Fender Rumble 25 Speaker Rattles - diyAudio
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Old 24th December 2006, 09:24 PM   #1
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Default Fender Rumble 25 Speaker Rattles

Hello Everyone,
Hope you have (Had) a Great Holiday.

Need some help with a Fender Rumble 25 Bass Amp that the Speaker Rattles like crazy when the volume go's over about halfway.
I tried another speaker and the same happens. I checked it out with a different bass and the same thing happens. I can also hear it through the headphones. So I figured maybe the power amp was faulty so I changed it (LM3876T) However the same problem. So I changed all the IC's (TL072CP and CA3080E)
No change. So I changed the three transistors that are in this model (MPSA63 and two 2N4401) still the same problem.
Any ideas??
Thanks, John.
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Old 25th December 2006, 05:24 PM   #2
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Try feeding it to an external speaker cab, it may be a mechanical vibration?. But how loud are you driving it?, you may just be overdriving it? - I presume you're only using it for practice in your bedroom?.
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Old 25th December 2006, 06:39 PM   #3
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Thanks for the feedback Nigel: Yes, I've tried another speaker and cabinet and still get this very distorted rattling sound like the speakers about to blow. This happens at about 50% volume, which is not very loud. I also tried the bass on another amp and it sounds ok. So there seems to be something that is making the Rumble Rattle. The amp got good reviews so it must work ok. I'm wondering if maybe a capacitor is not filtering correctly and at a certain frequency is breaking down. I can't find a schematic for this anywhere. I did find one for the Fender Frontman 25B which is similar, except it uses a different power chip, so that section is a little different. Not quite sure what to do right now.

John.
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Old 25th December 2006, 07:19 PM   #4
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I would suggest using a scope to see what's happening!.
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Old 25th December 2006, 07:22 PM   #5
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These chips just don't sound very nice when they go into overdrive. Actually, they sound like crap! And with a bass guitar, it's quite easy to trigger the SPIKE protection, which sounds pretty much like a rattling speaker. So I'm afraid you're out of luck. Short of keeping the volume down, there's really not much you can do.

Rune
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Old 26th December 2006, 09:40 PM   #6
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Gentlemen,
Thank you for your info. I am going to set up some tests with a signal generator and my scope in the next few days and see if I can find anything. Maybe some type of attenuater put on the input would help. Any ideas hear?
What I find odd is that using a Behringer BX108 (My friends unit)
everything is fine right up to 9 on the volume, and only a little distortion at 10, and its loud!! I would of thought that a Fender when working correctly would at least be as good in sound handling. If the chips are doing what Runebivrin says, maybe I should use different chips and do some other type of mods. However, I have no idea what would work there.
Thanks again for your help.
John.
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Old 27th December 2006, 01:19 PM   #7
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The numbers on the volume knob have no meaning at all, depending on the gain and the input levels one amp might clip at 5, and another not until 10.

Without a circuit of the Fender it's hard to suggest much (apart from scoping it), but the 25W circuit on their website has the usual Fender 'anti-clipping' circuit around the output stage, but I don't know if the Rumble has or not?.

What are you actually trying to do with the amp?, at 25W it's probably only good enough for practicing alone - if you practice with a drummer you're seriously underpowered and won't be anywhere near loud enough.
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Old 27th December 2006, 05:11 PM   #8
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Thanks Nigel. I am only using the amp for my own use, no gigs or anything. The Rumble manual states that it has DeltaComp adaptive limiter to eliminate power-amp distortion. I'm guessing that its this part of the circuit that is not working correctly. The manual for the Frontman 25B says it has the same circuit. So the schematic I have for the Frontman should show it. I found the schematic at Fender.com under schematics for amps. Is this the anticlipping circuit your referring to?
Is this after or before the power chip LM3876T ?
There is a feedback type circuit that I do not fully understand, that feeds back around the powerchip through a TL072, 4 switching diodes, resistors and electrolytic caps to the base of the MPSA63 transistor, whose collector go's to pin5 of the CA3080 chip. It's output adds to the input of the final TL072 that completes the feedback into the power amp chip input.
Maybe its this circuit thats not working.
My daughter is going to bring my scope up from the basement for me later. (I unfortunately am disabled with back problems and nerve damage in my leg, so I can't lift or do much very easily.)
Thanks again for your help.
John.
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Old 27th December 2006, 07:22 PM   #9
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Quote:
Originally posted by JohnR242

There is a feedback type circuit that I do not fully understand, that feeds back around the powerchip through a TL072, 4 switching diodes, resistors and electrolytic caps to the base of the MPSA63 transistor, whose collector go's to pin5 of the CA3080 chip. It's output adds to the input of the final TL072 that completes the feedback into the power amp chip input.
That's the limiter circuitry, it's the same in a 60W Fender Keyboard combo I have - which we use for guitar! (it's even been gigged with TWO guitars plugged in it!). The CA3080 is a transconductance amplifier - and is used as a voltage controlled attenuator, so it limits the volume 'nicely'.
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Old 27th December 2006, 08:29 PM   #10
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Nigel, thanks again for the help. So that is the limiter circuit. So I guess I have something there not working quite right. I shall concentrate my testing on this part of the circuit.
Today I tried a regular guitar on the amp and with the volume turned all the way up it sounds great until I hit either an open low E or open A, then that rattleing and distortion cuts in big time. So it looks like the low end of the frequency range of that limiter circuit is not working right.
I'm going to work on it tomorrow (Fingers crossed)

Thanks again for your help.
John.
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