Scratchy blackface bandmaster

Discussion in 'Amp Tech Center' started by nasdak, Jan 26, 2019.

  1. nasdak

    nasdak Tele-Meister

    Posts:
    236
    Joined:
    Mar 22, 2013
    Location:
    france
    Hi folks,
    i'm having a hard time with a vintage BF bandmaster !
    I changed all the electrolytics : filter caps, bias cap, cathode caps.
    tubes are new and biased. i tryed different preamp tubes.
    pots have been cleaned with deoxit.
    All coupling capacitors have been checked for leakage and value - they are ok, and only a few mV passing trough on the circuit.
    The amps works great, trem is good. All resistors are stock.
    BUT my 2 vol pots are scratchy and i without touching the pots i get an intermitent "scratch scractch" that does not seem to be related to volume. it come and goes. i tried to swap some resistors (plate load of V1, V2 and PI) with no luck. I even tried new coupling caps at some places, no luck. I don't want to check every component, i'd like to keep the original soldering...
    Any thought? it's driving me crazy !
     
  2. dogmeat

    dogmeat Tele-Afflicted

    Age:
    69
    Posts:
    1,391
    Joined:
    Oct 12, 2017
    Location:
    Alaska
    start poking things with a chopstick
     
  3. Wally

    Wally Telefied Ad Free Member

    Posts:
    34,687
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    Try reflowing the solder joints on those pots. Cold solder joints can cause that scratch.
     
  4. DADGAD

    DADGAD Friend of Leo's

    Posts:
    2,135
    Joined:
    Mar 18, 2003
    Location:
    Earth
    A couple of more maintenance items for old amps is to retention the tube sockets and retighten all jacks and potentiometers to make sure they are well grounded.
     
  5. nasdak

    nasdak Tele-Meister

    Posts:
    236
    Joined:
    Mar 22, 2013
    Location:
    france
    I'll try the pots solder joints but i think i've done them already

    i poked all i could poke with my favorite chopstick of course, nothing (only microphonic wires going to grid of V1 and V2 but it often happens)
     
    Wally likes this.
  6. nasdak

    nasdak Tele-Meister

    Posts:
    236
    Joined:
    Mar 22, 2013
    Location:
    france
    thx. did that already. will recheck :)
     
  7. Wally

    Wally Telefied Ad Free Member

    Posts:
    34,687
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    Okay...scratch even when not turning the pots. Do you get the scratch when you ARE turning the pots.
    Both channels......that seems odd.
     
  8. nasdak

    nasdak Tele-Meister

    Posts:
    236
    Joined:
    Mar 22, 2013
    Location:
    france
    i don't mind the scratchy pots (the're 50 years old...)
    but i do mind the intermitent scratch scratch... i have a scope, i tried to follow the signal, but that scratch is to weak, can't see it on the scope :/
     
  9. nasdak

    nasdak Tele-Meister

    Posts:
    236
    Joined:
    Mar 22, 2013
    Location:
    france
    Some update...
    Well it seems a change of PI tube did the trick :D
    And a second pass of deoxit helped with the pots.
    Sometimes it's too simple!!!

    Envoyé de mon Redmi 4X en utilisant Tapatalk
     
    robrob, corliss1 and Wally like this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.