Mackie cfx12 mixer -15v rail pulled down.

[nosaj]

I have another Mackie cfx12 mixer where the -15v rail is being pulled down to about 1.5v.
On the bottom of the mixer there are ribbon cables that tie the channels together i unplugged them and now voltage is up to 3v.
after about a 1 minute start getting dc on main out about .5v
What is a recommended approach on tackling a mixer.
Thanks,,
nosaj

[g1]

Do most of the boards have connectors? If so, you should be able to pull them all and see if the -15V rail comes up.
This is assuming you have not already verified that the -15V supply itself is good.
Once you have verified it is good, you should be able to connect 1 board at a time to see which one is pulling down the -15V rail.

[The Dude]

It looks like there is a main plug (J2) from power supply out to mixer. If you unplug that, does your supply return to normal? Most mixers have small value resistors from power supply to op amp supply pins so that if a single op amp fails, it doesn't take down the entire board. Of course, there could be a short in the mixer, but my bet is that there is something wrong with the supply itself and most likely a regulator. A bit of speculation, of course.

Edit: Scratch that last part. The schematic doesn't show any resistors in line with the op amp supply pins.

[nosaj]

Power supply has been rebuilt. disconnected from main board the power supply shows correct voltages. i have already unplugged any board jumpers i can , next move is removing all knobs an nuts so i can get at the other side. (also unplugged the digital effects module.
nosaj

[g1]

From the overview, you should be able to connect each board to see which one it is that is pulling down the rail. I would start with connecting the master, then FX, then slave.

[nosaj]

Right now I'm operating Master, Power, and AC Power.
Should I be looking at cutting traces or can i go to the farthest point away from power checking -15v rail(im guessing a shorted device will suck all the power) until I get to where I'm measuring the -3v?
Power comes in at connector by the eq faders.
Thanks,
nosaj

[The Dude]

Look for an IC getting hot. The finger test.

[nosaj]

I'll give it a spritz with isopropyl alcohol to see.
nosaj

[nosaj]

Ok alcohol shows u76 and feeling it gets rather hot. Thats a real neat trick for finding a shorted component.
It seems u76 relates to the tape in tape out. It looks like i can remove it and checkout the rest of the board for issues. Will follow up.
nosaj
page 7 of the main schematic

[nosaj]

Also found that c514 gets really hot removed it and seems to have settled things down on the voltage. Gonna see if i have a replacement for c514 and if i do put u76 back and see what happens.
nosaj

[g1]

It seems u76 relates to the tape in tape out. It looks like i can remove it and checkout the rest of the board for issues.

With IC's, it is often easier to just de-solder the (+/-) supply pins. Then if you find the problem is elsewhere, you can just resolder them, which saves the whole removal and re-install process.
However, with double-sided board it can be tricky, and you may need to verify with resistance check that the pin is no longer making contact with the trace.

[nosaj]

With IC's, it is often easier to just de-solder the (+/-) supply pins. Then if you find the problem is elsewhere, you can just resolder them, which saves the whole removal and re-install process.
However, with double-sided board it can be tricky, and you may need to verify with resistance check that the pin is no longer making contact with the trace.

It is SMD/SMT my first foray into this type. the cap was shorted i think 0603 size smaller than a grain of rice. On hand I do not have another .1uf but I do have some .22 uf how would this affect the power rail for testing the IC?
Thanks,
nosaj

[g1]

It is SMD/SMT my first foray into this type. the cap was shorted i think 0603 size smaller than a grain of rice. On hand I do not have another .1uf but I do have some .22 uf how would this affect the power rail for testing the IC?

With SMD you can sometimes lift the supply legs with a pin (& solder iron). It may or may not save time & effort depending on circumstance. With how delicate some traces are, it can sometimes be worthwhile.

If you look on pg.7 in the A5 sector, it shows the power rail setup for the IC's. C514 & 515 are just local decoupling for U76. Running with either removed is fine for testing.

[nosaj]

Everything hooked backup Looks like just c514 the decoupling cap was the problem. hooked up headphones tried all channels .Sounds good.
Just have to get a decoupling cap and button it up.
Now the decoupling cap in this situation what exactly does it do filter noise?

Thanks,
jason

[nosaj]

Double checked over onhand stock best I can do is .22 or wait till parts come in.
nosaj