Help debugging old MXR 6 Band EQ

[Eddie]

I need your help.
I have got one old MXR 6 band EQ. It worked the first hour I used it.
After that it distorts awfully, sometimes i hear oszillation at high boost levels.
I donßt know where the fault is.
I looked for bad solder joints, knocked on every part slightly while playing, cleaned the jacks, checked for fresh battery---with no luck.
One of the OP amps was changed later (TL1458), the vb voltage was nine volts on all OP amps.

Does someone own this 6 Band EQ ? Could you give me some voltage readings?
It would be a great help. Otherwise I would change every part on that pcb.

Pic: http://forum.musikding.de/gallery/displayimage.php?album=lastup&cat=0&pos=0


Thanks for your help!!

Eddie

[George Giblet]

Measure the voltages on the +in, -in and out terminals of *all* the opamps.  All voltages should sit at about 4.5V.  If not there is a problem.  Now be warned just because one opamp is out doesn't mean that particular opamps is faulty.  The circuit is DC coupled so the problem could be further up the chain towards the input.  That's why you need to check all of them.  When you find that point check for faulty tantalums and opamps.

[Eddie]

Ok, Imeasured the pinouts:

OPamp 1 (from the left):MC1458CP

pin4 to 8:9,41 V
to7:8,87V
t06:8,87
to5:8,83
to3:5,51
to2:8,85
t0 1:8,85

OP AMP 2:TL022CP
pin4 to:
8:9,41
7:8,86
6:8,86
5:8,82
3:8,82
2:8,86
1:8,86

OP AMP 3:UA1458TC:

pin4 to:
8:9,41
7:8,83
6:8,84
5:8,35
3:8,82
2:8,82
1:8,82


OP AMP 4:tl022cp:

pin4 to:
8:9,40
7:8,86
6:8,86
5:8,82
3:8,82
2:8,86
1:8,86

So is OPamp 1 the murderer?

Thanks!

Eddie

[George Giblet]

I think IC1 is the one.   Not because the '1' in IC1 is before the others,  as the opamp  wiring order is unknown, but because I can see IC1 pin1, 2, 3 are wired as a buffer.  If you have 5.5V on pin 3 you should be seeing 5.5V on pins 1 and 2 if the opamp was OK.

Try re-touching the soldering first in case the solder has a crack.

[Eddie]

Thanks George!


I resoldered it first--no luck.
Then I used a Tl072--it worked!!!
I get different voltage readings and it sounds like a EQ again.
Do I have to worry that I used the TL072 instead of the mc1458?

I think no, despite the higher power consumption.

Thanks for your help!


Eddie

[George Giblet]

>Do I have to worry that I used the TL072 instead of the mc1458?

IIRC they aren't that much different.  The TL072 is a better device - lower noise.