Roland kc-550, no output?

[Isidro]

I'm about to replace my caps/resistor now. I bought a multimeter and now I'm trying to figure out what I'm looking for in reguards to testing the speaker leads before installing the speaker. What is a normal, healthy voltage reading for the Kc550 speaker woofer leads? Should I test with unit on or off?

[The Dude]

1) Unhook the speaker.
2) Set the meter to read DC voltage.
3) Power up the amp with volumes down and no signal applied.
4) Read the voltage across the speaker leads coming out of the amp.
5) Report your findings. You should get only a few millivolts.

[Drewline]

The Dude Rules!
I couldn't have said it better.

[Isidro]

it reads DC -045.5 mV is that ok? The reading is actually fluctuating between -045.5 and -051.6 mV. When I set the meter to AC it reads at 0V. On the tweeter leads it reads around DC-365 mV and continues continues fluctuating but returns to around there. What you think? Safe to go ahead and pop the speaker in?

[The Dude]

You should be safe to hook up a speaker and see how the amp works.

[Isidro]

I really appreciate your help. I put the speaker in and it's still just farting exactly like before even after changing out the caps and resistor. I guess something else is going on and I'll have to dig deeper.... DAMMMMIT

[The Dude]

Yes, you'll want to thoroughly check the output section for shorted solid state components or open resistors. Until you get it sorted and as you change out defective parts, it would be best to leave the speaker unhooked and re-check for DC before you reattach it.

[Isidro]

thanks. is a negatve DCmV reading a great thing?

[The Dude]

DC voltage polarity is somewhat irrelevant at the speaker leads in that a speaker doesn't like either polarity. However, when there is too much DC at the output, polarity can be a troubleshooting tool for figuring out where the problem might be. Typically, a solid state amp has a positive and a negative rail. The output circuits must balance those rails so that no DC comes out of the amp. So, again talking in generalizations, if we have negative DC at the output, we should look for something shorted on the negative rail side of the amp, or possibly something open on the positive side causing an imbalance.

[Isidro]

kc550 fix

i finally gave up. I called roland and ordered a brand new amp pcb set for $280. the set comes with boards called 1/3, 2/3 and 3/3. i dont know what 1/3 is because the only ones i've tinkered with are 2/3 and 3/3. i hope ill be able to install these new boards painlessly. the only concern i have is it looks like the output transistors that are wired to the heatsink are somehow adheard and not just screwed. hopefully it'll all make sense once its in my hands. once i have it working i might sell both my 550s and invest in a solid pair of powered speakers that can handle abuse a little better.

[Jazz P Bass]

i hope ill be able to install these new boards painlessly. the only concern i have is it looks like the output transistors that are wired to the heatsink are somehow adheard and not just screwed. .

A word of advise.
Make a note of where every connector goes.
As you make a note, remove it.
That way there will not be any confusion when it comes time to reinstall the new boards.

The output transistors are most probably 'stuck fast' to the heatsink by the isolating 'silipads'.
A light tap in a downward motion at the top of the transistor plastic body will loosen them up.

Inspect the silipads for any cuts as these pads isolate the back plane of the transistors from the heatsink.
As the heatsink is most likely at ground potential & the transistor back planes are at the + & - power rails, you do not want any connection.
This should be verified with an ohmeter before powering up the new boards.

[Enzo]

And just another thought. When you give up on a repair, and we all have been there, instead of spending $280 for new boards, there are plenty of qualified repair shops who could have repaired it for a lot less. I rarely get bills of half that amount leaving my shop, for example.

[Isidro]

what kind of shops are you referring to? general electronics repair? I tried the two well know amp guys here (I live in new orleans) and they said they really only work on tube stuff and made me feel pretty hopeless about finding someone who'd be able to troubleshoot/repair my solid state board. I'm hoping that by spending so much money on brand new stuff it will be a simple 'swip swap'

[J M Fahey]

"Well known" or not, guys who "only work on tube amps" are not true Techs, sorry.
Their claim to fame is often that they did some work for some Guitar God, and afterwards everybody wants also some of the magic dust.

Leave them alone.

What you need is an unglamorous Tech who is Authorized Service Station for regular brands, such as Fender/Roland/Yamaha/Peavey/etc. and who *obviously* has to be able to repair SS stuff or he wouldn't have been authorized in the first place.

Get to the Roland page itself, they will list heir official service Techs.

IF you have to ship your amp to another State, just do it, although I'm quite certain a large important city as New Orleans must have at least 1 or 2 Authorized Techs.
Roland - Support - Service & Repair - Service Center Locator

Roland Repairs U.S.A., Roland Service Centers

Roland Repairs New Orleans (Louisiana), Roland Service Centers

[Enzo]

Sure, every town has their "I only work on tube stuff" guys. Some of them are real experts on it, others are guys who only know something about tubes and are not really electronics guys. But the guys with the big reputations are going to work on their vintage tube stuff because they can make a living doing just that. Music stores often have service departments, and there are independent general shops like mine. A quick look and I found McCann Electronics in Metairie and Baker Music in Baker. Those are listed as Roland authorized shops. I don't know either one myself. But many shops that are authorized by major brands can work on an amp like yours, even if they are not factory warranty shops. My shop is authorized by many major brands, but Roland is not one of them, nonetheless, we wouldn't hesitate to work on an amp like yours. (Sorry, I am in Michigan) So I look at some other brands. I see for example that McCann is also an authorized Fender service center. Allisons in Gonzales is authorized Fender and Peavey repair shop. Places like those are places that work on your kind of stuff. And another way to do this is to ask people who use equipment like yours who they trust for service. One of our members here, KB, is up in BR and works on such stuff.