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Post subject: Troubleshooting a Rumble 60 bass amp...
Posted: Tue Sep 25, 2007 4:14 pm
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I'm having problems w/my Rumble 60watt bass amp. I don't want to take it to a shop, and I contacted Fender via Email, but they were of no help.

>>The problem: When I turn on the amp, all you hear is a loud humming sound. I'm pretty sure it doesn't have anything to do w/the speaker...I think it is something electronical, but I don't know what. A fuse maybe?? I'm going to try to take it apart and look at it myself. BTW, does anyone know how to take the back off the amp? Any advice would be greatly appreciated, thanks!


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Post subject: Re: Troubleshooting a Rumble 60 bass amp...
Posted: Wed Sep 26, 2007 9:24 am
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LiquidSoul wrote:
I'm having problems w/my Rumble 60watt bass amp. I don't want to take it to a shop, and I contacted Fender via Email, but they were of no help.

>>The problem: When I turn on the amp, all you hear is a loud humming sound. I'm pretty sure it doesn't have anything to do w/the speaker...I think it is something electronical, but I don't know what. A fuse maybe?? I'm going to try to take it apart and look at it myself. BTW, does anyone know how to take the back off the amp? Any advice would be greatly appreciated, thanks!
You didn't mention if you have an instrument cord plugged in so, I'll assume no for the moment.

The loud hum sounds like a wire off, or, possibly a loose component on the mother board.

Because modern, hi speed production methods involve wave soldering it's now possible to buy products that have questionable solder connections.

So, the thing to do is, open her up and perform a visual inspection. Tug the wires and the components to see if any obvious problems jump out at you.

Then (if it was me) I'd start re-heating solder connections to flow temperature while holding the wires and components with hemostats or pliers, and let the connections cool. This is in case there are "cold solder joints" that escape visual inspection. A cold solder joint is one that appears normal (although the solder may appear frosty instead of shiny) but will not conduct electrons. This is what happens when the work isn't brought up to the correct temperature when soldered. And, the number one cause of these problems are poor solder connections that work at first and then break down later. Cold solder joints often pass inspection and cause problems later.

If you just melt solder and let it drip on the work for instance, it probably won't conduct electricity because it didn't bond properly. In order to effect a good solder connection all parts of the connection (i.e., the wire, the terminal or circuit board AND the solder) must reach the same temperature and then be allowed to cool slowly.

And with bass rigs the heavy vibrations and killer SPLs will bring out a weak solder connection quickly.

My son's bass rig began smoking and making noises during a metal gig so, he powered it down and switched to his backup rig.

The next day I opened it and found a bad solder connection on a large resistor on the board. I re-soldered it, adding a tiny bit of solder (even though in many cases it's not necessary because there's enough solder there to reheat and to make a good connection) and the problem was solved.

Had it been a guitar or PA rig the problem may never have surfaced, but bass is a sure way to find a weak spot in the system.

When you're looking the amp over, make sure that there are no single tiny hair filaments from a stranded wire floating around loose. If so, you can either trim it off or re solder the whole connection and add the "rogue" to the connection.

Let me know what you find.

BTW, the problem isn't a fuse. The purpose of a fuse is to burn up (thereby stopping the current flow) in the event of a power spike to protect the transformer and/or components, and a fuse either works or it doesn't. (If you have a transformer that will fry at 10 amps, then you'll install a 5 amp fuse, so the fuse burns up before the transformer) Fuses don't cause hum problems.

Also, if you use a soldering tool that's too hot for the task you may find that you damage components in the process. I'd recommend using a soldering iron no hotter than 25 watts when working at board level, and even then I may clamp on the leads of small zener diodes (the clear, glass-like components) and other small electronics with hemostats to act as a heat sink. This will keep the diode from melting while I'm reheating the connection. So, the hemostats (surgical clamps-available at Radio Shack just for this task) are clamped (between the diode and the solder connection) on the front while I'm reheating solder connections on the back. The hemos draw the heat away (because they're thicker metal) while allowing the solder connection to get hot enough to flow.

Got it?


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Posted: Wed Sep 26, 2007 2:17 pm
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Ok so I understand what you're saying, but first thing's first...why can't I get the head out?! I used to install car stereos...I should be able to do this. I took all the screws off top, front and sides, and even by pulling & pushing, still won't come out. Maybe there's a trick to it or something...


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Posted: Thu Sep 27, 2007 12:20 pm
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LiquidSoul wrote:
Ok so I understand what you're saying, but first thing's first...why can't I get the head out?! I used to install car stereos...I should be able to do this. I took all the screws off top, front and sides, and even by pulling & pushing, still won't come out. Maybe there's a trick to it or something...
The handle screws probably go all the way through and thread into the bottom of the amp chassis.

I'd remove the grille and the speaker (which is front loaded) and look inside.

Because the speaker is front loaded I'm assuming that the cabinet is glued together solid and there is no removable back. (I couldn't find any pix of the back of your amp. But, front loaded, solidly glued bass cabinets came into being to minimize vibrations, and that is a good thing.)

So, any service to the jack or the light at the bottom port must be done through the speaker hole, and given that it's possible that there are other screws, clips or means of solidly mounting the amp that are only accessible through the speaker hole.

Or, the amp may come out just by removing the handle and/or the grille.


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Posted: Thu Sep 27, 2007 4:43 pm
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Alright! So I finally got the amp out by removing the screws on the handle. I took a good look over the motherboards and I don't see anything out of place, disconnected or anything like that. None of the wires are soldered on, they are just held on using wire connectors. Visually, everything seems in place. Any ideas??


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Posted: Tue Oct 02, 2007 5:10 pm
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LiquidSoul wrote:
Alright! So I finally got the amp out by removing the screws on the handle. I took a good look over the motherboards and I don't see anything out of place, disconnected or anything like that. None of the wires are soldered on, they are just held on using wire connectors. Visually, everything seems in place. Any ideas??
If you have a VOM (volt ohm meter) then you can check the solder connections by setting the meter to "ohms" (the lowest range) and touching the solder connection on the back and the component lead on the front. If you read continuity then chances are they are good solder joints.

Also, you can check the four diodes that make the bridge rectifier with an ohm setting. Diodes only allow current to flow in one direction so you should read continuity with the probes touching the diode leads in one way but not the other. (Red on left lead-black on right then switch.)

I had an amp blow and just make humming noises once and it was a blown diode in the rectifier (which converts AC to DC)

A bad diode can either either be open and allow no current to pass (not even in one direction only) or it can allow AC to pass, and that's when your meter says that current is flowing in both directions. (when you swap the leads to opposite sides of the component and read continuity in both drections) Either way the rectifier is bad, and either one diode or the whole self contained bridge rectifier must be replaced.


Also, remember that capacitors can hold a charge and can shock you if you touch them (that's how police TASERs work. They store energy but unlike a battery, capacitors rapidly discharge like a bolt of lightning)

Back in the "olden days" kids playing in junkyards were sometimes burned or even electrocuted by capacitors in discarded TV sets. Caps can hold a charge and be dangerous for years after they were last charged. Most of the caps in amps aren't of the death dealing variety but they can definitely ruin your day, so please be careful and identify them, or stay clear of any unknown components and get a pro to help you.

Handle a PCB with caution, identify all caps and if necessary bleed them off with a resistor or an insulated jumper wire to remove a shock hazard.

I generally don't dead short a cap unless I'm going to remove it.

Also, you can try moving the components with one finger and those that are solidly soldered wont move, but a cold solder joint may pop loose and tell you if there's a problem. If you can wiggle a transistor, resistor, diode or cap (without actually bending the leads) then it may need to be re-soldered.


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