Roland KC-350

[lowell]

This came in with no power. Both 500ma secondary fuses were blown. First thing I notice was a handful of the ceramic caps from the low v supplies had literally exploded. Replaced the fuses and inserted my 40watt bulb, the 100watt allowed the fuses to blow again.

Through chunking sections of the amp I narrowed it down to the low v supplies being in over-current.

Found and replaced the following bad components.

Q1 and 2 (constant voltage sources?) in series with low v supply...both were shorted.
There was still no low v so found both base-zeners to be shorted, therefore the transistors were not ON.
AT THIS POINT I HAVE SOME VOLTAGE - However -15v and +5.6v.
Disconnected CN12 to preamp and the voltages were now both good.

Now I'm troubleshooting what's pulling the v+ down. I've disconnected many of the ceramic caps from +15v to ground. Still low.

2 QUESTIONS:

1) What does NIU mean? It says "inside area NIU" on top of pg. 29. That's the ACTUAL pdf pg #. I cannot find C300 anywhere.

2) How would y'all proceed? Do I need to disconnect every damn ceramic in here or is there a better way? Place a finger on each IC and see if any get hot?

Schematic is too big to upload

[lowell]

I shoulda heeded my own advice before posting. A bunch of the NJM4580 ICs are indeed very hot. Got some replacing to do. May as well do 'em all after all the stress they've been thru with exploding caps.

Never repaired these types of ICs before. These are correct right?
http://www.mouser.com/ProductDetail/...kNBImEhDaiz2QE

[DrGonz78]

Never repaired these types of ICs before. These are correct right?
http://www.mouser.com/ProductDetail/...kNBImEhDaiz2QE

Yup those are the right ones, SIP-8 packages. I am pretty sure the SIP-8 IC's are in all of the KC100-KC550 Roland amps. Don't see them that much in amps but do see them occasionally in foot pedals.

[lowell]

Thanks Doc. To add to my first 2 posts. I'd think it wise to toss a 20volt or thereabouts zener on each of the low v supply rails. This way if the voltage regulators blow the rails cannot exceed the zener voltage.

[Enzo]

NOT IN USE The 350 and 550 use the same boards and circuits, but certain parts on the 550 are not used in the 350. I could be wrong, but I might expect the circuit board to have traces and holes for the parts inside the dashed lines, but those places are empty of parts.

[lowell]

Ok cool... i thought Not in Unit. Yours is cleaner.

[DrGonz78]

Do all of the KC100-550 series amps use SIP-8 type opamps?

[Enzo]

Not in unit? Sure, equally plausible.