Behringer KX1200 loud hum only repair help please!!

[fausto]

I have a Behringer KX1200 amp that I would like to repair. When I plug it in it makes a loud humming noise even with nothing plugged into the inputs and all the volumes are turned all the way down. I took it apart and unplugged the wire coming from the input jacks board to rule out a short there but still just a loud hum. Can you give me some advice on how to repair this thing.

Well now I went a step further and took the board with all the volume controls out and I found a burnt looking thing which you can see in the picture. It is located between the "FX/MON" and the "volume 2" volume controls. I wonder if I could just remove this thing and just lose fuction of 1 of the inputs which is fine for me or if i could purchase a new volume control board?

[Jazz P Bass]

Not having the amplifier or the schematic, I am only guessing
That burnt item appears to be a TL074 jfet opamp.
Link: http://www.st.com/internet/com/TECHN...CD00000489.pdf
The scorch marks look they are right at the ic's power pins.
Pin 4 (+Vdc) & 11 (-Vdc).
Not knowing what the ic does (what circuit it is in), I can't tell what will happen if you remove it.

[fausto]

Thanks

Not having the amplifier or the schematic, I am only guessing
That burnt item appears to be a TL074 jfet opamp.
Link: http://www.st.com/internet/com/TECHN...CD00000489.pdf
The scorch marks look they are right at the ic's power pins.
Pin 4 (+Vdc) & 11 (-Vdc).
Not knowing what the ic does (what circuit it is in), I can't tell what will happen if you remove it.

First off I really appreciate your help or anyone else that jumps in to help. I don't have a schematic & from what I hear Behringer makes it as hard as possible to find one. That said maybe I should be think of replacing that IC instead of removing it. It may be hard to see in my picture but the numbers on that IC are :

074C
93N119

I tried to google these #'s in hope of finding it but had no luck. Is the one shown in the link you attached something that would work for me? If so where can I buy it?

[Jazz P Bass]

The ic appears to be a TL074C.
Surface mount.
Are you setup to do surface mount work?
If not, take it in to a tech.
There may very well be other issues.
The IC may have been damaged by am external source.
It may have simply failed.
The puffs of soot (ala Challenger) may be a tell.

[fausto]

The ic appears to be a TL074C.
Surface mount.
Are you setup to do surface mount work?
If not, take it in to a tech.
There may very well be other issues.
The IC may have been damaged by am external source.
It may have simply failed.
The puffs of soot (ala Challenger) may be a tell.

I have never done surface mount work before but i am thinking that i can carefully solder a new one in. Based on your comment it sounds like i may be getting in over my head. As far as other issues i am hoping that the reason it failed is from an external source that someone plugged into it but who knows if after all this my new IC will just burn out again.

[Jazz P Bass]

It is not so much the soldering in, as the soldering out (did I just print that?)
Err, the desoldering of the bad ic.
Your chances of ruining the board are pretty good.
As it is, there is a bad ic.
The circuit itself will have to be inspected & tested.
The power rails, the + & - VDC, will have to be checked.
All without a schematic.
Call a tech.

[km6xz]

That chip, IC7 is used for each of the 4 channels, with the channel gain controls in the feedback look of each of the 4 opamps in that chip, so no sound would pass any channel.

If you have a soldering iron, try to remove the old chip. One way to do it is to cut all the leads with a Dremel high speed cut off wheel. Cut right at where the lead goes into the plastic chip housing. That will leave the leads still soldered to the pads but easy to heat one at a time to loosen them and flick them off. Now, without the chip there, connect it all again and see if there is hum. There is a good chance that it won't hum with that chip gone because it probably shorted and pulling one of the low voltage rails down. If it does not hum. Remove that board again and get a new TL074 for $0.25 and install it. Go to an electronics store and get "Solder Wick" to remove the old solder from the pads. If your local electronics store has "Chip-Quik" you can use that very low temperature solder to remove the old chip very quickly and easily. There are Youtube videos showing how to use it.
Solder in the new chip making sure there is no solder bridge between pins and the chip is in the exact same orientation as your photo. Try to clean the burned board area. Once you get used to working with tiny surface mount, it is really easier than larger through hole pc board work.
Either all this or take it to a tech.

[fausto]

I just can't say thank you enough to all for taking the time to help me. I will follow your advice and let you know how this turns out.

[km6xz]

After looking at your photo blown up a bit in PhotoShop CS5.5 I think the board is fine, just some dark smoky soot from the chip plastic case burning.

For future reference here is the schematic for the KX1200:
kx1200.pdf

Here is one of many videos on using low temperature solder, "Chip-Quik" for removing surface mount parts without damage to the part or pc board: Using ChipQuik to Desolder Surface Mount Components - YouTube

A well stocked electronics supply store should have both the Chip-Quik kit and the TL074. Search for other videos on soldering techniques for installing the new IC.
The Chip-Quik comes with about 5 feet of the low temperature solder which is enough to remove dozens of ICs or difficult to remove through-hole parts from double sided circuit boards like pots, jacks and connectors. I have a $1200 desoldering station but use Chip-Quik most of the time because it is much faster, safer for the pc boards. Any tech working on modern electronics ought to use it.

[fausto]

After looking at your photo blown up a bit in PhotoShop CS5.5 I think the board is fine, just some dark smoky soot from the chip plastic case burning.

For future reference here is the schematic for the KX1200:
kx1200.pdf

Here is one of many videos on using low temperature solder, "Chip-Quik" for removing surface mount parts without damage to the part or pc board: Using ChipQuik to Desolder Surface Mount Components - YouTube

A well stocked electronics supply store should have both the Chip-Quik kit and the TL074. Search for other videos on soldering techniques for installing the new IC.
The Chip-Quik comes with about 5 feet of the low temperature solder which is enough to remove dozens of ICs or difficult to remove through-hole parts from double sided circuit boards like pots, jacks and connectors. I have a $1200 desoldering station but use Chip-Quik most of the time because it is much faster, safer for the pc boards. Any tech working on modern electronics ought to use it.

Thank you for the schematic and youtube video link. I ordered a few of those TL074 and the Chip-Quik which should be here in a few days so i'll let you know how things work out.

[fausto]

FAILED!!!

Well despite my attemped to carefully replace the surface mounted IC I did fail and cause damage to the board. That said do you know where I could purchase that entire board? Or is it possible to replace that board with another one even if it's not a match? Of course an exact match would be ideal but the truth is if I could just plug in to one or two inputs that would be better than nothing.

[km6xz]

How, or in what way was the board damaged? It probably is salvageable and not as bad as it looks. Can you take a photo of the damaged area? Was it in removing the old part or installing the new one? If the old one is removed OK, turn it on to see if the hum disappeared. That would verify the diagnosis.
Worst case would be to take the board and the new chip to a tech and pay him to do nothing but put the part in and verify all the connections are made. That ought to be cheap, 15-30 minutes time maximum.

[fausto]

I would say that I applied to much heat while installing the new chip which melted parts of the board so I then tried to add some pieces of wire in an attempt to make the connections. I did turn it on and i get no hum anymore but I get nothing at all out of it at this point. I spoke to a few tech shops in my area and there minimum fee is $95 and they felt other things on the board may be damaged. so i felt that buying the entire board may be better option but look at the photo and let me Know what you think.

[km6xz]

That looks a bit messy but not hopeless. Is the wire added from pin 7,(1st from the left on top row) going to C68, or just a close by via?
Time to get a volt meter out and plug it in and figure why all channels are down. Those are 4 independent amps on that chip so if there was a wiring error, one channel would be expected to be down. The only common section is the power supply leads to pin 4 and 11. Measure those to ground and see you are getting supply voltage minus and plus respectively.

$95 for a minor clean up? Humm seems high to me. This is the sort of thing I usually do for free for regular customers because it is already disassembled and a minor problem.
Take the measurements and report back what you get on each pin compared to ground. The clues will be on the pins.