Maestro PS-1A (Help!)

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aavatech1
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Post by aavatech1 »

Greetings, I recently found a Maestro PS-1A phase shifter pedal and it definitely needs some love.
I'm up to the task but I've got a couple of questions I was hoping some folks could chime in on.
Firstly after opening this guy up I noticed that the transformer had been disconnected. The 4 of the 5 electrolytic
caps seem to have been replaced but everything else looks original and nothing is missing or burnt.
I found the schematic I've uploaded here, online, but it's a "PS-1" schematic. But I've noticed some slight
differences between it and what I've got going here. For instance the bridge rectifier seems to be drawn wrong. (?)
I've never seen a bridge where the AC supply comes in between two sets of anodes. The board I have shows each secondary connections in between each anode and cathode of each set of diodes, like I'd expect.
My board has 5 electrolytic caps instead of 4 as shown on the schematic. One of my electrolytics is 22uF instead of the 4uF listed on the schematic. Cap disparities are "normal" as far as product changes in vintage audio equipment are concerned from my
experience but it's just another discrepancy I've noticed.
As far as the transformer is concerned I measured resistance between one secondary connection and the other and
got 86.0 ohms. And then approx. 43 ohms between each secondary connection and the center tap. Seems right to me.
The primary shows "OL". No continuity, no resistance value.
So that's about where I am right now. I guess that place to start with or without a correct schematic would be the transformer.
Is this guy toast? Are there possibilities for a replacement if it is?
Any help or comments would be greatly appreciated. I'd be great to get his phase shifter running again.
Thanks
Mark
P.S. I'm having trouble with attaching pics. They're only about 1 M or so each but they won't post for some reason. Apoligies.
Attachments
Oberheim_Maestro_ps-1.pdf
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Fender3D
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Post by Fender3D »

Welcome,
if one transformer winding shows no resistance, then it's open...
Replace it.

I got this schematic
phase shifter.png
It does not have transformer voltages, but, it has 12V zeners, then you can safely get any dual sec. transformer with the same zener voltage, 2x12V if your phaser is the same as my scematic or 2x15V if it's like yours.

aavatech1
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Post by aavatech1 »

Fender3D; thanks for the reply and the schematic! I think my phaser is like yours since it has the 1N759A 12V zeners.
I had been looking at 12-0-12 transformers and most seem a bit bigger than the one I have here tho they are available.
The higher the current the larger the transformer apparently. But mathematically how would you figure the current rating?
I know I've got 12VDC leaving the power supply circuit through a 270 ohm resistor and that gives me 0.055 mA current. Is that correct? Or should I double that? So maybe a 1 or 2 amp transformer would be sufficient. (..one thing I've learned about electronics is never to assume anything, just do what you can and ask lots of questions!) Thanks again Fender3D, appreciate the help.
Mark->

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Fender3D
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Post by Fender3D »

I guess the 2 zeners eat more current than the 4 op-amps....
As a thumb rule... they're 0.5W zeners, you'll need 1W transformer in order to burn them off...
Anything in that ballpark will be ok

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The G
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Post by The G »

If you don't need to keep the pedal in its original state, some alternatives could be to ditch the transformer+zenners and
  • use a normal 12V wall wart and a charge pump IC like LT1054 to provide -12V or
  • use a normal 9V wall wart and a boost-buck converter like this one on Amazon to convert it to ±12V or
  • use a normal 9V wall wart and LT1054 or the like to provide -9V; the circuit should work at ±9V after readjusting the JFETs' bias.

aavatech1
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Post by aavatech1 »

[Fender3D] unfortunately I'm not a the point where I can calculate how much wattage the power supply circuit will dissipate. I thought about it and I'm familiar with Ohm's power formula but don't know how to apply it in this case. I've looked around quite a bit but haven't landed on a transformer that I think will work, mostly size wise. The original had to be custom or possibly from some toy manufacturer or something because it's really small. So, the search continues. Thanks again for your help ; )
M->

[The G] I always like to keep things as original as possible hence my search for a viable transformer. But I had thought about the possibility of using a wall-wart as well. Your familiarity with those chips is far beyond mine but I'm grateful because I'll look up their spec sheets and familiarize myself with them. You never know, I may have to go that way in the end. I'm wondering if I were to go with the LT1054 if I'd end up with something looking like the boost-buck converter in the end. In which case I may as well do the B-BC. If I did use the B-BC would I bypass the old power supply circuit all the way past the 12V zeners? The +12V at the cathode and the -12V at the anode? And might I have to cut the circuit traces before that point? (....I sure hope not)
Thanks much for your kind suggestions.
M->

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Fender3D
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Post by Fender3D »

I don't know the exact power draw, but, as I said above, if you have 0.5W zeners on board, then that must be the max. power drawing (per rail).

I don't know either what's the smallest trafo available in stores, but one 2-3-5 W trafo will be enough.

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Post by aavatech1 »

Thanks Fender3D, got it!

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The G
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Post by The G »

I suggested LT1054 because it happened to be available to me and it worked just fine.
There is also the well known ICL7660S (mind the "S", the 7660 version oscillates at 10kHz, well into the audio range), but its current capability is ten times lower. It could work with this phaser, but I like me some room.
There are many other charge pumps.

You should not have to cut traces to bypass the power supply section, but you'd need to take out the Zenners and the 270Ω resistors from the circuit, or at least lift a leg of each one (which would lead to 4 parts hanging on one leg, not sure I like the idea). But the plug would be different, so I'm not sure converting the power supply to a switched-capacitor circuit can be reversible.

aavatech1
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Post by aavatech1 »

[The G] Thanks again for your input and info. I don't know why I didn't think about just removing some components from the circuit to isolate the unused components. Bench blindness maybe. Anyway thanks again for pointing out the obvious. : )
In regard to your statement "But the plug would be different, so I'm not sure converting the power supply to a switched-capacitor circuit can be reversible." I'm not sure I follow you. I looked up "Switched capacitor circuit" and I believe I understand what that is but not sure if I understand how that applies here. Are you referring to the wall wart and it's make-up?

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The G
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Post by The G »

The plug would be different when using a wall wart simply because the original connects directly to the power grid (120V or 220V AC)
maestro_ps-1a.jpg

, while most wall warts use the 2.1mm DC barrel connector
dc_jack_2.1mm_male.jpg
dc_jack_2.1mm_male.jpg (7.29 KiB) Viewed 120 times

which needs the corresponding female plug on the box
dc_jack_2.1mm_female.jpg
dc_jack_2.1mm_female.jpg (5.31 KiB) Viewed 120 times

The charge pumps (LT1054 and ICL7660S) are working by "moving charges into and out of capacitors when switches are opened and closed" (source: Wikipedia). So, all that was me trying to be fancy while saying that if you convert the effect to a charge pump power supply, you'll need to put a new hole into the box.

aavatech1
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Post by aavatech1 »

Thanks for the clarification "G". I'm still looking for a small 12-0-12 transformer that will work so I don't have to modify this guy.
A friend once told me "things can only be original once". So I go a long way in trying to keep things as original as possible.
But hey, ya gotta do what ya gotta do and your info is greatly appreciated. I may have to go that route or with the Boost Buck Converter. I don't work with a lot of IC's so building a little pc board and such for the chips would be new territory for me.
Either way we'll get 'er done eventually. I'll keep this thread open until it's decided one way or another.
Thanks again for your help. I appreciate it.
M

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