hey fam
so years ago, picked up a maestro mps2 phaser to repair one of these days.
i just found it under my bench, after kinda wondering wtf had happened to it.
so i pulled it out, its been opened up forever... battery was even still good in it <i detached the snap when i stashed it>
its dead as it was then, so i guess a long nap didn't let it heal ;)
so i've replaced the electros. voltages seem to be decent. i have to go thru and figure out which dang ic is which, cuz the schematic doesn't label them and i'm not super good at tracing stuff with my wonky ass eyes.... speed knob has no effect, can't seem to find any sign of oscillations as i'd expected to...
and dealing with the bane of my existence, fets. i @#$%ing HATE fets. they ALWAYS bust my stones.
this beast in particular is known for being a ball breaking sack of nixons, soooooo i'm thrice screwed.
(https://i.postimg.cc/sBF8VPN2/Maestro-MPS-2.gif) (https://postimg.cc/sBF8VPN2)
so my big questions....
where the hell do i start? what should i look for? passes signal just fine, so i assume the buffers and stuff work.
if i mess with the trimmers, i can hear marginal differences in the thru-put tone, but, heck, i can't tell if its off or on ffs. i'll have to beep it later to be sure, i guess.
i did also try cleaning the pot.
wondering if it could be the ground of the input jack? i just had a shin ei wah surf hurricane vol siren pedal arrive doa, and it was the sleeve connection to the input jack that laid it all low.
i'll gladly post voltages, but first i gotta know which ic's are which, etc. oyyyyyy.
makes me widdle bwain hurt.
any guidance for my dumb ass is gratefully accepted. yes, i did search for info. i found a plethora. but still can't seem to figure out how to start.
thanks peeps.
updated the schem slightly and numbered the chips
(https://i.postimg.cc/LnkKnbj1/Maestro-MPS-2-updated.gif) (https://postimg.cc/LnkKnbj1)
I'd say start by verifying that the LFO is O-ing. So set the rate control for something that looks like it ought to be a medium rate, and measure the AC voltage at pin 6 of IC5.
If that checks out, I'm not sure whether the R1 trimmer results in a greater or lesser amplitude at the junction of those two 390K resistors. But if you know you have a working LFO, set R1 for the highest amplitude, and then back it off just a bit. That still won't necessarily make the FETs sweep, so now you want to play with R2 to find the right bias. Remember that the LFO is going to sum with the bias voltage, which is why I suggested NOT aiming for the highest possible LFO amplitude.
If the unit behaves anything like the Phase 90, then R1 and R2 could actually be panel-mounted controls to set the sweep width and offset or "initial" notch frequency.
We'll leave verifying phasing for later, but for now let's verify that each op-amp - IC1 thru IC4 is passing signal. Feed something to the circuit input and measure AC voltage at pin 6 for each of those op-amps, in succession. They're ostensibly unity gain, so you should get something very close to the same AC voltage at pin 6 for all 4 op-amps. You say that the unit "passes signal", but we don't really know if that's wet, dry, or both. So temporarily lift one end of that 2k2 resistor by the output to eliminate the dry signal. This way, everything you hear will be wet-only, making it easier to make adjustments.
gonna try all that in a minute.
lfo is lfo-ing, so that's good
here's voltages:
8.86 b+
ic1
1 -0.011
2 3.57
3 3.79
4 -.047
5 0
6 6.37
7 6.52
8 0
ic2
1 -0.004
2 3.81
3 3.41
4 -.041
5 0
6 1.61
7 5.92
8 0
ic3
1 -0.005
2 3.80
3 3.72
4 .039
5 0
6 5.32
7 6.52
8 0
ic4
1 .002
2 4.3
3 3.7
4 0
5 0
6 1.8
7 6.4
8 0
ic5
1 0
2 3.7
3 3.7
4 0
5 0
6 modulating
7 6.4
8 0
ic6
1 0
2 3.7
3 modulating
4 0
5 0
6 modulating
7 6.4
8 0
q1-q4
d 3.5 modulating
g 3.8
s 3.8
q5
c 6.53
b 1.34
e 1.32
q6
c 8.73
b 7.20
e 6.53
q7
c 6.52
b 4.39
e 3.80
pot changes modulation speed
switch functioning
z1
7.2 stripe
lifting the 2.2k on the output kills the signal, so tho the osc is working, its not passing audio thru the wet path.
now i gotta find the psychotic break in the circuit lol
thanks mark
more shortly gotta find my audio probe and a signal gen
Quoteq5
c 6.53
b 1.34
e 1.32
Assuming your Q5 and the schematic Q5 are the same part, the Q5 voltages look wrong. That's why the voltage is off at all the opamp outputs.
Maybe easier just to replace Q5 and see what happens to the voltages.
good call, rob. betting its a mistake on my part. seems to me if q5 were bad, it wouldn't be able to pass audio for the dry OR wet signal tho, right?
can hear the emitter follower they used for q5, it seems to drop the output just slightly from unity so i assume that q is alright.
my gut says the most likely culprit to fail would be them tropical fish coupling caps. in my limited experience with them, they often seem to go open internally eventually.
soon as i find my audio probe and a 9v batt to power my sig gen, i'll walk thru the circuit path and try and find where it ends.
i will check the b connection to q5 again. sometimes funk that builds up over the decades can make reading wonky.
thanks rob ;)
stay tuned
Quotegood call, rob. betting its a mistake on my part. seems to me if q5 were bad, it wouldn't be able to pass audio for the dry OR wet signal tho, right?
can hear the emitter follower they used for q5, it seems to drop the output just slightly from unity so i assume that q is alright.
Well, the transistor is kind of biasing so it might still pass the dry signal. However, the bias voltage on the emitter is totally off and that's screwing up the phaser part. I suspect Q5 is low gain. That could be due to damage or a natural death. If it has been replaced in the past it's possible the replaced part has e and c reversed, that will also make it low gain - maybe worth checking the pin-out and PCB.
Actually the b and e voltages aren't even consistent with low gain. Maybe b and e are shorted or got shorted when you took the measurement.
:icon_biggrin:
or.... i didn't get a good connection.
i'll check it out tomorrow, or later tonite if i can't sleep like usual.
its the original transistor, so could be dead i guess. shorted b to e?
i gotta look up the tis97, see if i have something close.
replaced q5 with a bc549. voltage is better there, but still no audio thru the phaser stages.
i am assuming every 741 should be showing the same voltages, pin 5 on ic 2 and 4 is way off from the other two however. gonna replace the caps first, as they seem the most likely place to mess up. goin' in... stay tuned!
got it ALMOST passing signal... can hear the modulation, faintly and distorted if i strum the guitar hard. speed knob working, gonna reconnect the 2.2k mixing resistor. its making trem sounds, but very distorted.
just messed with it some more, but all i get so far is a very distorted trem... half voltage seems real low too, about 1/2 v shy
man this sucker is annoying :icon_mrgreen:
Quotereplaced q5 with a bc549. voltage is better there, but still no audio thru the phaser stages.
i am assuming every 741 should be showing the same voltages, pin 5 on ic 2 and 4 is way off from the other two however. gonna replace the caps first, as they seem the most likely place to mess up. goin' in... stay tuned!
Quotejust messed with it some more, but all i get so far is a very distorted trem... half voltage seems real low too, about 1/2 v shy
man this sucker is annoying :icon_mrgreen:
It's a crap design because the DC voltage on the emitter of Q5 needs to be close to the voltage on the JFET sources (ie. the 4V rail). There's nothing forcing that to be true so there's a high probability of failure!
Can I suggest lifting the end of the 100k that goes to the emitter of Q5 and inserting a 100nF cap in series with it. The 100k is the one between the emitter of Q5 and pin 2 IC1. That should fix the DC matching problem. If it doesn't work the problem is elsewhere but leave the 100nF in until you find the problem since it will remove a lot of freakin headaches along the way. When you do that the DC voltage on all the opamp pins should be the same as the DC voltage on the rail that goes to all the JFET sources (ie. 4V).
Hmm, voltage near power supply on IC1 and 3 output pins, is also suspicious...
i will try your suggestion when i get back to it later, rob.
i agree federico, that voltage looks weird. everything about this sucker is weird ;)
Quote from: Rob Strand on May 16, 2022, 03:14:35 AM
Can I suggest lifting the end of the 100k that goes to the emitter of Q5 and inserting a 100nF cap in series with it. The 100k is the one between the emitter of Q5 and pin 2 IC1. That should fix the DC matching problem.
Rob, don't go launching poor ol' pInK down a nasty rabbit hole now ...
by doing what you suggest the signal path will have no DC reference to GND
if/when the FET's open up completely ... nasty noises will ensue
the most direct (and safest) way to fix this is by fudging the 4v reference circuit
ie., by replacing the 22k/47k biasing the base of Q7 by a 50k or 100k trimpot
set the existing BIAS trimpot half-way first, then set new trimpot to taste
---
incidentally, this is a Maetro sub-unity gain design that sucks
if you convert to direct bypass ...
just so ya know
QuoteRob, don't go launching poor ol' pInK down a nasty rabbit hole now ...
by doing what you suggest the signal path will have no DC reference to GND
if/when the FET's open up completely ... nasty noises will ensue
the most direct (and safest) way to fix this is by fudging the 4v reference circuit
ie., by replacing the 22k/47k biasing the base of Q7 by a 50k or 100k trimpot
What I'm suggesting prevents the DC from Q5 stuffing up the opamp biasing.
It doesn't cause any more problems. What you are suggesting won't solve that issue
but it does solve yet another issue - I'm all for it, it can only help.
These old pedals often have design issues. They certainly don't help when
you are trying to debug problems.
If the bias on the opamps can be made right then it's possible to trace the signal through
the opamp chain and see where it stops, then take it from there.
The 2k2 in the dry path looks wrong to me. My guess is the intended value should be perhaps 22k. Otherwise when the phasor is kicked in it has virtually no effect.
At the moment you have a bigger problem in the phase shift path, since after following Mark's good suggestion of lifting the 2k2 there was no audio.
Quote from: Eb7+9 on May 16, 2022, 12:39:18 PM
Quote from: Rob Strand on May 16, 2022, 03:14:35 AM
Can I suggest lifting the end of the 100k that goes to the emitter of Q5 and inserting a 100nF cap in series with it. The 100k is the one between the emitter of Q5 and pin 2 IC1. That should fix the DC matching problem.
Rob, don't go launching poor ol' pInK down a nasty rabbit hole now ...
by doing what you suggest the signal path will have no DC reference to GND
if/when the FET's open up completely ... nasty noises will ensue
the most direct (and safest) way to fix this is by fudging the 4v reference circuit
ie., by replacing the 22k/47k biasing the base of Q7 by a 50k or 100k trimpot
set the existing BIAS trimpot half-way first, then set new trimpot to taste
---
incidentally, this is a Maetro sub-unity gain design that sucks
if you convert to direct bypass ...
just so ya know
thanks brother ;)
yeah, i know, tb means no bueno
but i love these shitty old phasers.
right now, its acting like a dirty tremolo. i found/ordered more jfets if i need 'em, but next trick will be likely lifting resistor legs to measure... replacing the caps in the actual phaser stages made an improvement..
but i think the big issue is the freekin voltage is just plain too low in the circuit. it won't pass signal without the bias full up, and the osc most of the way up too.
i will try your suggestions when i get back down the dungeon!
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.
so 741's will be coming out, sockets going in, and may end up converting to ne5532 cuz i dunno if i have enough 741's unless i use old metal can ones i usually save for more mojo builds.
more later... thanks brother
stay tuned
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.
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.
Quoterob... 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 :icon_mrgreen:.
Quote from: Rob Strand on May 16, 2022, 08:05:44 PM
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
Quote
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
Quote from: Rob Strand on May 16, 2022, 08:37:49 PM
Quoterob... 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 :icon_mrgreen:.
oyyyyyyyyyyyyyy
(https://i.postimg.cc/PNhnHyfK/mp2-1.jpg) (https://postimg.cc/PNhnHyfK)
(https://i.postimg.cc/KR8XkYMM/mp2-2.jpg) (https://postimg.cc/KR8XkYMM)
(https://i.postimg.cc/14Hh3FfS/mp2-3.jpg) (https://postimg.cc/14Hh3FfS)
pixes
Quoteoyyyyyyyyyyyyyy
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.
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.
https://www.youtube.com/watch?v=Wfaxk4rNUwo
Quotehttps://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.
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
video or it never happened
dungeon extra trashed cuz redoing it lol
https://www.youtube.com/watch?v=TdPfNOA4N2I
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.
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
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
Quoteit 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.
Quotejust 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.
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 @#$% was tom oberheim thinking here? i don't get it. but then, i don't understand or pretend to understand these things in the first place ;)
the more i look, the more my widdle bwain hurts ;)
Following this with interest - is this the same circuit as the Oberheim Phasor P-100?
I'm soon going to work on a repair for someone locally who has one that was on a pedal board that "had a big power surge and all the pedals got fried" -- thinking I'll just follow RG Keen's "what to do on fixing a fried pedal" thread in this forum and then come back to this thread if I get only part of the way through it. Figure all the electro's and IC's need to get replaced for starters
Quotei just looked at the 2 22k resistors over to the right of ic4 like mark suggested, and now am suddenly wondering why in @#$% 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
Quote from: Strategy on May 23, 2022, 02:39:25 PM
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.
QuoteBased 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
Possibly a dumb question but can any modern 741 IC stand in for these old RC741D... IC's?
thanks!
Paul
QuotePossibly 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.
i used cheap ass bojack 741's from amazon, fired up great!
071's might work as well. preforming quite the generic function here.
probably the LM308 would as well, given accommodation for the comp cap is jerry-rigged on. would give the phaser that ''slew rate smoothness'' haha
cheers
Hi all, back with the Oberheim P-100, "almost" the same circuit as Jimi's MPS. I've had progress, following RG's "how to trouble shoot pedals fried by overvoltage" thread, and also thanks to wiring photos from Jimi, I have this gem passing dry signal and phased. BUT, no sweep, have tried all manner of trimmer tweaks and it's like playing the phaser manually, can't find the sweep. I'm bumping up agai st the edge of my troubleshooting experience, can anyone share some suggestions? Measure voltages on those first two 741's perhaps?
...can a potentiometer get damaged in a "catastrophic" overvoltage event? The owner of this pedal said it was on a pedal board that had a "power surge" in which every pedal died. I had to replace all ICs, electro's, and transistors to get it this far.
Thanks all!
Strategy
(https://i.postimg.cc/s1n4pPrg/20220820-162411.jpg) (https://postimg.cc/s1n4pPrg)
QuoteBUT, no sweep, have tried all manner of trimmer tweaks and it's like playing the phaser manually, can't find the sweep. I'm bumping up agai st the edge of my troubleshooting experience, can anyone share some suggestions? Measure voltages on those first two 741's perhaps?
It would be a good idea to make sure the LFO IC5+IC6 is working.
If you set the speed to low you might be able to see IC6 pin 6 going on and off.
Increase the speed and see if it increases.
If that's too hard to see, wire a 10k resistor and a RED LED and connect it from IC6 pin 6 to ground.
The LED should go off and on with the speed.
(https://i.postimg.cc/gxRYzQHL/Maestro-MPS-2-sch-markup.png) (https://postimg.cc/gxRYzQHL)
IC5 pin 2 about 4V
IC5 pin 3 about 4V
IC5 pin 6 will have triangle wave (centered around 4V) you might be able to measure that.
IC6 pin 2 about 4V
IC6 pin 3 will bobble up and down (maybe 3V to 5V, something around 4V anyway)
If the LFO isn't working then there's no way you will get any sweep, regardless of the trimpots.
If your LFO is working progress to the trimpots and perhaps check your JFET orientations.
Ok, a covid reinfection had me off track for a few weeks but I'm back and tested ICs 5 and 6. The LFO is not O'ing, and i get way higher voltages across the IC pins, up around supply voltage:
IC5
1 - 8.35
2 - 8.13
3 - 8.34
4 - 8.36
5 - 8.36
6 - 8.29
7 - 8.29
8- 0
IC6 -
1 - 8.36
2 - 8.35
3 - 8.29
4 - 8.36
5 - 8.36
6 -8.26
7 - 7.96
8 - 0
There's not a lot upstream of these ICs, so I feel there's a potential to hone in here but I'm puzzled. Advice sincerely appreciated!
Those voltages look totally whacked.
Upfront: Are you sure the IC's are inserted correctly? Are you sure you are reporting the IC pins correctly?
This is the pinout when viewing from the top of the IC (not the solder side of the PCB).
(https://upload.wikimedia.org/wikipedia/commons/7/72/Generic_741_pinout_top.png)
You should be able to use your DMM continuity beeper to confirm pin 4 is connected to ground and pin 7 is going to +7V. In fact pin 4 of all ICs should connect together and to ground and Pin 7 of all ICs should connect together and go to +7V.
If that's not the case check the IC orientations. If you want to check further follow some of the IC pins and make sure each pin connects a part that matches the schematic.
It might also help to check the voltages on the other IC's. That might highlight an error, or a problem on a larger scale eg. a wiring error, broken track, bad solder joint.
Thanks Rob!
I counted the IC pins on the correct order.
They are correctly oriented.
I will try the other troubleshooting you recommend.
As I mentioned before as well, effected signal passes and I can modulate phase by turning the trimpot - it sounds really nice. So I believe the issue is confined to the LFO.
So, the 741s I used for IC5 and IC6 are from a different source and batch from the ones in the ohasor section. Could bad ICs turn up a result like i'm seeing? Sadly Im all out of 741s, so I'd need to order more to test
Thanks again for all the help!
Quote from: Strategy on September 13, 2022, 11:38:25 PM
Thanks Rob!
I counted the IC pins on the correct order.
They are correctly oriented.
I will try the other troubleshooting you recommend.
As I mentioned before as well, effected signal passes and I can modulate phase by turning the trimpot - it sounds really nice. So I believe the issue is confined to the LFO.
So, the 741s I used for IC5 and IC6 are from a different source and batch from the ones in the ohasor section. Could bad ICs turn up a result like i'm seeing? Sadly Im all out of 741s, so I'd need to order more to test
Thanks again for all the help!
The fact pin 4 doesn't have 0V is a sure sign of a large scale problem. If the ground/0V isn't getting to the ICs then the circuit has no hope of working. Check the continuity of 0V everywhere, jacks, DC inlet, pots. Something isn't right there!
Quote from: iainpunk on June 01, 2022, 10:11:22 AM
071's might work as well. preforming quite the generic function here.
probably the LM308 would as well, given accommodation for the comp cap is jerry-rigged on. would give the phaser that ''slew rate smoothness'' haha
cheers
308 would work fine. i don't use tl0xx for anything, i don't care for the sound of jfets personally
i know your prob, strat... replace the trimmers. they're hosed.
woah, really? one of 'em gives me phasing when I turn it. did you have to replace the trimmers on yours?
not on this one, i was lucky to get them back to life with a bit of effort, but the last one i'd had a few years ago, yep, it was the damn trimmers, doing the same kinda thing... i could kinda hear it phasing, but i couldn't get it to fire right. one is the bias for the jfets, the other is the amp for the oscillator if memory serves... effectively, one you set for the "cleanest" signal, the other you set for the least distorted sweep.
Don't overlook the fact if the opamp voltages are way off there's no hope of the thing working. The 0V issue you have is a very significant problem that has to be fixed.
Yeah as Rob suggests, before I go at those trimmers I need to trace through the entire LFO section of the circuit and double check ground throughout that section. I haven't had time the last week or so, getting ready for a gig.
Depending on what happens with that, I'm going to test the speed potentiometer as well as the trimmers, I've had new potentiometers fail in new projects, who knows with a cheap pot in a 50 year old pedal that got 'fried' in a catastrophic overvoltage event...
But again I think the issue is confined to the LFO Section of the circuit, ground for the audio section seems to have continuity. Passing effected audio great. Just not LFO-modulated effected audio.
Quote from: Strategy on September 27, 2022, 12:29:48 AM
Yeah as Rob suggests, before I go at those trimmers I need to trace through the entire LFO section of the circuit and double check ground throughout that section. I haven't had time the last week or so, getting ready for a gig.
Depending on what happens with that, I'm going to test the speed potentiometer as well as the trimmers, I've had new potentiometers fail in new projects, who knows with a cheap pot in a 50 year old pedal that got 'fried' in a catastrophic overvoltage event...
But again I think the issue is confined to the LFO Section of the circuit, ground for the audio section seems to have continuity. Passing effected audio great. Just not LFO-modulated effected audio.
ended up selling mine, so i can't quite remember which trimmer is which, but one sets the bias to the jfets, and the other is the for the gain of the lfo. on the one i'm talking about, corrosion had built up enough on the trimmer, and people had f'd with it so much in the past, it just couldn't make good connection, it was maddening, cuz i was def getting lfo sweep i could read on the chip for it.
i ended up kinda bending the trimmer so the wiper made better contact and caiging the bejesus out of it.
if that trimmer goes open, no lfo at all, at least not useable.
i socketed and replaced all the 741's in the circuit, and replaced all the tropical fish with green meemies instead, as it seemed fish weren't too fresh.
fired right up after that. i DID buy a set of jfets for it from smallbear, so i may build another one on perf or something.
hope ya got it sorted out bro!
Thread wrap up: Oberheim Phasor FIXED, finally. Took a break from it this winter due to work and gigs. Anyways, I wasn't getting supply voltage on Pin 7 of the 741s in the LFO section because there was a break in continuity around one of the electrolytic caps I replaced. I tested continuity after soldering in new parts, but must have been in a hurry/skipped over this one. Lesson learned.
If there's a PCB pattern for this phase shifter out there I'll likely build one. It does only one, simple thing but does it really musically.
thanks to all who helped with troubleshooting pointers.
-Strategy