Fender Pro Junior problem

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by SaMurray, Oct 13, 2008.

  1. SaMurray

    SaMurray Member

    Messages:
    4
    Joined:
    Oct 11, 2008
    Hi all
    NOTE: I posted this in the wrong forum before so am posting again in the technical area, where i am hoping for some pointers:

    I have an early '90s tweed pro junior. It sounds great but has always been a little unreliable. It has recently developed the following fault though and I was wondering if anyone else has experienced this or if it sounds similar to common amp faults etc:

    After playing the amp for maybe five minutes at volume a VERY loud hum/buzz comes from the speaker and cuts out the guitar signal altogether. I have tried replacing the speaker and valves but same thing happens. Also, since my previous post, I have checked solder joints on both the main board and the valve seats. All appear OK. The only way to stop hum is to turn the amp off.

    In my previous post I had a reply from Tuberattler who advised me to "Check all caps that are possibly hot glued to make sure they're stable"

    What I am unsure of is what a hot glued cap is exactly and how to go about checking them. I can certainly change caps and have plenty of experiece soldering etc. Any clues out there?

    Best
    Sam
     
  2. PRNDL

    PRNDL Member

    Messages:
    502
    Joined:
    Aug 25, 2007
    Location:
    SW Florida
    It's most likely the power supply electrolytics, which are usually big caps on the board or cap cans (I believe the Pro Jr has 2 cap cans). There are also 2 electrolytic caps for DC heaters on the board.

    To test, you discharge them and measure their resistance, which should rise as they charge.

    I'd recommend reading up on this, as these are high voltage caps that cause big shocks.
     
  3. slider313

    slider313 Silver Supporting Member

    Messages:
    7,480
    Joined:
    Jul 28, 2005
    Location:
    NC
    Have you checked the input? A crack in the trace from the input would cause this.
     
  4. HeyMrTeleMan

    HeyMrTeleMan Colonel of Truth Gold Supporting Member

    Messages:
    2,936
    Joined:
    May 21, 2006
    Location:
    Room 10-09
    Ha!

    For once, I may have the solution! Double Ha!

    I had one, and thanks to the guys here at TGP, I found out that there is a problem peculiar to the Pro Jr. that is easily remedied, and you don't have to be a rocket surgeon (like me) to fix it.

    There is a screw on the back panel which, due to being a tiche too long, touches the chassis and causes the hum. I believe it's the middle screw at the top, but may be the middle screw below it. Back out the screw and see if that cures it before taking it to a repair facility.

    If that works, you are welcome. If not, sorry! I hope you get it resolved.

    HMTM

    [​IMG]
     
  5. SaMurray

    SaMurray Member

    Messages:
    4
    Joined:
    Oct 11, 2008
    PRNDL and slider 313, thanks for the advice. I shall go about looking at both suggestions. HeyMrTeleMan, if only it were that simple! The common hum you refer to is not what's happening - what i am experiencing is a sudden and alarming sounding hum/buzz that appears after a few minutes of playing, which removes the guitar signal altogether.

    Sam
     
  6. Nolatone Ampworks

    Nolatone Ampworks Silver Supporting Member

    Messages:
    2,736
    Joined:
    Jun 4, 2006
    Location:
    Flowery Branch, GA
    Sam, did you sort this out? What was the problem?
     

Share This Page