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Post subject: Fender BXR 100c Help!
Posted: Wed Dec 14, 2011 2:20 pm
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A guy is trying to sell me a Fender BXR 100 combo amp.When I got the amp home it had a terrible crackle sound come out any time I hit a string. It does not sound like a blown speaker as there is almost an electronic buzz over tone that happens. I get the same sound when i turn the volume up or down. I'm tempted to have the amp checked out and see if it it would be worth fixing. Has anyone experienced this problem? Is this amp even worth the effort?


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Post subject: Re: Fender BXR 100c Help!
Posted: Wed Dec 14, 2011 10:14 pm
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It sounds like you are describing the same thing I'm experiencing with my Rumble series amps. (see post about "Rumble 25 Crackling Sound.") This seems to be a fairly common complaint across many of the Fender bass amps.

I find it hard to believe that Fender can't....or won't....publish a fix.


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Post subject: Re: Fender BXR 100c Help!
Posted: Fri Dec 16, 2011 3:28 am
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Diagnosing a problem like this one with your BXR100 via a forum post is nearly impossible. Same with the Rumble 25 fuzz tone. It could be any number of things. Bad preamp chip. Bad connection somewhere. Bad power amp module. Bad jack. In each case you can't know till you diagnose it.

On a Rumble 25, which is basically a home practice amp, unless it is a really easy fix I'd just buy a new one. That is why I didn't bother to reply to the Rumble 25 post. On a BXR100 it would be worth looking at to see if it is something simple. In either case figure $100 minimum on labor, some places more and other places maybe a tad less. Plus you have to pay for parts if you decide to go ahead and fix it. The BXR was a great series in it's day and as a larger amp it is more worthy of a look. Also know that if you do fix it, likely something else will eventually go wrong again. Old amps are like that.

One thing you can try on BOTH amps to sort of narrow it down is this. On the BXR there is an effects RETURN input jack. On the Rumble 25 there is an a "CD" Input jack. Both these input jacks bypass the preamp completely and go directly to the power amp module at FULL VOLUME.

1. With the amp turned OFF, patch a CD player or IPOD/MP3 player into the jack.
2. Turn the volume down on the player, and start it up playing some song with a strong bass line ("Another One Bites The Dust" for example).
3. Power up the amp.
4. Gradually turn up the volume up on the player.

If the music player sounds just like your bass with that fuzzy sound then it probably is in the power amp section. If the music player sounds fine with clean bass through the amp at a moderate to full volume then you just narrowed down the problem to the input jack, the jack connection to the circuit board, the preamp itself or the connection from the preamp section to the power amp module. Preamp chips/parts are way cheaper than power modules or output transformers. Labor will run about the same either way. I'd be more prone to repair a small amp with a preamp, jack or connection problem than a power amp/transformer problem.

There actually are no "user serviceable parts" inside the amps so unless you know exactly what you are doing I'd seek a qualified repair person. A radio TV shop might be willing to give it a look, but expect a minimum charge. One local music store here will only work on amps they sold for the original owner. With TV's and radios basically becoming disposable it is getting harder to find a TV/Radio repair shop anymore. One local TV repair guy here worked on some amps for local players but recently he died so I have been looking for a new repair guy willing to help players out, but I'm not finding one.


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Post subject: Re: Fender BXR 100c Help!
Posted: Fri Dec 16, 2011 10:36 am
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A very helpful reply, Dave. Coincidentally, I reasoned out the same process of using the CD input on the R25 to narrow the problem down. I did it very quickly and not very critically, but I'm pretty confident that the sound was much cleaner thru the CD inputs. I'm actually going to run the same tests using a bass thru my mixer, into the CD inputs.

My point about this being a "known" problem that Fender should be aware of is based on reading posts here and across the internet about similar problems with both brand new and used Rumble 25, 60 and 100. Many people report the problem right out of the box, returning the amp to the seller and having exactly the same problem with the replacement. Others seem to have no problems at all. In my case, all my Rumbles were used.

For these reasons, I'm convinced there is a design flaw in the Rumble series. I have the schematics for the 25 but haven't done any serious troubleshooting in over 40 years and no longer have the test equipment or solid-state knowledge to dig too deeply into these amps. I am, however, competent to do the repair if anyone can supply a known fix.
Again, thanks for taking the time to post. You gave me some good food for thought.


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Post subject: Re: Fender BXR 100c Help!
Posted: Fri Dec 16, 2011 11:37 am
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Good news! After doing a few tests and narrowing it down to the board, i decided to take the board apart (This is usually not a good idea!). I found that there are 2 fried resistors on the board. An easy fix if I can find out which resistors they are. Perhaps someone on here can help me. On the schematic, Page 2 all the way to the bottom left corner of the board there are 2 parallel resistors marked "R112 and R111". R111 looks like it caught on fire. R112 is just black from the debris of R111 but should probably be replaced. I looked up R111, and R112 resistors, but did not find what I was looking for.


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Post subject: Re: Fender BXR 100c Help!
Posted: Fri Dec 16, 2011 3:22 pm
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If you are looking at the actual schematic....(lots of strange symbols)....not the parts layout.....(drawings of what the parts actually look like and their location).....it should specify the resistor values in ohms.


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Post subject: Re: Fender BXR 100c Help!
Posted: Sat Dec 17, 2011 3:58 am
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http://www.eserviceinfo.com/download.php?fileid=46401
That is the schematic for the BXR100.
R111 and R112 are both 15 Ohm half-watt resistors. They are in the circuit for the direct out, so the fault that fried them may fry them again just as soon as you replace them and power it up. Possibly a bad filter capacitor? That is only a guess. Won't hurt much to replace them and try it though. Since they are in the direct out circuit I doubt that they are the root fault. The schematic gives the voltage reading before the resistors as 6.28 volts, so if it is way out of tolerance they will just blow again. I'm suspecting you'll see a much higher voltage running at that point.


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