Intermittent volume drop when I tap the amp (old Fender).

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by efnikbug, Dec 2, 2017.


  1. efnikbug

    efnikbug Member

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    I have a SF Drip Edge Dual Showman head that sputters.

    I took the chassis out and it plays okay until I tap the chassis and then the volume drops by about half, and then goes back to full power randomly. I'm about to say that physical jarring causes the power to drop and come back.

    I'm ready to fire up the soldering iron and touch every point.

    Whoever had the amp before me had it serviced as it has new(er) filter caps: F&T and some no-namers for the high voltage ones under the pan. The bypass caps have all been replaced too.

    Please help.

    Thanks.
     
  2. zenas

    zenas Member

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    I don't care for the touch every solder joint method. I'd rather start by looking at every solder joint often you're going to see a bad one. Look at the resister that sits 90 degrees from the rest first.
     
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  3. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    Look at these two indicated joints in the phase inverter VERY CLOSELY. If they are bad, F that reflow idea. CLEAN OUT the old solder with braid, then re-solder.

    [​IMG]
     
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  4. Oatie

    Oatie Member

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    That's right Jeff, sometimes the eyelet boards warp and break a solder joint that runs left to right on the board. I found a broken resistor just under the solder ball, so use a wooden chop stick to carefully put a small lift pressure on the resistors. If the eyelet board has a big hump- warp def check that area. They were all perfectly flat from the factory.
     
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  5. efnikbug

    efnikbug Member

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    Damn it, I read this too late. I put everything back together and came home. Next time.

    What you guys have said makes sense. The board is warped.
     
  6. Blue Strat

    Blue Strat Member

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    Not only this, but make absolutely sure that the 100 ohm resistor (horizontal orientation) isn't cracked.
     
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  7. paulg

    paulg Supporting Member

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    Here’s an odd one. I had a Bandmaster the would cut in and out. It turned out a blob of solder hanging off a lug on the bias pot would ground out as the amp heated up. So it was intermittent. When I (finally!) figured this out it occurred to me that this was the case since it was built in the 60’s.
     
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  8. pdf64

    pdf64 Member

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    Yes, that 100 ohm resistor is the most likely suspect, but any of those old CC resistors can develop fractures.
    So if the 100 ohm proves good, the 100k plate resistors and 470 screen grid resistors would be next in my list to check.
    If you're competent to work inside the chassis of an operational amp, tap/nudge them with a insulating probes, eg chopstick, sharpie.
     
  9. Steppin' Wolfe

    Steppin' Wolfe Member

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    I have had two 1964 BF SR's on the bench that had a factory error on the solder joint to the right that Jeff shows us. IN each amp, the leg of the 100ohm resistor that went to that eyelet had a perfect circle around it....the solder had flowed away from the lead when it was soldered at the factory. the other leads in that eyelet had made a good solder joint. When I saw the first of these amps, I thought to myself that there must have been a drop of oil contaminating that lead and the solder was 'avoiding' the lead because of that contaminant and that I would never seen that again. . I saw the second one of those amps about 9 years later. A drop of oil must have fallen into that bin that held the 100 ohm resistors on that worker's bench that day, right?
     
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  10. efnikbug

    efnikbug Member

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    Damn, you're good. It was EXACTLY that resistor. One end just wasn't soldered in. It was just touching the point.

    I sucked out both ends of solder, removed the resistor, straightened the leads (because it was kinda short), bent the ends, and soldered it back in neatly.

    Glorious 4x6L6 Fender cleans for the F'n win!

    Thanks, man!
     
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  11. tele_player

    tele_player Member

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    That’s the first place I check for intermittent volume drop on a Fender amp. I’ve seen it a few times, caused by board warp, sometimes breaking the resistor.

    When installing resistors going the ‘long way’ on a Fender fiber board, it’s a good idea to leave extra length on the leads, so the part and joints won’t be stressed by warpage.
     
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  12. MKB

    MKB Silver Supporting Member

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    I don't think I've owned a single Fender amp that didn't need a good whack every now and then to stop some sort of minor noise. My coworker a year or so ago bought two new Deluxe Reverb reissues, both of those needed smacking also for very minor noises. I tried to talk him into keeping them as they sounded terrific, but he's anal about such things and returned them both.

    I had a text in college that was discussing troubleshooting, it clearly stated that punching a recalcitrant device is a valid diagnostic procedure, but should not be considered a permanent fix. :)
     

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