Roland kc-550, no output?

[methodofcontrol]

hey guys,
i've got 2x roland KC-550's here. my friend asked me to take a look at them for him.
both amps have the same symptoms, and he swears they were never abused.
here's the deal: everything powers on, but no sound to the headphone jack or speakers.
i pulled out the speaker and examined the power amp in each one and i found that both had burn
damage marks behind R41. i can't find a schematic anywhere, so i'm not sure what R41 does.
i tested the Left and Right power transistors in each amp for shorts and found nothing.
all fuses are also intact.
my plan right now is to replace R41 and see what happens... but, i'm wondering why it would've blown.
any suggestions? i'll report back w/ any new developments after replacing R41.

[Jazz P Bass]

Call Roland.
They charge for schematics, but if you have two to work on, it may be well worth it.

[methodofcontrol]

thanks for the suggestion. i'm finding now that R41 still reads on a meter @ 25 ohms. so i'm not convinced that replacing it is going to fix anything.
hmmmm.....
thoughts? is this a common thing for these amps? google search is not getting me anywhere, so far.

[methodofcontrol]

just got off the phone w/ roland. they will only sell their schematics to authorized service people. but i was able to get info on R41 out of them. it's listed as a 27 ohm resistor. so the one i have has seen some heat, but otherwise it's in spec. open to suggestions.... thanks guys.

[52 Bill]

What does R41 connect to? Power supply? Speaker output?

Check all of the simple things first, break jacks, speakers, etc. Try one of the line out jacks any signal there? Like anything else, try and isolate the problem to a specific section of the amp. Is the main power supply working? How about the power supply to the preamp?

[mozwell]

ROLAND KC-350 KC-550 Service Manual free download, schematics, eeprom, repair info for electronics
you need to register, but its free, you need to answer basic electronics questions to register.....

mountains of info on this site, heartliy recommended....

[methodofcontrol]

i replaced R41 in both amps, one of them is working normally now. although, maybe a little quiet. the other is still mute.
i suspect the resistor replacement was less of a fix then perhaps re-seating some of the wiring harnesses.
i'll try again tomorrow.
and yeah...there's two sandbox resistors, one coming off each power transistor, at the point where these resistors
meet, R41 goes from there to a pair of caps. i'm not sure...

[Jazz P Bass]

Could be the zobel network.
If it is, replace both of the caps that attach to R41.

[Jazz P Bass]

Roland KC 350-550 Service Manual

Show nuff, it's the zobel.
Replace the caps.
Check the bases of the ouput transistors to see if voltages are correct.
The outputs should barely be turned on at idle.
I would scope the output, looking for oscillation.
That may be where the power is going.

[methodofcontrol]

thanks for posting the service notes.
to be clear, the caps I was referring to are C26 and C27.
are those the ones you're referring to also?

[Jazz P Bass]

thanks for posting the service notes.
to be clear, the caps I was referring to are C26 and C27.
are those the ones you're referring to also?

Yep.
Technically, it may be a zobel or it may be a stability network.
I do not know what "took out" R41.
I simply would not trust those caps. (C26 & C27).
The output section must be checked real close.

[methodofcontrol]

well..turns out replacing the R41 resistor in both amps ACTUALLY DID fix the amps. the one amp that seemed to still NOT be working also had a bum speaker. i put my meter ont it just as an idiot check, and i got nothing. hooked everything up to an external speaker, and it's working! just need a new speaker now.

[Jazz P Bass]

Which begs the question "why did R41 burn up?"
I would still replace those caps as insurance.

[BigCJ]

Where would I buy resistor R41

[Jazz P Bass]

R41 is a 27 ohm, 1/2 watt resistor.
Digikey
Mouser
Allied Electronics
Newark in One
Or try a local repair shop.