Results 1 to 10 of 10

Thread: Strange switch behaviour

  1. #1
    Supporting Member Randall's Avatar
    Join Date
    Jan 2013
    Location
    Cape Coral, FL
    Posts
    2,569
    Thumbs Up/Down
    Received: 607/0
    Given: 109/0
    Rep Power
    10

    Strange switch behaviour

    I'm looking at a Dr Z amp that runs four 6V6s, and has a Hi/Lo switch that switches a grid on each pair from either signal or 100K to ground. It was making a fair bit of noise in the Hi power position, so I got some deoxit in there. While I was investigating, I found the center tab measured a short to one side of the switch in one position, and 190K ohms to the same point in the other position. This true for all four cases, 190K to either side when in the off position, for both switches. Just to be sure I lifted the most accessable connection to the center lug, and the same is true. I realize switches go bad, but the same resistance in all four cases?

    Before I replace this switch, is there such a thing as a resistive DPDT switch? So strange.

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

  2. #2
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    13,074
    Thumbs Up/Down
    Received: 609/3
    Given: 314/0
    Rep Power
    28
    It seems to me that what you are reading is two grid resistors in series.

    1 Not allowed! Not allowed!

  3. #3
    Supporting Member Randall's Avatar
    Join Date
    Jan 2013
    Location
    Cape Coral, FL
    Posts
    2,569
    Thumbs Up/Down
    Received: 607/0
    Given: 109/0
    Rep Power
    10
    "It seems to me that what you are reading is two grid resistors in series."

    No, I really don't thinks so. The grid resistors are 1.2K, and I don't see a path for any series connection. Plus, the center wiper, or common, whatever it is called, is disconected. Another thing I just realized, the two outside terminals of the switch measure 189K and 190K across each of them respectively.

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

  4. #4
    Supporting Member The Dude's Avatar
    Join Date
    May 2010
    Location
    Sioux Falls, SD
    Posts
    6,717
    Thumbs Up/Down
    Received: 1,515/1
    Given: 1,122/1
    Rep Power
    17
    Do you have a schematic so we can see what's going on in the circuit?

    0 Not allowed! Not allowed!
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  5. #5
    Senior Member
    Join Date
    May 2006
    Location
    Cheshire, UK
    Posts
    2,307
    Thumbs Up/Down
    Received: 614/4
    Given: 314/0
    Rep Power
    16
    Perhaps it's like this?

    Click image for larger version. 

Name:	Switch.png 
Views:	36 
Size:	1.7 KB 
ID:	56535

    2 Not allowed! Not allowed!

  6. #6
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    12,380
    Thumbs Up/Down
    Received: 2,032/24
    Given: 5,182/11
    Rep Power
    24
    Might as well just disconnect completely and measure.

    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

  7. #7
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    32,983
    Thumbs Up/Down
    Received: 2,274/7
    Given: 0/0
    Rep Power
    55
    It seems to me that what you are reading is two grid resistors in series.
    No, I really don't thinks so. The grid resistors are 1.2K,
    I THINK he meant two of the 100k resistors in series. As in perhaps the grid return resistors. I could imagine two resistors to ground, and when the switch opens measuring resistor to resistor yields 200k, 190k close enough, the fact that they go to ground in the middle doesn't change that. Possible?

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

  8. #8
    Senior Member
    Join Date
    May 2006
    Location
    Cheshire, UK
    Posts
    2,307
    Thumbs Up/Down
    Received: 614/4
    Given: 314/0
    Rep Power
    16
    This would make it 190k

    Click image for larger version. 

Name:	Switch2.png 
Views:	27 
Size:	1.9 KB 
ID:	56538

    0 Not allowed! Not allowed!

  9. #9
    Supporting Member Randall's Avatar
    Join Date
    Jan 2013
    Location
    Cape Coral, FL
    Posts
    2,569
    Thumbs Up/Down
    Received: 607/0
    Given: 109/0
    Rep Power
    10
    It is wired like the one in post #5 if you change the leftmost 100K to 90K. It's a Dr Z so no schematic. I was reluctant to do much disconnecting given the immaculate wiring and red marker dots on every solder point, but once I did the switch acted normally, so it seems I actually have no problem here.

    I still really am not seeing why I get 190K across the switch with it all connected.

    edit: Oh wait, light bulb goes off, now I get it. The 100K and 90K are connected at ground putting them in series, but being grounded really doesn't have anything to do with it. DOH.
    Attached Images Attached Images  

    1 Not allowed! Not allowed!
    Last edited by Randall; 01-07-2020 at 06:17 PM.
    It's weird, because it WAS working fine.....

  10. #10
    Member
    Join Date
    Dec 2009
    Location
    Canada
    Posts
    342
    Thumbs Up/Down
    Received: 180/0
    Given: 6/0
    Rep Power
    11
    This may be the schematic (reverse engineered). I don't remember where I found it.

    https://music-electronics-forum.com/...1&d=1578431216
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	Dr Z Remedy.gif 
Views:	42 
Size:	125.4 KB 
ID:	56543  

    1 Not allowed! Not allowed!

Thread Information

Users Browsing this Thread

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

Similar Threads

  1. Strange guitar pickup behaviour
    By plithner in forum Guitar Tech
    Replies: 5
    Last Post: 09-24-2019, 11:07 PM
  2. 5E3 tone pot strange behaviour
    By Bluefinger in forum Maintenance, Troubleshooting & Repair
    Replies: 4
    Last Post: 02-17-2017, 11:34 PM
  3. Peavey Vypyr 100 strange behaviour
    By Nyqusit5 in forum Maintenance, Troubleshooting & Repair
    Replies: 0
    Last Post: 10-14-2016, 07:53 AM
  4. Replies: 25
    Last Post: 03-27-2016, 11:47 PM
  5. Presence control strange behaviour
    By GainFreak in forum Debugging Your Build
    Replies: 1
    Last Post: 09-20-2012, 05:09 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
  •