# Fender Rumble 75



## THRobinson (Jun 29, 2014)

Hey guys... needed a bass amp and someone local for once, had a Rumble75 for sale cheap because one day it worked, the next day it powers up but no sound. Managed to grab it for $60... thought it sounded like a deal. Even if a blown speaker... buy a new Legend BP12 for $135CAD.

Apart from obviously checking to see if the speaker is fine/blown... are there any known issues with these I should look at first?

Amp looks mint, though worried, because they emphasized that no one ever opened it to take a look at it, yet one of the screws and washers is missing off the top that holds the amp in place. Didn't see that until I got home.

And, before someone says it.... yes, I know that caps hold a charge and can kill you. Got a scar on my wrist from when I was 12 and learned that lesson.


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## Paul Running (Apr 12, 2020)

The Rumble 75 is an all solid-state amp. The highest DC voltages are ±30VDC however, it is always a safe practice to be careful around any electronic circuitry. A visual inspection of the circuitry is advised, checking for signs of any stressed components (overheating, bulging caps...a good eye-ball check). Here's the schematic:


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## Latole (Aug 11, 2020)

" one day it worked, the next day it powers up but no sound"

IMO it is not the speaker,
You have to trouble shout the amp. there are something wrong in power supply rail


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## THRobinson (Jun 29, 2014)

Thanks for the schematic. I looked for one, found other models but not the 75.

Saw a video that had a broken solder spot on the jack. Hopefully something simple like that.


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## Latole (Aug 11, 2020)

THRobinson said:


> Thanks for the schematic. I looked for one, found other models but not the 75.
> 
> Saw a video that had a broken solder spot on the jack. Hopefully something simple like that.


Input Jack ?
Steel input jack are very tough. Plastic one are fragile
Schematic sheets show they are plastic ( with steel nut ? ) 
Tight nut by hand only, not with wrench you may damage the threads


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## THRobinson (Jun 29, 2014)

It was where the jack pins were soldered to the board, one was a cold solder joint and broke lose in the video.


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## Latole (Aug 11, 2020)

THRobinson said:


> It was where the jack pins were soldered to the board, one was a cold solder joint and broke lose in the video.


It's the issue sometime


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## jb welder (Sep 14, 2010)

Do you get sound with headphones?


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## THRobinson (Jun 29, 2014)

jb welder said:


> Do you get sound with headphones?


Well... FINALLY plugged the amp in to test it a bit. Few health issues and the repair kinda got pushed to the back-burner. About to clear coat my PBass after a new paint job and figured better get the amp running again. 

Tried input 1 and 2, no sound. Plugged in the headphones, and both inputs work. So, assuming 1st thing to test is the speaker and speaker connection. Failing that, maybe the switching jack? though with 2 inputs, I'd doubt both went at the same time. 3:10am right now, might wait until morning.

Just curious though... reading some stuff about the Rumble 75, few people upgraded to an Eminence BassLite S2012 which hopefully my speaker works... but also quite a few people using 1/2" sound dampening foam inside. Anyone recommend that? How does it affect the sound... or is it more to reduce rattle/shake in the cab?


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## THRobinson (Jun 29, 2014)

OK... speaker is out. Tried with 2 multimeters to get a reading. One is autoranging, the other I have set to 20k Ohms.

I have the speaker removed and unplugged, and when testing the meter isn't at 0, but it isn't staying at a solid number either. Constantly changing between 1.5 and up to around 15 back and forth. I used a 9v battery on it and heard nothing but not sure if because dead? or battery too small to make a difference.

I had a 4" speaker for an Arcade cabinet I plan to build and hooked it up to the Rumble and worked fine.

So... dead speaker? Why are the numbers constantly changing and not 0? Anything else I can do to officially declare the speaker dead?


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## Paul Running (Apr 12, 2020)

THRobinson said:


> Why are the numbers constantly changing and not 0?


It may be intermittent. A full-capacity 9.6VDC battery would push or pull the voice-coil to full range. I would speculate that somewhere in the loop, there is a restricted passage for electrons to conduct, so much so that not enough current is allowed to flow to create a reaction with the magnet. It could be a poor connection at the voice-coil termination points or the voice-coil itself has been compromised. I would inspect the connections at the frame and at the cone.


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## THRobinson (Jun 29, 2014)

@Paul Running I came across this video in the wee hours last night... I'm thinking it's the next step.

The dome is one really well and the seam very smooth. I doubt I can work it up with a knife/scalpel. Video mentions solvents, then every solvent mentioned may ruin the cone so... not sure what to try. I have Acetone-Free nail polish remover... maybe that and a Q-Tip?


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## THRobinson (Jun 29, 2014)

Ok...saw a few vids where they just had to cut the dome/cover out... so cut it out. Solvent wasn't working and could not get a knife under the edge at all.

Took a scalpel and scraped the black glue off a small part of where the main leads come through the cone and the thin coil wire. seems the joint between the thick and thin wire on either side is ok, continuity check and got a beep.

I put the leads of the multimeter on both sides, and if I move the cone in/out I get intermittent beeps.

Plus... I have this coated wire sticking out. Scraped it a few times trying to get a bare spot but no luck getting a beep, even with leads on same wire next to one another... guess varnished?

What is with this wire floating inside? Anyone know where it should go?

*EDIT - A-ha... maybe figured it out? Thought the wires on both sides were on the outside of the coil... maybe not? The "+" lead though coming into the cone, the thin wire was REALLY hard to find and scrape compared to the "-". I heated up that wire and scraped again and used my meter for continuity and with the lead on the "+" and that wire... nothing... when I put the lead on the "-" and touched that wire I got a solid beep (continuity).*
*
So... still a bit confused... why is that wire inside the coil? why no indication it was ever attached to the "+" wire coming through the cone? and why when I move the cone in/out do I get intermittent continuity?
*
*Hoping someone can let me know because I solder that to the wire through the cone. I don't wanna ruin the amp as well as the speaker.*


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## greco (Jul 15, 2007)

Following with interest. Good Luck with the repair!


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## Paul Running (Apr 12, 2020)

THRobinson said:


> What is with this wire floating inside? Anyone know where it should go?


That wire should be located on the other side of the voice coil, question is how did it get there and where does it originate?.


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## THRobinson (Jun 29, 2014)

@Paul Running yes, that's my thoughts.... my theory is this, the + wire broke at the coil. When I move the cone in/out it makes intermittent contact. The loose wire came out through the seam in the metal cylinder. Question though is... was that intermittent connection enough to make the amp work long enough to shake that wire loose and through the seam? That's where I'm stumped.

My worry is if I solder it, and I still have the intermittent connection, when they both connect something bad may happen.

Maybe, cut the thin + wire where it joins the thick wire at the cone... then solder this loose wire to the thick wire... then use some 15min epoxy to glue the wire to the inside wall of the cylinder?


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## THRobinson (Jun 29, 2014)

Well... cut the thin wire from the thick, soldered the loose wire to the thick one at the cone. Since that sleeve moves when the cone moves, I didn't glue it to the wall and left 3-4mm gap at the top where it went up/over. Put it in the cab and well...

Good - It worked!

Bad - It sounded absolutely awful. Fuzzy, buzzing, just awful. When I move the cone in/out with my finger, you can hear some sorta scraping.

So... sadly, didn't want to spend the extra money but at this point I think it's better to buy a Eminence BassLite S2012, then to either try to re-coil this speaker, or spend the same amount buying a used Rumble 25.

What do you guys think?


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