Maestro Stage Phaser -- No Phasing!

[Zero]

Good morning everyone

I bought a Maestro Stage Phaser from the late 70's off Ebay a couple of days ago... And it turned out to be defective. I'm having some luck these days 
The tone changes somehow when not bypassed, but there's no phasing (read below for more details).
It's one of those huge "wedge" units with the balls and speed foot controls and a speed selector switch. Overall, a fine piece of engineering indeed!

I got the schematic and very detailed manual from Mark Hammer's site (thanks, Mark!). I wish we had these kind of documents more often.
I hosted the .pdf on site, in case you want to take a look: http://people.ee.ethz.ch/~lbucher/MaestroStagePhaser.pdf

I proceded to disassemble the unit (again, I'm was grateful for that manual) and started checking for the obvious.. I followed the instructions in the manual, searched high and low on this and other forums... But I haven't had any success..

Let me sum up the situation (I always tend to write too much  ):
* Power supply is OK: 9V and 4.5V on respective supply pins (as in schematic)
* LFO seems to be the problem. When I manually turn the "center frequency" trimpot (R67 in the schematic) I can hear a nice phasing effect. So the phasing works, but only manually   
* Oscilloscope shows me no triangle/ramp signal before the schmitt trigger..
* I replaced IC8 (dual opamp) with a TL072 and also replaced electrolytic caps C15, C16 and C18. No change...

As far as I understand, the LFO circuit works in the following manner: C15 is charged by a constant current proportional to the one entering pin 5 of the OTA (IC7) coming from the speed control. The voltage I measured across R55 (12k) is around 300mV, so there should be about 25 microamps of current coming into the OTA. Is that more or less ok?

IMO, something is wrong with the "trip points" of the schmitt trigger, which doesn't generate the oscillator signal...

Any hints on what I could try? I have a scope, but no too much experience with it.
Is there a way I could find out it the OTA is working?

Thanks in advance for any suggestions

[StephenGiles]

Multimeter on the LFO output?

[Zero]

Hi Stephen and forum

Sorry about the delay... I had trouble getting some readings, 'cause they seemed to change at random.
Something seems to be moving in there... I couldn't get any reading of a time-varying signal with the scope though.
Maybe this is why: The voltage over the "ramp" capacitor C15 increases very slowly... I mean, it takes minutes to fully charge.
But then, it won't discharge.. at least not within my patience interval 

I've taken the affected part of the schematic to help illustrate:


*The current coming from the speed control (through R55 on the lower right side) is supposed to control the amplification of the OTA, and therefore the rate at which C15 charges. This phaser uses a "slow start" to simulate a motor speeding up.. So when I hit the bypass switch, the voltage measured over R55 (and therefore the current through it, I hope) increases in about a second. So that part of the circuit should be working OK, if you ask me.

*Then, at pin 1 of the OTA. If turn the "motor" on, the cap starts charging .. but as I said, very slowly (much too slowly) and doesn't discharge. The output (pin 6) of the OTA reads an increasing voltage similar to one over the cap C15...

Now correct me if I'm wrong: In the smitt trigger (IC8B), the negative input (coming from the OTA) will rise until it reaches the level of the positive input (pin 5). The, the output (pin 7) will "switch" and pull the positive input (pin 5 of IC8) low, and at the same time pull pin 3 of the OTA to low, thus discharging the cap C15.
At least that's what I've grasped on how it should work.

IMO the next suspect is the OTA... As I've already replaced C15, C16 and the opamp (IC8).
An OTA has always been a little blurry for me.. specially the current/voltage behaviour and when which signal is current or voltage-driven, etc...
Why would the cap be charged so slowly?

Any kind of help will be greatly appreciated.

>Lorenz<

[Rectangular]

where'd you find that schematic on Mark's site ? I just flipped through it but I couldn't find any of the schems from this foot-control series of maestro pedals.  I've been looking for the parametric filter and fuzz-tain schematics for a while..

[Zero]

Hi

I just did a google search on his site: ("maestro site:hammer.ampage.org" in google search field)
There you should find a link to maestroworld.zip

I don't know if the parametric filter is in there though... good luck!

[Rectangular]

no, the maestro stage phaser isn't in that package. did you get this schematic from mark hammer, or find it somewhere else ? I'm really hoping the others have been scanned

[Zero]

I got from Mark's site... edit: Yeah, I think you're right... that zip just has the older phaser (with the color rocker switches)
But I also found some Maestro Schems here: http://www.dredgetone.com/Schematics/

[Rectangular]

hey zero, this is great, thanks for tracking those down ! I presume this is where you originally found it ? unfortunately its not the entire series, but I can't complain 

the thing with these foot-control series pedals though, is that they're useless to clone, because so much of what makes them special is the ultra heavy-duty machines cases/dials

[Zero]

You're welcome. I think I got the link from some thread in this forum. It was somewhere in the depths of my Firefox bookmarks

the thing with these foot-control series pedals though, is that they're useless to clone, because so much of what makes them special is the ultra heavy-duty machines cases/dials

You're absolutely right! I got mine off ebay... and I had read these things were huge... but, I mean this thing is HUGE 
And with the lights on the sides, it just looks awesome! There are actual light bulbs in there! Which explains why they are not active when you power it with batteries. I'll try to cover these bulbs with a small piece of colored cellophane or something to make it looks even cooler.

But first I have to get it workin'...

[Zero]

Alright! I've got some progress to report:

I desoldered the CA3094 (IC7) and replaced it with a CA3080 plus 2 2n5089 transistors in darlington (mounted them on a breadboard).
Now I finally get some sort of triangle wave on my scope! I can change the frequency of this wave when I use the speed pot.

But something is still not right. Here's what happens:
1) When I turn the effect on (an the LFO ramps up...) I get a phasing effect (I hear the sweep) for exactly one rising period, i.e. for about half a second. then it's "stuck" and doesn't sweep anymore.
2) The LFO waveform I get on pin 7 of the opamp (IC8) has a swing of about 4V peak-to-peak and a period of 150 microseconds (-->~7KHz!!) at the lowest speed setting. I can also hear some whistling ... I'm guessing somewhere above 12kHz (probably a harmonic of the LFO)...
**This would mean the "LFO" is actually above the audio-spectrum instead of below...**

According to the schematic, I should be reading  a period of 50 milliseconds at that location, which would be a frequency of about 20 Hz.. which seems much more logical to me. I triple-checked and even exchanged the 10 nF capacitor in the schmitt trigger (C17) ...

The voltage I measured over R55 (coming from the speed control) is between 60mV and 3.5V over the entire range of the speed pot. This would mean there's a current between 5 and 290 mircroamps controlling the OTA through pin 5.

I don't know if I should look for the problem (the LFO-frequency that's way too high) before the OTA or around the schmitt trigger/op amp after the OTA.

Does anyone see something suspicious? Any suggestions as to what else I could try??
I'm getting closer, but I really need some help--

Thanks,
>Lorenz<

Here's the detail of the schematic again for reference:

[Zero]

bumpin' on sunset  -- Wes Montgomery   

[Rectangular]

hey zero

I'm not really sure if my advice applies here. I was having some problems with the LFO of my phaser getting "stuck" when the ramp reached its peak, it was due to the board being under powered, so maybe X cap wasn't filling up and resetting, not sure. double check all your input voltages, make sure you're running everything at the appropriate levels. I've never seen a matched pair 3906/3904,  are they housed together or is there just some thermal epoxy between them ?

I might look into cloning one of these, I've heard one before, its really smooth effect

[Zero]

Hi Rectangular

I've checked the power, and there's about 9V on all supply pins of the ICs. But there are a couple of Zener Diodes that might need replacement. I've heard, the sometimes can have a higher voltage drop with age. Thanks for the tip... I'll check that out just in case.

I've never seen a matched pair 3906/3904,  are they housed together or is there just some thermal epoxy between them ?

They're not only housed separately, but are located like 2 inches apart on the PCB, so there's no kind of coupling.

[Zero]

SUCCESS!!!
It's alive, it's alive!!


I was already losing hope... The thing was first oscillating at around 6 kHz, then it would phase but just on the ramp-up...

It turned out to be a fried CA3094 OTA after all ... It was the first thing I had suspected. I replaced the OTA with a CA3080 and 2 n5089 in Darlington... I mounted this whole thing on a breadboard. I thought it was OK, but it turns out one the solid-core wires from the effect to the breadboard was broken inside       ****grrrrr*****

After that, it worked like a charm!
I then built myself a little McGyver-ish construction (excuse the blur):


It's basically CA 3080 and the aforementioned Darlington pair soldered (sandwich style) onto a socket to have the same pinout as the CA3094. It ain't pretty, but it sure works!
I was very happy to find out that my local electronics shop still sells 3080 OTAs for around 2$ (!). Nevertheless, I'll order a CA3094 from Smallbear soon, to tidy up things in there a little bit.

Anyways... enough tech talk. Let me tell you about the sound:
This thing's just awesome!! I'll be in the studio with my funk band next week recording an album, and this phaser is just heaven-sent! Can't wait!
It never wanted too many univibe sounds from this phaser, I prefer the slower settings. It's the coolest, funkiest phaser I've ever heard! I even prefer it over my Pigtronix EP-1 for chord work-- and the enclosure and the lights alone are a guaranteed jaw-dropper!

This forum has been a great source of information. Specially reading the posts about replacing a 3094 with a 3080 + 2 trannies helped me a lot. Thank you!

You can't imagine the amount of joy in my heart right now 

> Lorenz <

[Rectangular]

hey zero

glad to hear you finally got it working ! I might have to clone one of those afterall. 3094s are way too expensive, so I'd have to mock up 6 of those 3080/darlington monsters and hope for the best

too bad Maestro stopped making those, they're some of the best pedals around

rec

[StephenGiles]

Glad you got it working. I question the moans that 3094s are too expensive. My wife and I had a meal with her nephew and girlfriend recently in London - the bill was £212, that's $400 at the current exchange rate!!

[Rectangular]

one reason not to eat out, especially in england 

for me,  every dollar is a potential pot, IC, ldr/led,  or vintage transistor