Help with repairing old MXR script phase 90

[Elvisofnazareth]

Hi,


I got this pedal..... it's a 1975 or 76..



Right now it sounds like a thin raspy terrible distortion Box....and does not phase. Adjustments to the pot do nothing to the signal.

I believe it has a bad chip, (?) (It sounds exactly like a different pedal I fixed with that problem.)

How can I figure out which chip of the 6 chips is Bad?
And How come even with the bad chip it doesn't phase?

I have a meter, I've done some step by step mods, and built some things by layout...but I'm a beginner when it comes to trouble shooting.

Thanks!

[Robert M. Martinelli]

Hi,
the first step for a good troubleshooting, especially if you're not experienced, is to get the schematic diagram. Google for "mxr phase 90 schematics " and download the schematic diagram.

Since you stated the thing "doesn't phase", the first place I would look for troubles is the oscillator section ( the chip alone in the lower part of the schematic ) which generates the frequency at which the phasing cells ( four of the other chips ) work. The first chip right after the input is a buffer stage.

Best of luck!

Best regards

Bob

P.S. - just as a footnote, this thread should have been started in the "effect" section of the forum, to facilitate other people's search.

[olddawg]

Do you have a scope? If not, unless you have an obvious problem with a pinout voltage (if the schematic has them) you might be pissing in the wind. You would probably be better off with a vintage pedal, without an electronics background, to send it to Keeley or someone like that and have it restored before you mess it up.

[Elvisofnazareth]

Hey thanks for looking over my post you guys.....

One thing I do have going for me is I also have a Script Logo 1974 custom shop reissue here. Voltages should be similar I imagine.

- Could you guys tell me what points to place my meter leads (to test the chip etc.?)

I don't have a scope BTW.


I've heard that op amps can be fried just with a little static charge..... Am I being over cautious?

I don't want to put a bunch of money into this ...

I've built some tube amps and hand wired germanium fuzzes from scratch with step by step instructions, and using my ears to determine what sounds best.

I'd like to do this myself rather than send Keeley money I'm sure I need way more than him.

[Robert M. Martinelli]

Hi,
Olddawg is right, a scope would make things alot easier, OTOH there are some simple measurements you can carry out with a DMM, especially if the reissue version you have at hand is an exact reissue....

As to the "static" issue, this caveat is not applicable with BJTs and old op-amps built with BJT technology, you have to worry about static only when working on MOSFETs and the like ( wearing a conductive bracelet tied to GND cures this issue, anyway ).

I assume you know how to operate a DMM and I also assume you have good soldering abilities, otherwise you'd better take the FX to a qualified tech as per Olddawg's advice.

First place to look is the bias circuit, at the lower left part of the schematic, you'll find a Zener diode paralleled with a normal one, measuring across it should read about 3 VDC, if not, you should try to change the Zener diode.

If memory serves me well, the "phase 90" had 741s in it, this means power supply is applied on pins 7(+) and 4(-), so you should find 9V across these pins.

If your pedal has three double op-amps ( 4558s, TL072s, TL082s, NE5532s and the like ) instead of six single ones, the power supply pins are 8(+) and 4(-).

As I told you, the first chip to look for trouble is the oscillator chip, and, since you don't have a scope, the fastest thing to do is take it away, install a socket for easy replacement and replace the osc chip ( 741s are very cheap ).

If this doesn't cure the problem, You should either get yourself a scope ( and know how to use it ) to go any further, or otherwise take the damn thing to a qualified tech.

It seems to me the FETs used in the "phase 90" had to be selected/matched
( hand-picked ) for similarity, so if it turns out that the op-amps are good and a FET needs replacement, you should have all of them changed with a "matched set".

Hope this helps

Best regards

Bob

[David Schwab]

I had to fix one of these back in the late 70s. I couldn't figure out which 741 went south, so I pulled all of them, installed sockets, and replaced them with TL071s.

[Elvisofnazareth]

We're getting there!

Yeehaw!!!!! ( voxrules rules!)

The thang is phasing!!!
& the tone is now just awesome.

I changed that chip out .

I got a 741 from Radio shack.


Now for the bad news.....

The Pedal still has an overpowering raspy unmusical distortion when I do anything other than finger pick as lightly as I can...

Is the Buffer fried too?

(when I walk on eggshells with it.... it is awesome.)


Any more suggestions?



wow..... Picked lightly It already sounds better than my 1974 custom shop reissue.

(More open and natural sounding......still has the upper harmonic content and the guitars natural "shimmer" in tact)

I believe I''m getting a small taste of how cool this pedal can be ..

I have seen the mountain top....and it is good.

[Robert M. Martinelli]

Hi,
glad I've been able to help,
( the few times it happens it's always a good sensation ).

Well, to go any further you'd probably need to waste a lot of time, so I would take David's hint of changing all of the 741s and installing them on sockets, only thing I would add is that there are better substitutes for the 741 nowadays; I would give a try to NE5534s and OP27s, they're pin to pin compatible, so they're a drop-in replacement, and they showcase an infinitely better performance.

If you don't want to change all of the ICs at the same time, you could change them one by one ( or track down the problem with a o-scope ) but, given the low cost of op-amps and BJTs I would not bother myself, all the more so because of the big performance improvement you'd get with the above mentioned op-amps ( very low noise and higher "headroom" ).

Hope this helps

Best regards

Bob

[Elvisofnazareth]

I noticed something else...

You guys are great.
I can't say thanks enough for sharing the knowledge & cool ideas.


I noticed something else by the way...


I get a reasonably loud (but not overly alarming) pop when this pedal is in the chain if I hit the standby on my amp. (when I'm done playing, not when starting up)

It only does this with the effect bypassed, not with the phase active.

Leaky old cap or something?
The pedals tone sounds great so I don't really care ( If this isn't related to the buzzy distortion issue in some way, or hurting anything, I can live with it.)

I'll probably just end up throwing in a TB switch to fix that. (?)

I haven't put it in my TB looper yet though for the old bypass transparency test... & would rather not mod if it's not tone sucking.

As far as sockets and better chips....
I'm going to try to to keep this circuit as original as possible ... I don't like to make improvements on things that have already got the magic ya' know?.

All I know is My 70's small stone was way better than my new one, and this phase 90 is already much nicer than the other phase 90's and the current production phasors I've had.

( I've had several boutique phasors, reissue's and block logos, R28 modded etc).

Who knows, 20 years from now I may be able to swap it for a Harley!...I mean geesh If a POS 12 lbs. 1976 strat is worth 4k now.. why not?

[Elvisofnazareth]

FIXED!!!!!!!!

Thanks you guys.

1st.
I swapped out the Buffer chip.... (It brightened the pedals tone a wee bit, & didn't fix the fuzz... So I re-installed the original.)

2nd.
I swapped the 2 OP amps on the left (From the top).....

Problem solved.

The new chips don't seem to have quite as smooth a high end as the originals, but 90%.

It's only noticeable in my Goodsell when playing very bright and clean.

With a Marshall jmp 50.... It's straight Zep!


You guys are great....let me know if you're ever in Austin...

HAPPY NEW YEAR!!!!!!!!

[Robert M. Martinelli]

Congratulations!
I'm truly happy you did it....I bet using your Phase 90 from now on will be even more satisfactory, 'cause you've been able to fix it yourself!
Best regards
Bob