Roland KC 60 amp

[bsco]

I had this or should I say have this unit to repair....it was blowing the two 100mA pc mount fuses for the low voltage power supply....so something was drawing too much current...checked the regulators and they were ok...did resistance checks on the 15V rails and couldn't find any low resistance....


removed the pc board fuses and soldered two 12V brake lights in their place....power light came on but was very dim and the voltage dropping resistors for the positive 15V rail was overheating...but couldn't find the source....
de-soldered every pre-amp chip's pins from the 15V rails and the supply voltages came back and the power light glowed nice and bright....then I soldered the supply pins on the ic's one ic at a time and checked the 15V rails...I found one ic chip that was causing the problem....it was a NJM4556AL.......great learning experience......all I have to do now is find a replacement.....Hopefully this helps somebody out in the future...

[g1]

Did you check if it was getting hot when the bulbs were in?

[bsco]

What was happening is that the voltage dropping resistor for the positive 15V rail was getting hot even with the blubs in....when I located the defective chip and removed it, I did a resistance check and the output, Pin 1 had a low reading to both pin 4 and pin 8 but not a dead short....so it was drawing enough current to blow the 100mA fuses...but not enough to light the blubs....and I didn't have other blubs on hand to test it with....as soon as I dis-connected the bad chip, the 15 volt rails cane back up....
Cheers

[g1]

What I was wondering was if you would have been able to detect the bad chip by finger checking for heat while using the bulbs. If it was enough to blow fuses I would think it would have been heating up.
Just another method to consider for next time.

[bsco]

What I was wondering was if you would have been able to detect the bad chip by finger checking for heat while using the bulbs. If it was enough to blow fuses I would think it would have been heating up.
Just another method to consider for next time.

I tried that but couldn't find anything overheating except for the series voltage dropping resistor feeding the +15V regulator.....and both 100mA fuses were blown....so I figured
it had to be a low resistance on some pins on at least one chip...and maybe more than one...
so de-soldering all the + and - supply pins on the chips was the only idea I could think of.....and I had to use the blubs as it was blowing these low current fuses....but thank you
for replying to the post....and you are right....you would think the bad component would
have been heating up....
Cheers,
Bernie

[g1]

The other possibility is it was a very low resistance short when running. As someone pointed out recently, the lower resistance the short, the less heat created. So the finger checking method is not fool-proof, but can be helpful.

[Jazz P Bass]

It would be interesting to see the resistance readings of the failed ic.
+V pin to -.
+ to input & output pins.

[bsco]

It would be interesting to see the resistance readings of the failed ic.
+V pin to -.
+ to input & output pins.

Actually I still have the chip....I'll check the readings and let you know what they are....

[Jazz P Bass]

The pinout & internal diagram are on page 1.
http://www.njr.com/semicon/PDF/NJM4556A_E.pdf

[bsco]

Here you go Jazz....-V to + V is 52 ohms......
Pin 1 to -V is 12.3 ohms
Pin 1 to +V is 59 ohms
Pin 1 is the output of amp 1
all other pins check ok.....
Cheers

[Jazz P Bass]

Well, there you have it.
+V is shorted to -V .
Pin 1 is shorted to -V.

[bsco]

Well, there you have it.
+V is shorted to -V .
Pin 1 is shorted to -V.

Pin 1 was also shorted V+

[J M Fahey]

Originally Posted by Jazz P Bass
Well, there you have it.
+V is shorted to -V .
Pin 1 is shorted to -V.

Pin 1 was also shorted V+

Well, of course.

If A is shorted to B and B is shorted to C, A is also shorted to C
And so on.

[Jazz P Bass]

Pin 1 was also shorted V+

Well sure.
But it was the -V to pin 1 that was less resistance.

[bsco]

Well sure.
But it was the -V to pin 1 that was less resistance.

...
You are totally correct Jazz.........funny how the voltage dropping resistor for the +15V line was the one that was overheating...the other resistor in the -15V line was not overheating.......anyway.....that's it I guess......I got the part ordered..
hopefully I'll have it in a week or two....
Cheers.....