Debugging MXR 6band EQ - help appreciated

[lopsided]

Hello,

i found this forum about a month ago,  build the beginner project and then some other boosters and fuzz pedal. This forum was a big help for me. It also helped me understand my other stomp-boxes. I have an old MXR 6 band equalizer, which isn't working properly and i thought i use my newly gained knowledge, to find out what's wrong and maybe try to repair. I have made a hypothesis about what's wrong and I hope somebody could help me confirm or falsify it.

Effect type: MXR 6 BAND EQUALIZER, the old, blue, (probably) eighties one, without a foot-switch. I found a quite accurate schematics here: http://freestompboxes.org/viewtopic.php?f=19&t=3988 , except mine has three TL022CPs and one CA1458G (at the input/output position labeled U2A and U2b)
You can find some photos of the circuit here:
http://i1027.photobucket.com/albums/y334/lopsided2/mxr6EQ01.jpg
http://i1027.photobucket.com/albums/y334/lopsided2/mxr6EQ02.jpg
http://i1027.photobucket.com/albums/y334/lopsided2/mxr6EQ02.jpg
http://i1027.photobucket.com/albums/y334/lopsided2/mxr6EQ04.jpg

Effect history and problem: I bought it secondhand a couple years ago it worked fine for a year but then the 1,6 kHz slider stopped having any effect on sound.


What i have tried.
After i have tried cleaning the mechanical parts of the slider, I have measured the resistance of the sliders, they all give the same resistance from 0 - 3 kΩ, so there shouldn't be a problem with the slider pot.

I have checked the circuit, and although it doesn't look as good as new, i haven't found any shorts or broken connections.

I have measured voltage on some points and this is what i got:


So there seems to be voltage going through the not working part. To be honest I don't understand the circuit to the extent that i could tell if the values signify something broken, but what I have noticed that voltage between the R19 resistor and the 7th pin of the U4B is approximately 2 Volts higher then on the parts dealing with all other frequencies. It has 7,18 V all others have 5,29.

Could this be the crucial symptom? Does it mean that the integrated circuit U4 is broken and gives to much voltage? Do ICs brake like this? Or should I look for the problem elsewhere?

I would really appreciated any thought anybody could give me on this problem. Thank You.

l_s

[Fender3D]

Swap that loosy b***ard with a working IC 
op amps leak DC voltage when broken usually 
... then push all central controls up and play like Van Halen 

[familyortiz]

1) Double check the supply voltages at your chips.
2) For U4-B, pins 5,6 and 7 should be at the same voltage, so I would concur with the previous post to change that chip out if all else is ok.

[lopsided]

Guys, thaks a lot for your advice! Today I have switched the opamp for a 8pin socket. I didn't have the original opamp but I've tried a TL072 and the voltage dropped down to 5,3 and the equalizer works again.
So it was a borken opams fault.

Thanks again.
l_s

[MZANNETOS]

Hi
I have come across one of these old mxr 6 band blue box
It seems though to have a problem and when I opened I noticed a few shreded wires.

Can someone provide the wiring the way it should be, I am not sure the diagram m-109 is the same as the old Blue one.
And I am not that great with reading schematics.
Basically I need the layout of the 4 wires on the circuit board and where they go in reference to the input, output and battery connector
Thanks