jan 2013
    Location
    Cape Coral, FL
    Posts
    2,659
    Thumbs Up/Down
    Received: 640/0
    Given: 114/0
    Rep Power
    10
    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?

    0 Not allowed! Not allowed!
    It's weird, because it WAS working fine.....

  9. #44
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    33,459
    Thumbs Up/Down
    Received: 2,528/7
    Given: 0/0
    Rep Power
    57
    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.

    2 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  10. #45
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    16,458
    Thumbs Up/Down
    Received: 3,077/5
    Given: 3,719/0
    Rep Power
    32
    Quote 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:

    Quote 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?

    0 Not allowed! Not allowed!
    "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

  11. #46
    Supporting Member Randall's Avatar
    Join Date
    Jan 2013
    Location
    Cape Coral, FL
    Posts
    2,659
    Thumbs Up/Down
    Received: 640/0
    Given: 114/0
    Rep Power
    10
    This would be a lot easier to find out if it works if the issue weren't so fleeting, as in sometimes days between. Because as has been said maybe the issue doesn't happen because this mitigated it, or maybe just because of the random unknown nature of the failure.

    0 Not allowed! Not allowed!
    It's weird, because it WAS working fine.....

  12. #47
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    33,459
    Thumbs Up/Down
    Received: 2,528/7
    Given: 0/0
    Rep Power
    57
    Intermittents are always the hardest to solve, by their nature you can't sit there and watch it in failure mode.

    0 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  13. #48
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,638
    Thumbs Up/Down
    Received: 2,185/24
    Given: 5,505/11
    Rep Power
    24
    I thought it had yet to be proven that the audio fault always corresponded to the oscillation seen on the scope.
    I think that assumption may be preventing or at least slowing down solving the problem.

    0 Not allowed! Not allowed!
    "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

  14. #49
    Supporting Member Randall's Avatar
    Join Date
    Jan 2013
    Location
    Cape Coral, FL
    Posts
    2,659
    Thumbs Up/Down
    Received: 640/0
    Given: 114/0
    Rep Power
    10
    At first I thought it was always following the oscillation, but I have since stated that a few times I have seen it do the opposite. The latest failure mode did indeed have a collapsed wave, but I have seen the opposite.

    0 Not allowed! Not allowed!
    It's weird, because it WAS working fine.....

  15. #50
    Member
    Join Date
    Jun 2018
    Posts
    266
    Thumbs Up/Down
    Received: 179/0
    Given: 225/6
    Rep Power
    2
    Quote Originally Posted by Randall View Post
    At first I thought it was always following the oscillation, but I have since stated that a few times I have seen it do the opposite. The latest failure mode did indeed have a collapsed wave, but I have seen the opposite.
    Have you tried simply cleaning and maybe re-tensioning the tube sockets? You keep mentioning that touching a probe to tube pins makes things change. Maybe you're jiggling something into being bad and good.

    0 Not allowed! Not allowed!

  16. #51
    Supporting Member Randall's Avatar
    Join Date
    Jan 2013
    Location
    Cape Coral, FL
    Posts
    2,659
    Thumbs Up/Down
    Received: 640/0
    Given: 114/0
    Rep Power
    10
    I have jiggled and jostled and pounded, it's not mechanical. Touching several HT places makes it reset, not just tube pins. As mentioned even probing B+ feed to OT makes it reset.

    0 Not allowed! Not allowed!
    It's weird, because it WAS working fine.....

  17. #52
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,638
    Thumbs Up/Down
    Received: 2,185/24
    Given: 5,505/11
    Rep Power
    24
    I guess no difference with footswitch connected or not?
    Have you eyed up the layout drawing for any wiring dressed out of whack?

    0 Not allowed! Not allowed!
    "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

  18. #53
    Supporting Member Randall's Avatar
    Join Date
    Jan 2013
    Location
    Cape Coral, FL
    Posts
    2,659
    Thumbs Up/Down
    Received: 640/0
    Given: 114/0
    Rep Power
    10
    Footswitch is not in play. Reverb is disconnected, and tremolo tube is out, because one tridode was shorted when it came in, so I pulled it. Customer has a replacement at home he wants to use.

    I have buttoned this back up, I have other work I need to get to, and I have spent over a week with this on my bench. I fixed the complaints it came in with a long time ago. It only does this issue about once in 10 - 15 times of powering up cold, and is corrected by flipping the power switch a time or two. The customer only plays at home, so I guess in this case it is what it is. This is a very weird issue that I must accept I don't know how to solve, and move on.

    0 Not allowed! Not allowed!
    It's weird, because it WAS working fine.....

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Fender Twin Reverb, Deluxe Reverb input jack issues
    By nevetslab in forum Maintenance, Troubleshooting & Repair
    Replies: 9
    Last Post: 04-07-2019, 03:15 AM
  2. Heater voltage strangeness
    By Ampzone in forum Maintenance, Troubleshooting & Repair
    Replies: 8
    Last Post: 05-18-2018, 03:59 PM
  3. Fender Deluxe Reverb AB763 reverb pot adding gain/mids?
    By acorkos in forum Maintenance, Troubleshooting & Repair
    Replies: 32
    Last Post: 06-23-2016, 01:36 PM
  4. Replies: 3
    Last Post: 02-23-2012, 01:54 AM
  5. Pickup strangeness
    By MaxAnderson in forum Guitar Tech
    Replies: 4
    Last Post: 07-13-2010, 05:18 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •