Roland KC 300 problem

[jarcorbo]

I just purchased a Roland KC 300 that is not working. It powers up fine, but no sound. There is a clearly audible low frequency hum upon power up that fades out after a couple seconds, but after that no sound. I checked to make sure the speaker terminals were hooked up properly, and found one of the wires to the tweeter was disconnected. Reconnected it, powered up, and still the same problem.

My knowledge of electronics is very minimal, but I'm not shy about poking around. I bought this thing from Goodwill for $40 and hoped it would be a simple solution, or at worst take it to a repair shop and still come out with a good deal.Any guidance/advice will be much appreciated. Thanks!
-Jacob

[oc disorder]

Try plugging a signal in to the stereo link in , this should bypass the pre-amp or at least some of it.
I suspect they can be used as trolleys by resting keyboards on top as the ones I saw are on wheels.
I have seen some where the knobs have been pushed in from the top which has forced the pots (volume controls) to start to disintergrate.
Its possible to remove the board and carefully squeeze the pots back together
and bend the little tabs firmly around the pot body.

A schematic may be helpful here.

If you have no soldering gear or multimeter and if its not the above I'd take it in for a quote.

[cjlectronics]

These amps are notorious for the speakers going bad. Ohm the speaker and the horn. Suspect the speakers before anything else.

CJ

[J M Fahey]

Does that hum fade smoothly or get cut off abruptly?
You might have a blown power stage and a DC protection relay doing its job after a couple seconds.

[jarcorbo]

Thanks for all the quick and helpful replies!

I tried the stereo link in, but still no luck. I have a soldering iron, no multimeter,but it's about time I get one anyway.

@ J M Fahey, The sound fades out smoothly. I can also hear what I can only describe as ambient noise coming through after the fade out. If I do have a blown power stage, what would be involved in the repair?

[J M Fahey]

It'ss guessing game here, having so little information, but if the unit has a chipamp (Japanese seem to love them) and most have some kind of mute/thermal protection, maybe it's overheating (consistent with the original hum you hear) and self muting.
Check whether it has a power chip (a chocolate bar attached to a heatsink or the chassis itself) with 9 to 18 legs, or discrete transistors (plastic stamp-sized candies with 3 legs each), also attached to heat sinks.
Apply your trusted, NASA calibrated Finger-O-Matic Thermal Probe to the plastic case to check whether it quickly overheats.
A couple sharp, readable pictures will help.
The exact model too, maybe there's a schematic somewhere.
Good luck.

[jarcorbo]

Update:
I located the transistors attached to the heatsink (which are mounted directly on the chassis), but felt no heat at power up. However, on the main circuit board, There are two rather large white rectangular resistors that got very hot, so hot that I could not keep my finger on them. One of them showed signs of burning, so I'm wondering if that's my culprit.

I took the chassis out to take clear pictures, but my camera battery ran out of juice, so I'll post pictures tomorrow.

Model is: Roland KC-300. Looked online for schematics to no avail. I'll try contacting Roland...

[oc disorder]

When you get your multimeter maybe check the + and - 15 rails to the preamp.
The two resistors getting hot may be part of a zener supply.
If it is, one end of the resistor should connect only to one diode.
Well its still guess work at this stage.
Another way to try to isolate the fault between "pre" and "pwr" would be to take the line out (forget what they call it) and feed it to an external working amp.
That way you can see if the basic mixer/pre is working .

[Dawg28269]

I jst got mine KC300 fixed for the same issue and it was the low voltage power supply circuit. Cost me $82 to get it fixed......

[jarcorbo]

Thanks for the tip!! I took mine to a repair guy last week, and he told me it was most likely the same issue. Hopefully mine will cost around the same..