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Save my MXR 6 band EQ!

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5/9/1999 3:54 PM

Save my MXR 6 band EQ!

hehehe

Anyway, my friend gave me an old MXR 6 band Graphic EQ (the blue box) so I could do experiments with post and pre EQ on distortion.

Anyway, when I plug it in, I hear this horrible jet-like sound (noise) and sometimes a faint crackling.

I looked inside and see 2 TL022 chips. Would replacing them with TL072 be much quieter?

Any ideas about the faint crackling? I thought it was dirty sliders but even if I move the sliders to different positions, I still hear the faint crackling.

Thanks,

Aron

5/10/1999 7:50 AM

You need to give it the standard treatment for geriatric pedals.

(1) replace the electrolytics with fresh ones
(2) remelt every solder joint, touching the heated joint with a bit of rosin core solder
(3) Get the dust and dirt out of it.

Only then start debugging. The noise is probably a capacitor leaking or a carbon comp resistor gone hissy. Could also be the opamps, or a transistor if there are additional transistors in it, or a poor solder joint that's corroding (hence the shotgun approach to solder)

5/10/1999 10:22 AM
R.G.	As an aside, it might be easier to co-opt a stereo graphic EQ for the up front work. You might find someone who will let you borrow one more simply than rebuilding a pedal.

5/10/1999 12:40 PM
aron	Yes, I have a Rane 31 band stereo graphic EQ, but I'm lazy to take it out of the rack and connect it to the pedals But if I must, I must! First I will fool around with this thing. Thanks R.G.! Aron

5/11/1999 10:25 AM

I don't know if I'd be so quick to switch devices. It would depend on the purpose. I know around here, 5 and 7-band Radio stereo EQ's go for as low as $10 Cdn in the pawn shops and second hand stores (modal price $15-20), so you're right about the practical aspect of simply using a standard box like that.

On the other hand, I found my own long-gone MXR 6-band to have certain endearing qualities when the sliders were up full tilt. The silk-screening on the chassis indicates +/-18db cut and boost. As Nigel Tufnel failed to comprehend, there is often a difference between the actual design parameters and the numbering on the chassis or knob. In the case of the MXR unit, though, I think the boost capabilities intended exceed the headroom afforded by the combination of a 9V battery and humbuckers. The result is that it does double duty as a band-selectable overdrive, with soft clipping in select bands. I used to use mine with a compressor ahead of it, set for boost, and the combination of the two was sweet. Come to think of it, in all the years I owned it, I never even HAD any type of overdrive or fuzz.

This is not to suggest it is some sort of "magic" box. It isn't. However, if the intent is to add flavour to the sound, rather than to simply tame it, then it is quite capable of that.

5/10/1999 11:12 AM

If it is an "old" MXR 6-band, then I would think there shouldn't be a TL022 anywhere near it. Being a mid 70's beast, the 6-band pre-dates these particular chips, and my reasonably accurate recollections of my own long-gone but still dear 6-band identified only 4558's wedged in between all those chunky white caps and slider pots. It's possible that MXR changed to BiFet's during production but given the large number of opportunities they had to use BiFet's in later effects....and never took advantage of the opportunity to do so...these smell like a retrofit.

There are plenty of occasions where a BiFET op-amp will not necessarily outperform a standard 4558 in the noise department. This may well be one of them. That the unit includes chips which appear to be retrofits suggests that maybe the crackling has to do with what transpired during the retro-fitting. Bear in mind that the original did not come with socketed chips, so there was a lotta heat that needed to be applied to the board to stick new ones in. You have to admit, stuff is packed pretty close in there.

5/10/1999 12:44 PM
aron	Mark, This thing is OLD! It's gotta be close to an original. I see TL022CP and MC1458P chips on here 2 each. Then there are 11 of those white blocky caps, 4 tantalums? and 1 film cap. I don't see any electrolytics. Do you think that replacing the TL022cp with a TL072 will help anything among the other tips that R.G. gave? Thanks, Aron

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