Roland KC 60 amp

[g1]

...
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

I thought that to be inconsistent with the readings too, but those are static readings, and the resistances when the bad chip was powered may have been quite different.

[Jazz P Bass]

BSCO, do you have the KC60 schematic?
If so, can you post it?

[bsco]

Back again.....and so is this amp.......seems that the same problem has occured....will have to take it apart today to see what is going on here...
Cheers,
Bernie

[drewl]

Aside from a bad chip that may be pulling too much current, look for any corrosion on the PC boards.

You may have a bad trace on one of the boards.

I had this problem with one of the Vox AC100 CPH's.

Corroded shorted trace pulling down the low voltage supply.

[drewl]

Aside from a bad chip that may be pulling too much current, look for any corrosion on the PC boards.

You may have a bad trace on one of the boards.

I had this problem with one of the Vox AC100 CPH's.

Corroded shorted trace pulling down the low voltage supply.

[bsco]

Ok. Thanks drewl.....I haven't opened it up yet...I am going to do that later today...From what I can remember from the last time I had this opened up...I didn't see any corrosion but I will check again....I am thinking a component connected to the chip might be intermittment....as it did work for awhile.....anyway I'll check it out and post my findings.....
Cheers,
Bernie

[bsco]

Ok drewl......I checked it out......The pc mount fuse for the +15V line was blown...the one for the -15V line had a resistance of 9 ohms....I removed both fuses and re-checked them....same thing.....When this was in last time, I had to replace IC4....Pins 1, 4 and 8 were shorted to each other 50 ohms or less depending on what pins you measured...I replaced the chip. ran if for over an hour and then sent it back.....The owner had used it for only a few minutes???? not quite sure how long exactly but that is what I was told.....SO I have IC4 removed from the ckt and I am running the amp with this out....This IC supplies the head phones, sub out and line out...I checked the ic chip for shorts but there are none.....And I checked the readings with a brand new chip........same thing......I have it running now for about 40 minutes...I am going to let it run for another hour or so and if it works ok, I am going to test the components tied to IC4...if they check out then I will install a new IC and re-test it.....Also, I should mention that I have two 12 auto brake lights connected up instead of these fuses to see if something is going to happen....I only have two of these fuses left and don't want to risk blowing them....I might try to see if I can find a couple of 12V blubs that are rated for a couple hundred MA.......if you have any ideas, feel free to comment.......
Cheers,
Bernie

[bsco]

Just noticed something.....Pin 1 of IC4 is connected to the headphone out jack as well as the 15V rails.....I wonder if the jack could be an issue....I'll have to check this out as PIN1 on the original IC was shorted to pins 4 and 8....

[Jazz P Bass]

Not having the schematic, I checked out the other KC amps.

The KC150 has +15Vdc On the headphone jack.(to -15 through a 47K R)
Breaking it causes the amp to mute.

That jack would have to be really mangled to have the two terminals touching each other..

kc-150 Hd Ph.pdf

[bsco]

I just sized that up when I was downstairs....the circuit you posted is the same....I installed a new IC4 and I have the amp running with all the controls wide open and a speaker connected......The 15V rails are normal.....nothing is overheating...I am going to leave it on for another hour or two before I shut everyrthing down for tonight.....then go back at it again tomorrow morning...I was thinking of replacing those brake light blubs with another blub type that would light with less current....like a couple hundred ma.....a blub from an old expired Xmas light set maybe would work....(20 or 35 light set)....anyway I'll try something tomorrow......
Thanks for the replay Jazz P......
Cheers

[bsco]

Did some more checking.....connected two meters where these 100ma fuses go and checked the current....there is a slight difference of about 10 ma between the two 15V rails......at idle with all the controls wide open and everything connected, current draw for the +15V rail is approx 60ma and the -15V rail is sitting at around 50ma.....When touching the regulators they do feel kind of hot to the touch after you leave your finger there for 10 or 20 sec......I am beginning to think that is normal....but that doesn't explain why the fuses were bad again....unless there was something wrong with them to begin with...I am going to run a guitar through it after for a bit to see what the current draw is like......I'll post the results.....
Cheers,
Bernie

[Jazz P Bass]

I would also try the headphone jack into a load.

[bsco]

Ok. Ran a guitar through the amp....even at close to max volume....current draw varied by only a milliamp or two....so the current draw is well below the 100ma rating..........maybe the fuses were inferior.....I am gong to install a couple of glass fuses..... I don't particularly like these round pc fuses...I'll hook up a set of headphones tomorrow Jazz and let you know what I find.....
Cheers,
Bernie

[g1]

I would also try the headphone jack into a load.

And also a mono plug in that headphone jack just in case...

[bsco]

Hey G1....How is everything?? Glad to hear from you....I'll try this stuff later this morning...I'll let you know what happens....
Cheers......