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Vintage threads from the first ten years

Roland KC 500 popping fuses

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8/27/2005 12:31 PM
DM	Roland KC 500 popping fuses Hey guys, yes I know its not tube, but hoping someone can help me out here. The thing pops fuses as soon as the power is turned on, right now im running it off a 6VAC wall wart trying to find the problem. Is this enough voltage to be able to do any troubleshooting? The voltage drops to zero at the rectifier, from there on im having to follow it in MA.

8/27/2005 8:51 PM

quote:
"The voltage drops to zero at the rectifier,"

And what does that suggest to you?

8/28/2005 9:28 AM
DM	Well lol, it suggests a bad rect,, but only running it on 6VAC only gives me 3.8VAC on the secondaries of the PT, 3.8VAC at the input legs of the rect,,, is this enough voltage to actually know if the rec is bad?

8/28/2005 9:39 AM

DM, the rectifier may not be bad at all. Have you taken ohmeter readings front to back and back to front across the diodes? This is Diodes 101 stuff.

What R G was gently suggesting to you is that if the voltage drops to zero after the rectifier then something on the power rail after the rectifier must be shorted to ground!

Either the diodes are blown open so you don't get any voltage through them or there's a short across the diode output. There's really no other possibilities.

I'd unsolder or otherwise disconnect the output of the rectifier diodes and measure if the voltage is ok when the actual amp circuitry is disconnected. That will confirm if the power supply is ok.

If you have something shorted across the power rail(s) (you may have two rails in a SS circuit, one side positive and one negative) it could be any diode, IC or transistor in the amp but the most likely culprits are the output devices.

If you're not already comfortable with this stuff DM it's gonna take a LOT of typing to walk you through the repair!

---Wild Bill

8/28/2005 12:23 PM
DM	ok last question, Ive ohm'd put about everything on this board, only things I cant get a reading on appear to be some sort of cap ( stands up on board looks like silver covered in glass) Can u guy tell me the name of this device so i can research it and see if its bad or just outta range of my meter.

8/28/2005 12:58 PM

DM, it could be a diode, a cap, a surge suppressor, or almost anything.

If you think that measuring everything in sight will help you solve the problem you are doing yourself a big disservice. Even if you find the bad part and replace it the new one will probably blow up too! Besides, all those connections between parts will screw up your meter readings. Unless you're willing to unsolder each part from the circuit before you measure it! If you do that then you've got far mor patience than I do.

Amps are not like mechanical devices, in that when your car dies it's usually because an engine part wore out. Electronic parts don't normally wear, except for tubes getting old and electrolytic caps drying out. If you had a poor/cheap design where the resistors in some spots were not rated high enough for the power involved they can break down over the years but in general with modern stuff parts blow up because one part did fail and it took out some others.

If you don't understand how every part relates to the others in the circuit you can really spin your wheels and end up saying a lot of bad words!

In every town there's some auto chain repair shop where one real mechanic "supervises" maybe 25 uncertified "technicians". Often when you take your car in they just look for something that seems broken and keep replacing parts until the car finally works right. Then they just add up all the parts and give you the bill!

I learned the hard way about such places years ago. Now I take my car to a guy who has a deep understanding of how things all work. He quickly zeroes in on the problem and replaces only what's really necessary. He saves me money and I've never had to bring the car back in after a job.

Not meaning to be insulting or patronizing but you seem to be just getting in to working on amps. The faster you soak up your theory then the faster you can fix things. It's really hard to help someone when he has no real focus...

I'd hit some books first. It'll really pay off in the long run.

---Wild Bill

8/29/2005 6:18 PM
Enzo	Your transformer seems to be working, so if it blows fuses, it is most likely one of two things. Either the power output transistoras are shorted, or the power supply is shorted. FInd the power transistors and check each of them for shorts between their legs. Then check the power supply rectifiers. Also check for shorted main filter caps.

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