MXR 6-band EQ clips...fixable?

[bobbletrox]

I have a vintage MXR 6-band EQ that I use to add some kick to my p-bass...except it's begun to clip/distort.  It's just a recent thing too as far as I can tell because it's been fine in the past.  There are no electro caps in it to have gone bad (they're all tants), so the only thing I can think of is the opamps.  The opamps are 2 x TI SN72L022P and 2 other unmarked opamps.

Does anyone know what could be causing the clipping in this thing, or what the opamps are?

Signed,
Confused.

[Paul Perry (Frostwave)]

I would be very suprised indeed if it was an op amp.
Maybe a resistor has changed value & shifted bias or gain?

[Prive]

I'd like to see some schem.

Saludos, Marcelo.

[spongebob]

Is the amount of distortion independent of the input volume? I also don't think an opamp would go bad with results like this, but you could always replace the opamps with modern low noise ones (NE5534, TL072, etc.). Did you check for bad/broken solder joints?

[bobbletrox]

Is the amount of distortion independent of the input volume?

Yes.

I'd like to change the opamps to newer low noise ones, but it beats me what the original opamps are in the first place.  Does anyone have a schematic for this thing?

[Mark Hammer]

Those are the same opamps used in a lot of Maestro units from that era.  As near as I can tell, they are likely the same thing as a TL022 which is simply a dual lowpower op-amp with a normal 4558 pinout.

I used to have one of those little blue beasties.  Before you go tearing anything apart bear in mind that 6 slider pots on the floor facing upwards provide an open door to dust and dirt.  I would clean those things properly before making any assumptions about anything electronically wrong.

[Rob Strand]

To state the obvious:  low battery, dirty input/out sockets.

[Mark Hammer]

Incidentally, the earliest ones I had seen used 4558's tghroughout, as well as those spiffy "white-box" Philips plastic caps.

[Ammscray]

All of the ones I've seen and owned had 2 TLO22CP's and 2 1458's...it's possible it's an IC but unlikely...change 'em all to 5532's for mega noise reduction but it'll eat batteries a little quicker...but be careful it's a double-sided board! I think that's why there's no schemes, because it would be hell to reverse-engineer...then there's the little obstacle of finding the values of the inductors...

I still have and use my original I bought in 76 and it's still one of the best pedals to use to kick the front of an old 4 input Marshall top...awesome pedal, noise or not one of the secrets to the Boston guitar sound and EVH used one with his rig on the first LP as well...

[Boofhead]

values of the inductors

The one's I've seen used Gyrators instead of inductors - that's why there's so many opamps.  Is there an inductor version as well?

I started tracing this from a pic at one stage but there were a few details which I couldn't work out from the pic.  It's pretty much a stock standard gyrator based EQ.

[bobbletrox]

Thanks for the input guys.  The battery measured at 9.26v, I cleaned the sliders, I measured all the resistors and they check out.  This really is a mystery!  I've got some burr browns, so could socket them and see what happens.  It's a bit of a shame to ruin this pedal's 'mojo' with new chips...but oh well.  My pedal is in real bad cosmetic shape so it's not like I could ever sell it!

[Rob Strand]

If you measure the DC voltage on the inputs and output of each opamp it could reveal a lot - the outputs in particular.  In most circuits they will be at about half the supply, the ones that aren't are either faulty, or there's something unusual about the ckt. Whatever the case, voltage measurements can narrow things down considerably.

If all looks OK look for shorted tants.

[bobbletrox]

All the Vcc+ are 8.94v and all the outputs are around 5.21v

I guess that means they're fine!  If I get an opportunity to turn my amp up loud, I'll make some audio clips.

[Mark Hammer]

It would seem there are multiple versions of the same device out there!

Although it has been 25 years since I owned one, my experience with them was that whatever "mojo" they had stemmed from an apparent attempt to produce more boost than could be reasonably expected from anything running off a 9v battery.  You will note that the markers for the sliders read +/-18db.  In most instances, few designers working with a +/-15v supply would try to go past +/-12db, so tacking on more than that with a considerably more limited supply (and headroom) seemed to be band-selective clipping more than anything else.  Indeed, that is what I used to use it for.

Of course, what gets painted on a chassis is not the same as what takes place in the design itself.  Otherwise Nigel Tufnel's arguments about "going to 11" would make more sense than they do.  Still, I don't know why MXR would have opted for that +/-18db designation (as opposed to +/-10 or some other arbitrary number) unless something deliberately radical were going on inside.

[Ammscray]

values of the inductors

The one's I've seen used Gyrators instead of inductors - that's why there's so many opamps.  Is there an inductor version as well?

I started tracing this from a pic at one stage but there were a few details which I couldn't work out from the pic.  It's pretty much a stock standard gyrator based EQ.

Sorry, my bad, that's what I meant...thanks for the correction