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Intermittent Problem with Fender Bassman

Discussion in 'Amps [BG]' started by neckdive, Dec 11, 2013.

  1. neckdive

    neckdive

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    I have an intermittent problem with my Fender Bassman 350w combo. Normally, I have no problems playing at a "normal" volume -- volume 7-8, gain at 3-4. If a dig in too much and send it into overdrive the signal starts to crackle loudly. If I back down my playing and even the gain or volume it still crackles and doesn't return to the quiet it was before. Last night the crackle was so loud, I had to turn the amp off for a moment, then turn it back on to clear the noise.

    If I turn the amp on its side, or upside down the problem goes away completely and I can crank it as high as I would like without any crackle or other noise. Flip it rightside up again and the crackle/noise comes back. If I play the next day, there is usually no crackle right side up.

    I have had the amp for about 6 months and this is the second time this scenario has played out. The first was about 3 months ago with no problems in between incidents.

    My theory is that there is a problem with the jack but before I opened it up, I wanted to get others' opinions. Then, again it could be with the pre-amp so I don't know at this point. Other than this glitch I like the amp just fine.

    Thanks in advance for helping me troubleshoot this.
  2. B-string

    B-string Supporting Member

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    Try first using an instrument cable to "loop" from the effects send to effects return jack. Could be the contacts inside the effects return jack, especially if you don't use it. The cable will bypass those contacts.
  3. beans-on-toast

    beans-on-toast

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    If the cable doesn't fix the issue, it is possible that you have a loose connector inside the chassis. This would require opening the amp and reseating all the connectors inside the amp. This is somewhat dangerous to undertake as you can get quite a zap, even with the amp turned off and unplugged. If you aren't comfortable doing this, have some look at it for you.
  4. neckdive

    neckdive

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    Right away, we tried a few different working cables and even switched guitars so we definitely isolated the problem to the amp itself. It was after trying other troubleshooting methods that we tried flipping on its side and head.

    You mentioned that you can get zapped even if off and unplugged. Where would that charge come from?
  5. B-string

    B-string Supporting Member

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    Power supply "storage" caps in the amp head. Their function is to smooth out the rectified AC (DC with AC component remaining) to "pure" Direct current and provide a reserve for the output amp.
  6. neckdive

    neckdive

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    Is there a way to safely purge the residual power from the capacitors?
  7. BassCliff

    BassCliff

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    Hi,

    Check the circuit board components, wire connections, potentiometer connections, etc, for cold, loose, or broken solder joints.

    I know a couple of great Fender amp guys if you are in Southern California.


    Thank you for your indulgence,

    BassCliff
  8. B-string

    B-string Supporting Member

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    Yes requires a resistor like a 100K 1 or 2 watt and some jumper leads. Most amps have "bleeder resistors" in the power supply but certainly not all and you should not blindly assume they are functioning. Knowing how to safely attach the jumper leads and to where is best taught in person, not with words on the internet.:)
  9. beans-on-toast

    beans-on-toast

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    I type way too slow.
    _________

    Yes. The charge is stored in power supply electrolytic capacitors. In an amp, these are the ones with a "-" or "+" marked on them. A procedure for discharging them is described here.

    You can start by playing a low note hard on your bass, then pull the power cord plug. You will hear the note taper off as the caps drain. That will get you most of the way there. To be safe, you should still discharge the caps.
  10. will33

    will33

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    His amp is one of the newer solid state "bassman" combos. It'll still have some juice in it, but likely not the deathly amount of a tube amp. Likely also has a bleeder resistor, but yes, never just assume those are working anyway. Can probably poke around inside this one as long as you're careful to just handle insulated connectors. Keep one hand in your pocket anyway...it's a good habit to have.
  11. beans-on-toast

    beans-on-toast

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    A zap is still a zap. If you get zapped with one hand in your pocket, it could be very painful. :p
  12. will33

    will33

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    Yes it could. :p

    OP, I'd start by hitting the note and unplugging it as beans said, then wait a few minutes, then set your meter to high DC voltage, the 1000 volt setting, then check for voltage across the caps, holding both probes in one hand, with the other hand away from the amp.
  13. B-string

    B-string Supporting Member

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    I got my thumb in the way of a 12vdc arc before, that hurt pretty bad.
  14. will33

    will33

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    I can grab both posts of a car battery and not feel a thing, but when I lay a wrench across there it hurts. :p
  15. B-string

    B-string Supporting Member

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    If you could lick both terminals of a car battery at the same time you'd get lots of dates with the ladies :D. Ever put a 9volt on your tongue to see if it was dead? :cool:
  16. neckdive

    neckdive

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    Thank you everyone for your opinions on how to troubleshoot my amp. I'll tear into when I get the chance, perhaps after the holidays. Until then, I may have to just play it upside down.
  17. will33

    will33

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    It's likely going to be a mechanical/bad connection problem rather than a bad electrical part, as when you set it upside down, gravity pushes the loose parts together, making them contact.

    Unfortunately, that may not narrow it down very much. Could be a jack, molex connector, ribbon cable, socketed IC, or a preamp tube if it has one. And that's without getting into bad solder joints that can be a total PITA to find. Whatever it is will likely be simple, cheap, and very frustrating. :p

    Good luck.
  18. rfslick

    rfslick

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    A wooden chopstick works very well as an insulated device for poking around in amplifiers.

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