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Author Topic: Loud Pop when turning off Fender Stage 112 amp  (Read 3959 times)

Bloodbath

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Loud Pop when turning off Fender Stage 112 amp
« on: August 09, 2008, 08:25:48 PM »

My friend gave me his amp to try and fix it. It was humming like a mother and there was a crackling when the guitar is played. I fixed that. It was 2 large electrolytic caps. Replaced them and it is dead silent and no crackling at all. Now there is a loud pop when I turn it off. Turning the volume down before turning it off doesn't help. Any suggestions?
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teemuk

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Re: Loud Pop when turning off Fender Stage 112 amp
« Reply #1 on: August 10, 2008, 05:14:35 AM »

Does it have a delay/mute circuit built-in? (Relay in output or FET in PA input or something like that). If yes, check out that it works flawlessly. Many amps don't have one, though, and the standard feature of these is to "pop" annoyingly.

The pop originates from the AC component of charging/discharging the capacitors "finding its way" to the signal path. The effect is magnified if this happens in the early stages of preamp after which there is typically tons of gain. Hence turning down the volume control can often decrease the "popping" significantly. The volume control is not always right before the PA, though.

So other than the first suggestion of checking out the delay circuit (assuming one even exists), you might try looking out for DC offsets that seem out of place and which might cause the pop when power is turned down. If the supply caps were dead there's a fair chance that some caps have become leaky and are now bleeding small amounts of DC to a wrong place, which during the power down might cause a discharging phenomena that is "bled" to the signal path and fed to the speaker as the "pop". Though, that is just a wild guess. As said, that "pop" is a "feature" in some amps and can't really be helped by no other means than adding a mute circuit. Good luck for troubleshooting, though. Hope you'll get it sorted out.
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Bloodbath

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Re: Loud Pop when turning off Fender Stage 112 amp
« Reply #2 on: August 10, 2008, 02:37:36 PM »

Thanks. I will have another look at it. I did notice the two large 3300uf electrolytics that I had to resolder still contained voltage when the unit was off for a while. Maybe I'll start by replacing them.
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Bloodbath

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Re: Loud Pop when turning off Fender Stage 112 amp
« Reply #3 on: September 28, 2008, 01:23:55 PM »

Turns out the amp has always popped. Man, that pop is annoying and loud. Seems it should be pretty easy to get rid of. I wonder why Fender would not take care of that?
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Enzo

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Re: Loud Pop when turning off Fender Stage 112 amp
« Reply #4 on: October 02, 2008, 04:02:27 AM »

Because while it might be annoying, it is not a threat to the amp, so they don't go to the extra expense.  The pop is limited to the power rail voltage, just like the signal is.  SO the pop will never be any louder than the peak signal can be.

Isolate the problem.  If you want to see if it is the power amp settling, plug something into the power amp in jack to disconnect the preamp signal, then cycle the power.  If it no longer pops, the preamp was the popper.  If it still does, then it is power amp.

Likewise, you can send the preamp out signal to some other amp to monitor it and see if the pop comes out of the preamp.

SOme amps pop because the power switch arcs as it opens.  The power supply charge lasts long enough to amplify the resulting spike.  To test for this, leave the power switch on and just pull the power cord from the wall outlet instead.  If it doesn't pop that way, then the power switch is sparking.

In my experience, when I see any of the Fender solid state amps with that wedge shaped chassis - front panel short, rear panel tall with sloping chassis bottom - come into my shop, I expect to have to resolder the two main filter caps sticking up from the main board.  They never seem to be bad, but they love to crack free of their solder.  If yuo still have the amp, do your customer a favor - glob a bunch of silicone sealer or hot melt glue between those caps to glue them together,  They'll vibrate kess.  Then run a bead of glue around the base of each.
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4by12

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Re: Loud Pop when turning off Fender Stage 112 amp
« Reply #5 on: October 03, 2008, 02:08:31 AM »

I used to have the same amp and it did the same thing.
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