Trying to repair a Roland KC-350 amplifier, no speaker output.

[Hensinger806]

I have read and posted on some forums recently,

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trying to get an idea of how to test what to replace to give this thing new life. I followed a separate thread on here and tested the fuses #4 & 5, select resistors, and tested the main output transistors. I found some varying results from what others said was acceptable parameters for him. Just looking for a direction to go or start from. I have the schemactics found on the forum. Thanks for the help so far already. Here's where I am... I have all input channels working through headphones but nothing out of the master/ woofer&tweeter(horn) whatever. From what I believe the op amps are not suspect if aux in works through headphones? I failed to test prior to digging in. Lol so that got me to testing the rest. Any help appreciated.
Thanks, Brett



The + or - below the measurements for the main output transistors, is how much the major differences were between my measurements and another KC-350 on here with apparent working speaker output.
...also first post so.....woohoo!

[The Dude]

I don't see anything that looks broken looking at your measurements. Did you try the insert jacks? See this thread.

https://music-electronics-forum.com/...ad.php?t=48339

It would also help if you post a schematic or schematic link.
I'd consider reassembling and trace signal through the amp.
It's also best not to remove parts unless you suspect them defective. You can damage traces or inadvertently reassemble something wrong and create new problems.

[Hensinger806]

Updated for your clarity

This it the thread I was using for comparison,

https://music-electronics-forum.com/...ad.php?t=43716

And his measurements....

I have read and posted on some forums recently,

---

trying to get an idea of how to test what to replace to give this thing new life. I followed a separate thread on here and tested the fuses #4 & 5, select resistors, and tested the main output transistors. I found some varying results from what others said was acceptable parameters for him. Just looking for a direction to go or start from. I have the schemactics found on the forum. Thanks for the help so far already. Here's where I am... I have all input channels working through headphones but nothing out of the master/ woofer&tweeter(horn) whatever. From what I believe the op amps are not suspect if aux in works through headphones? I failed to test prior to digging in. Lol so that got me to testing the rest. Any help appreciated.
Thanks, Brett



The + or - below the measurements for the main output transistors, is how much the major differences were between my measurements and another KC-350 on here with apparent working speaker output.
...also first post so.....woohoo!

[Hensinger806]

Duly noted, will reassemble and clean all jacks that seem to have an issue. Appreciated.

Schematic?

https://music-electronics-forum.com/...4&d=1485971084

[Hensinger806]

I thought I shouldn't see any dc voltage on the speaker outputs? Was I mistaken?

I don't see anything that looks
broken looking at your measurements. Did you try the insert jacks? See this thread.

https://music-electronics-forum.com/...ad.php?t=48339

It would also help if you post a schematic or schematic link.
I'd consider reassembling and trace signal through the amp.
It's also best not to remove parts unless you suspect them defective. You can damage traces or inadvertently reassemble something wrong and create new problems.

[The Dude]

No, there shouldn't be DC on the speaker output. Do you have DC and if so, how much?
Edit: A few millivolts is nothing to worry about.

[The Dude]

Also, now that we have a schematic, it looks like your tweeter is also bad/open. I had initially thought it might be a piezo tweeter, which would read open. The schematic shows it as an 8 ohm tweeter with a voice coil. I would first worry about getting the amp working. You can replace the tweeter later.

[Hensinger806]

Referring to my first picture of measurements I've taken. The woofer output tested , resting @5.0 to initial 7.8 mv DC, so probably okay. The tweeter however tested, initial @ 121.2 after a few minutes up to 132.6 mv DC slightly increasing. I figured the tweeter was fried and sourced a replacement, but over 120 mv DC seems more than a few on the tweeter output... Thanks for the update on piezo. I actually tried to message you directly when I got this account. I'm glad you're the one helping me. I'm a little outta my element, Donnie.... But I should be able to keep up. Thanks.

[The Dude]

The tweeter output is capacitor coupled via the crossover. The only way there could be DC there is if the cap (C18) was shorted, which I doubt. Even then, there would have to be DC coming out of the amp, which you don't have. It's certainly possibe the cap is shorted, so check it if suspect so. I'm more likely to believe it's just some "phantom"voltage you're reading since the tweeter is open and there's no current path.

[Hensinger806]

Okay i will check cap c18, reassemble and clean the 1/4 jacks then report back, Thanks again!

[Hensinger806]

Testing the cap from zeroing out my multi-meter and testing the leads watching it rise "infinitely". Reassembled everything and cleaned the 1/4th jacks and shes purring like a kitten with the exception of the tweeters. Thanks a lot The Dude. Sorry for an obvious fix I could have caught with better searching. Now looking into how to test ESR for my applications. Tweeter replacement on the way!