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Keyboard Amplifier Troubleshooting Question
February 11, 2014 2:11 PM   Subscribe

I feel like if I knew more about amplifiers the symptoms of my Roland KC-550 would narrow down my problem significantly. Only the tweeter works (and it works fine), and the two other speakers are totally un-driven across all channels. Does this indicate anything about the issue? I'm trying to figure out if it's worth having repaired and/or if I can repair it myself.
posted by cmoj to Technology (5 answers total) 1 user marked this as a favorite
 
You may find this thread interesting:

http://music-electronics-forum.com/t27953/

The last post contains a link to a PDF of the service manual/schematic for the KC-350/KC-550 amps.

Can you try it with a different speaker? Does the headphone out work as expected? I don't have any experience with this amp, so without navigating the circuit with a 'scope or multimeter I don't have anything brilliant to suggest.
posted by mosk at 3:15 PM on February 11


There may be some kind of crossover component that routes highs to the tweeter, and mids and lows to the other speakers, which has failed. I'm not that well versed in amp tech, though. I do know not to go playing around inside of tube amps, which have capacitors that store high charges, unless you know what you are doing. I think these Roland amps are solid state, but they may still have some components that are dangerous.

I don't know if there is a way to say this without sounding condescending. So here goes. If you are asking this question, no, you should not try to fix it yourself (unless you have a lot of electronics background and are just new to amplifiers, and, even then, $50-$100 for an experienced repair person is possibly still the way to go).
posted by thelonius at 5:45 PM on February 11


two other speakers

Ummmm, everything I'm finding on that amp says there's one other speaker besides the tweeter - a 15" woofer.

In any case, here's the support page for that amp, from which you can download the owner's manual. On page 13 of the manual there's a block diagram of the amp, and from that it looks like signal comes out of the power amp section and gets sent to the 15" and to a high-pass filter and then to the tweeter.

A high-pass filter is a collection of a few electronic components that prevent low frequencies from getting to the tweeter, which would blow the tweeter. A high-pass filter can be part of a passive crossover, which sends the right audio frequencies to the right speakers, but from a quick peek at the schematic at the forum mosk linked to, it doesn't look like there's really a full crossover, just that high-pass.

So, if your tweeter is working, then your amp is mostly fine, and if there are no other crossover components, then likelihood #1 is that your 15" woofer is blown.

The schematic shows a couple of internal fuses, but I think if they were blown you'd be getting no sound at all out of the amp.

if it's worth having repaired and/or if I can repair it myself.

You can replace the speaker, which can be a DIY project with a screwdriver and maybe a hex wrench (a.k.a. Allen key) and removing a couple of wires from the old speaker and connecting them to the new one. But not everything in every amp is exactly the same, so if you're an utter noob at this kind of thing I'd see if you can enlist the help of a friend (guitar players tend to have some experience mucking about with speakers) who might have some more knowledge & experience - it's a little hard to explain over the internets.

You should probably be able to get a replacement speaker for about $100. But I can't give you an idea of exactly which speaker to get, because there's some tech info (wattage & impedance) that I can't find anywhere online. That info should be on the back of the speaker itself, so if you're willing to take a whack at DIY then the first thing to do is take the old speaker out and look on the back for wattage rating and impedance (which will be given as "ohms" or the omega symbol and will most likely be 4, 8, or 16.) This will give you some important info about what your choices are for replacement speakers.

Speakers can also sometimes be repaired (a.k.a. reconed), but between parts and labor it's often just as expensive, if not more, than buying a straight replacement. Reconing speakers is not a DIY project.

If all this is making you nervous, go ahead and take it to a repair person. Note that you'll want someone who works in pro audio and/or musical instrument gear repair, probably not home stereo or car audio.

Even then I can't see it costing you more than $200 all told, and considering those amps sell for $650 new and about $400 used on ebay, I'd say it's worth repairing.
posted by soundguy99 at 6:49 PM on February 11 [1 favorite]


Oh yeah, and if you do decide to try the DIY speaker replacement route, there are a couple things to keep in mind;

1) Speakers have a polarity, a positive connection and a negative connection, often color coded with red for positive and black for negative. When you remove the original speaker, note which wire goes to which connection so you can put the right wire in the right place on the new speaker.

2) Once the original speaker is out, there are a couple of quick tests you can do with a multimeter or a 9 volt battery (these tests should be easily Google-able) to double-check that the speaker is actually dead - no point spending money on a new speaker if the old one is fine and something else is wrong with the amp.
posted by soundguy99 at 6:53 AM on February 12


Update: Tested the speaker, found infinite resistance, so I dug around in forums to find the manufacturer and consumer-market equivalent of the original speaker and bought one. Cost me about $80 all told.
posted by cmoj at 5:51 PM on February 25 [1 favorite]


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