maestro mps2 ..... where to start?

[pinkjimiphoton]

As for the schematic:

There's a pic of the PCB here,
https://tagboardeffects.blogspot.com/2014/02/maestro-mps-2-mini-phase.html

I think the obscured 2k2 at the top right is for the power supply.

thats the 2.2k between b and e of q6

Just under that near the green wire and above the switch is a 43k and a 22k. 
I think that 22k is incorrectly shown as 2k2 on the schematic.
Tracing the PCB starting from the output jack wire should confirm it.

So for the Mark's 2k2 lift test you want to lift that 22k.

that 22k i think is indeed mismarked, and is the one i just re-connected last nite.
gonna have a play  with it soon... life...lol

[pinkjimiphoton]

rob... no worries... but so we're on the same page, we're now passing signal and have oscillation... but it sounds like a distorted trem.

OK cool.

Not sure if the distortion is from the JFETs or clipping or a fault.   

There's no linearization (RC networks on the gate) on JFETs so the distortion depends on the JFETs.   

However, it could be just outright clipping.   The 4V bias point isn't bad but it's non-optimal.  Then the last opamp  stage (IC4) has a gain of 2 set by the 2x22ks.    If you decreased the 43k after the phaser switch, say to 22k, then that will let you dial back the gain of the last opamp.   To dial back the gain of the last stage decrease the value of the 22k on pin 6 of IC4.   With the 43k replaced with 22k, you might even be able to just short out the 22k on pin 6 of IC4.

Before you bother with such a mod you would want to make sure the distortion isn't caused by a faulty opamp (or the JFETs).  I guess you will find that out pretty soon  .

oyyyyyyyyyyyyyy








pixes

[Rob Strand]

oyyyyyyyyyyyyyy

Awesome pics.

No doubt the 43k and 22k near the green wire are the output mixing resistors.
So the 2k2 mixing resistor on the schematic should be 22k.

[Mark Hammer]

On the Phase 90, and a whole lot of other FET-based phasers, there is usually a fixed resistor in parallel with the drain-source path.  Since the FETs are in parallel with the resistors, the resistors set a maximum resistance to ground, and the FET simply drops the resistance below that.  The MPS2 lacks those, so I wonder what the impact is on the sweep range.

[pinkjimiphoton]

https://www.youtube.com/watch?v=Wfaxk4rNUwo

[Rob Strand]

https://www.youtube.com/watch?v=Wfaxk4rNUwo

Hello jimi.  Crikey! yeah, sounds like a fuzz pedal.

What absolutely stumps me is you click the switch footswitch to clean and it's still fuzzy.
What the hell's doing that?  It's only a buffer (Q5).

Can you post your Q5 bias voltages again?

The only other place it could distort is the JFET Q1.   Somehow the JFET is biased on and acting like a clipping diode on the output of the buffer.  Even that I'm struggling to believe.

[pinkjimiphoton]

 it was the amp that was distorting. i didn't notice the gain and volume were the same setting.
but it was still distorting.

i looked at it, realized it couldn't be the buffer, and it couldn't be the oscillator, so the only option was one of the opamps in the phase shift circuit or the jfets.

i took a gamble, bought both. ....err... the small bear "equiv" anyways.

took out the 4 741's. replaced with sockets. put some brand-new 741's in that sucker and it came right in, phasing... poorly.

took mark's trimmer advice, and messed with tthe trimmers at various speeds til its dialed in just about how i remember these sounding.
they sound way better than you may expect, got a quite pronounced "hickup" in the sweep that reminds me quite a bit of my univibe, a "throbulating" sound <ha, i used to play with a band called the throbulators once in a while.. the idea was to just play everything OTHER than "music">

so its working... yay~! sounds great. now i gotta put it back together, if i can remember after all these years.

thanks all for the help... i'll try and get some video of it now that its' working.

do ya think i should put the original tropical fish caps back in? i think the poly's sound better, and i think something failed that maybe blew out the 741(s).

since its a 4 stage phaser, thinking about ... is it worth hacking in a feedback pot for shits n giggles?
could literally just put a 50 k pot in series with a small resistance from the last stage back to the first, or something like that, right?

ugggh... panky drank too much coffee today. one small victory at a time. no need to BUM now lol...

thanks thanks thanks!

PjP

[pinkjimiphoton]

video or it never happened

dungeon extra trashed cuz redoing it lol


https://www.youtube.com/watch?v=TdPfNOA4N2I

[Mark Hammer]

A preliminary mazeltov, Jimi.

Keep in mind that the LFO sums with the bias.  They BOTH feed the gates of the 4 FETs.  So I will suggest that, for "dialing in" best functioning, you set the LFO speed to a medium rate that should be audible; a Goldilocks setting - not too fast and not too slow.  The R1 trimmer that sets sweep width probably shouldn't be more than maybe 3/4 up.

Once that's done, THEN you can adjust the R2 bias trimmer so that the entire LFO wave meets the FETs' requirements for changing their drain-source resistance.  What I heard on the video was that *part* of the wave met that criterion, but not all of it.  The solution is to identify the trimmer settings where the entire wave is in the ballpark

I don't know who Maestro hired to do their technical drawings, but by now, those drawings are like Sunday-morning newspaper-comics "Can You Spot the Errors?" quizzes.  It seems there is always something amiss with a Maestro schematic.  So far, you spotted the erroneous dry mixing resistor value on the output.  But shift your gaze over to the far right.  Just what the deuce is pin 2 of IC4 doing connected to the junction of those two 22k resistors?  So, as much as it might seem at first blush that a little bit of feedback might be nice, and easy to add, I'd skip that until you find out more about that last phase shift stage.

[pinkjimiphoton]

toda, brother.

thanks... i did mess with it a little more last nite.
the thing sounds fantastic on a REALLY slow sweep, but pretty much dissappears into a very mild vibrato at high speeds. so it definitely needs a bit more playing with. thanks for the advice, i will have a play with it.

yeah, maestro stuff i think they read rg's dirty trick article.. shoot, the guy i build for has "extra parts" in some of the circuits just to throw off copiers. lol. its rarely right.

i'm wondering if the mixing stage drawn as shown was modified to give a less intense phasing, the 2.2k resistor in the schem would make the dry signal a bit stronger than the wet, i'd think... but its definitely 22k's, not 2.2k.

i will look at that part of the schem, i hadn't even noticed it, i was too busy trying to figure out where the signal broke down.

more later... thanks brother

[pinkjimiphoton]

just got to spend a wee bit of quality time with the phaser abusing it's twisty things until it gave up the phase shift.

noice n chewy with a touch of formanty goodness that seems to go yahyahyahyahyahyah up high. i think i got it.

thanks mark!

when i get a chance, will revisit and post voltages etc.

thinking about making one on vero, since i have the spare parts

stay tuned-ish

[Rob Strand]

it was the amp that was distorting. i didn't notice the gain and volume were the same setting.
but it was still distorting.

Ha, no prob.  That puts an end to overthinking the problem - not that I had anything to offer.

just got to spend a wee bit of quality time with the phaser abusing it's twisty things until it gave up the phase shift.

Cool jimi, looks like it's on its way to working.

[pinkjimiphoton]

working 100% now bro!
i just finished a quick production run around 3 am this morning, when i get back down there i'll try and get voltages, etc.

sounds great, in fact

i just looked at the 2 22k resistors over to the right of ic4 like mark suggested, and now am suddenly wondering why in @#$% they're not only there, but why you'd need to add a half voltage that's already half voltage to the output of the phase shift network.

what the @#$% was tom oberheim thinking here? i don't get it. but then, i don't understand or pretend to understand these things in the first place

the more i look, the more my widdle bwain hurts

[Strategy]

Following this with interest - is this the same circuit as the Oberheim Phasor P-100?
I'm soon going to work on a repair for someone locally who has one that was on a pedal board that "had a big power surge and all the pedals got fried" -- thinking I'll just follow RG Keen's "what to do on fixing a fried pedal" thread in this forum and then come back to this thread if I get only part of the way through it. Figure all the electro's and IC's need to get replaced for starters

[Rob Strand]

i just looked at the 2 22k resistors over to the right of ic4 like mark suggested, and now am suddenly wondering why in @#$% they're not only there, but why you'd need to add a half voltage that's already half voltage to the output of the phase shift network.

what the @#$% was tom oberheim thinking here? i don't get it. but then, i don't understand or pretend to understand these things in the first place

The two 22k resistors on IC4 provide a gain of 2 (roughly, perhaps a bit more) at end of the phase-shift chain.  That lets the design use a higher mixing resistor (43k) on the phase shift path and still achieve a 1:1 blend.
[remember the 2k2 mixing resistor on the schematic is wrong and should be 22k]

When the phaser is active the level of the dry signal is dropped by the 22k+43k resistive mixer.
Using voltage divider equation: dry signal mixing loss = 43k/(22k + 43k) = 0.66

For the phase-shift path the 22k+43k resistive mixer drops the level but it loses more signal.
phase-shift mixing loss = 22k/(22k + 43k) = 0.34.
When we factor in the gain 2 from the 2x22k's on IC4 the overall phase-shift path signal gain is 2*0.34 = 0.68.

So voila the dry signal path gain and phase-shift path gains match like a normal phaser.   There is an overall loss
of 0.67 in level.  Perceived loss is less.

The whole idea of making the 43k large as possible is to minimize the dry-path signal loss (in phaser mode).   However the down side is we need to add gain to phase-shift path to compensate.   We don't want to make the 43k really high as that means we need to add even more gain to the phase-shift path which means IC4 will clip.  Given the opamp supply is already reduced from 9V we don't want too much gain.

If you look at the old Ibanez PT900/PT999 phasers (see schem on fsb) they using the same value resistors on the resistive mixer but have an opamp at the front-end to boost the overall signal.  The resistive mixer has an extra resistor to ground.  You still have to limit the opamp gain to prevent clipping at the first opamp.

EDIT:
This one looks OK,  mixer is R5, R6, R27  (bernarduur's schem is the older simpler circuit but it's missing the 30k to ground on the mixer; the equivalent of R6 on this one.)
https://www.dirk-hendrik.com/maxon_phasetone_pt999.pdf

[Rob Strand]

Following this with interest - is this the same circuit as the Oberheim Phasor P-100?
I'm soon going to work on a repair for someone locally who has one that was on a pedal board that "had a big power surge and all the pedals got fried" -- thinking I'll just follow RG Keen's "what to do on fixing a fried pedal" thread in this forum and then come back to this thread if I get only part of the way through it. Figure all the electro's and IC's need to get replaced for starters

Based on the PCB pic on FSB, the PCB looks identical.  However, I'm not so sure about the part values.   In the FSB pic, the odd thing is the mixing resistors near the green wire are both 22k.   From what I can see the 2x22k gain resistors on IC4 are still present.   That's going to weight the behaviour of the phaser more towards a vibrato.

[Rob Strand]

Based on the PCB pic on FSB, the PCB looks identical.  However, I'm not so sure about the part values. 

Here's a pic of another Oberheim Phasor P-100 PCB.  It's got the 43k,
https://www.effectsdatabase.com/model/oberheim/p100#pictures

[Strategy]

Possibly a dumb question but can any modern 741 IC stand in for these old RC741D... IC's?
thanks!
Paul

[Rob Strand]

Possibly a dumb question but can any modern 741 IC stand in for these old RC741D... IC's?

The circuit will should work with any 741.   Well, provided it's not an ebay unit which isn't really a 741.

The interesting thing is the internal circuit in RC741 datasheet shows an extra diode compared to the Fairchild datasheet.  The diode speeds-up recovery if overloaded.    I wouldn't read too much into it though as those internal circuits are not always exact.

[pinkjimiphoton]

i used cheap ass bojack 741's from amazon, fired up great!