Page 3 of 3 FirstFirst 123
Results 71 to 78 of 78
Like Tree6Likes

Thread: Marshall VS100R hum problem

  1. #71
    Member
    Join Date
    Nov 2017
    Location
    Madrid, Spain
    Posts
    35
    Quote Originally Posted by nickb View Post
    It should only make a difference if the second shorting contact were broken also as that one mutes the signal input to the tube. Earlier you said that it hummed with all the knobs at zero and no guitar plugged in. All the tests were supposed to done that way. If you changed them at any time it invalidates everything.

    Right now I have zero confidence in any of the results - there are too many contradictions. I suggest you fix the jack and carefully check the operation of both switching contacts and we'll re-evaluate the situation.
    Hey Im back and I fixed the jack. These are the new tests:

    Everything on zero, no guitar plugged in, all the values are in AC volts from measuring across the speaker:

    1) Nothing, just turning on the amp: CLEAN CHANNEL=0.24 OVERDRIVE CHANNEL=0.184

    2) R48 TEST: I placed one end of the jumper cable on the main ground and the other end on the side of R48 that connects with R49 (just to make sure im doing it right)
    CLEAN CHANNEL=0.001 (no hum) OVERDRIVE CHANNEL=0.152

    3) R77 TEST: I placed one end of the jumper cable on the main ground and the other end on the side of R77 that connects with R78 (just to make sure im doing it right)
    CLEAN CHANNEL=0.145 OVERDRIVE CHANNEL=0.002 (no hum)
    Last edited by avoverdu; 12-11-2017 at 07:34 PM.

  2. #72
    "Thermionic Apocalypse" -JT nickb's Avatar
    Join Date
    Dec 2009
    Location
    Devon, UK
    Posts
    2,475
    OK Thanks. Can you try the DC heater supply once more please and measure the ACV out across the speaker?
    Experience is something you get, just after you really needed it.

  3. #73
    Member
    Join Date
    Nov 2017
    Location
    Madrid, Spain
    Posts
    35
    Quote Originally Posted by nickb View Post
    OK Thanks. Can you try the DC heater supply once more please and measure the ACV out across the speaker?
    Everything on zero, no guitar plugged in, all the values are in AC volts from measuring across the speaker:

    DC Supply: CLEAN CHANNEL=0.070 (approximately, it fluctuates up to 0.040) OVERDRIVE CHANNEL=0.012

    Thanks

  4. #74
    "Thermionic Apocalypse" -JT nickb's Avatar
    Join Date
    Dec 2009
    Location
    Devon, UK
    Posts
    2,475
    Quote Originally Posted by avoverdu View Post
    Everything on zero, no guitar plugged in, all the values are in AC volts from measuring across the speaker:

    DC Supply: CLEAN CHANNEL=0.070 (approximately, it fluctuates up to 0.040) OVERDRIVE CHANNEL=0.012

    Thanks
    So the DC supply helps quite a lot. You can
    (a) Fit a DC supply
    (b) Try elevating the heater DC voltage [cheap to do] or
    (c) keep trying different tubes until you find one you are happy with
    (d) Live with it.

    What's it to be?
    Experience is something you get, just after you really needed it.

  5. #75
    Member
    Join Date
    Nov 2017
    Location
    Madrid, Spain
    Posts
    35
    Quote Originally Posted by nickb View Post
    So the DC supply helps quite a lot. You can
    (a) Fit a DC supply
    (b) Try elevating the heater DC voltage [cheap to do] or
    (c) keep trying different tubes until you find one you are happy with
    (d) Live with it.

    What's it to be?
    I didnt give up!!!! Ive been busy at work this week but today im gonna get some parts. Im going to try all your suggestions until I find the cure LOL. By the way I did some tests that I will ppost later in detail.

    Thanks guys!!

  6. #76
    Member
    Join Date
    Nov 2017
    Location
    Madrid, Spain
    Posts
    35
    Quote Originally Posted by nickb View Post
    So the DC supply helps quite a lot. You can
    (a) Fit a DC supply
    (b) Try elevating the heater DC voltage [cheap to do] or
    (c) keep trying different tubes until you find one you are happy with
    (d) Live with it.

    What's it to be?
    I didnt give up!!!! Ive been busy at work this week but today im gonna get some parts. Im going to try all your suggestions until I find the cure LOL. By the way I did some tests that I will ppost later in detail.

    Thanks guys!!

  7. #77
    Member
    Join Date
    Nov 2017
    Location
    Madrid, Spain
    Posts
    35
    Quote Originally Posted by nickb View Post
    250mV rms is a lot of hum. I don't recall these being especially bad in that respect. Am I mistaken?

    For certain HK leakage is a component but how is it getting into the ground? Unless of course the test in post 19 is wrong. I wonder if there has been a misunderstanding? I took it that with R48 and R77 shorted and with the tube in the hum was there. On a re-read maybe that is not the what was meant. Perhaps the OP could clarify?

    A cheap alternative to a DC supply would be to elevate the heater supply up to 100V or so with a divider* off the plate supply and isolating pin 9 from ground.

    *PS
    i.e. isolate pin 9 from ground by cutting track. Add 100K 0.5W from ground to pin, add 390K 1W from pin 9 to to pin 1, add 2.2uF 100V ( bigger uF and bigger volts would be OK). Check voltage across 100K after rework. Aim for 75V to 100V.
    Hi Nick. Im ready to perform what you just suggested. What Im not sure is where to put the 2.2uF cap? In parallel with the 390k resistor between pin 1 and 9?

  8. #78
    "Thermionic Apocalypse" -JT nickb's Avatar
    Join Date
    Dec 2009
    Location
    Devon, UK
    Posts
    2,475
    Quote Originally Posted by avoverdu View Post
    Hi Nick. Im ready to perform what you just suggested. What Im not sure is where to put the 2.2uF cap? In parallel with the 390k resistor between pin 1 and 9?
    Better (updated) instructions:

    Isolate pin 9 from ground by the cutting track ( or whatever it take)
    Add 100K 0.5W from ground to pin 9
    Add 390K 1W from pin 9 to to pin 1
    Add 2.2uF 100V across the 100K (bigger uF and bigger volts would be OK).
    Check voltage across 100K after rework. Aim for 75V to 100V by tweaking the 100K if necessary
    Experience is something you get, just after you really needed it.

Page 3 of 3 FirstFirst 123

Thread Information

Users Browsing this Thread

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

Similar Threads

  1. Marshall ValveState VS100R white noise / buzz problems
    By MarshallMallow in forum Maintenance, Troubleshooting & Repair
    Replies: 14
    Last Post: 11-10-2015, 02:36 AM
  2. Marshall Vasvestate VS100R Head
    By JohnnyVieira in forum Maintenance, Troubleshooting & Repair
    Replies: 12
    Last Post: 07-15-2013, 06:56 PM
  3. 117 volt transformer for a Marshall Valvestate VS100R
    By Sethkomar01 in forum Maintenance, Troubleshooting & Repair
    Replies: 12
    Last Post: 02-01-2013, 04:55 AM
  4. Marshall VS100R Schematic Mixup?
    By ConnorW in forum Schematic Requests
    Replies: 3
    Last Post: 09-28-2011, 03:46 AM
  5. Marshall Valvestate VS100R Transistors Blowing
    By neilm in forum Maintenance, Troubleshooting & Repair
    Replies: 10
    Last Post: 06-23-2009, 01:00 AM

Tags for this Thread

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
  •