Grant Muller

Maestro PS-1B Teardown

maestrops-1b-1 A friend of the band recently had a problem with his Maestro PS-1B, a phase shifter from the early 70’s designed by the famous Tom Oberheim, and asked me to take a look at it. Based on the problem, a loud humming at the output with no sign of any other signal, I figured it should be an easy fix. It was probably just a ground fault or a bad output or something, so I took it home with me and cracked it open.

After getting the case off (which was bolted together with rarely used square head nuts), I flipped it and took a look at the circuit board. I was a little awed, first by the simplicity, then by the age of some of the components. The board was downright sparse. I thought to myself “they could have made this a lot smaller if they had just move everything closer together…”. Here is a shot of the top board:

maestrops-1b-5

I noticed that the ground pin on the plug was broken. I spliced in a new power cable and turned it on, plugged in a guitar and my Ritz amp and had a go. Seemed fine. Nice and wobbly, quiet and phase-shifty. Ha, Knew it’d be an  easy fix. I ran the new cable into the back, soldered everything into place and turned it on to test one more time. BUZZZZZ.

That’s odd, I just tested it a second ago and it seemed fine. After checking the solder joints I just made, I turned the unit on again to see if it was still buzzing. Yep. Then I noticed an anomaly, there were two components on the board that didn’t look…native. At the top left there were two capacitors that looked far newer than the rest of the components:

maestrops-1b-12

This has been repaired before.

So I did the naturally stupid thing and poked them. No buzz. For once, doing something stupid yielded a solution. The problem was a loose solder joint, one of the caps was literally falling out of the socket. I must have inadvertently been pressing the capacitor back into its slot when I had the cable spliced in. Lucky, lucky catch (though I would have found it anyway, after poring over the other side of the circuit board).

I pulled the board out and turned it over to fix the joint, and was greeted with the strangest circuit board traces I have ever seen in a production machine:

maestrops-1b-7

It looks like the solder traces were drawn on with a pencil. Like the design was made casually by hand, without a ruler on a cocktail napkin with a crayon. Incredible. I looked closely at the verbiage on the board (hoping to find a production year), and instead found the name Oberheim

maestrops-1b-8

The same Tom Oberheim of Oberheim synthesizers, who went on to create the first polyphonic synthesizer, the OB-X, and the DMX drum machine (not the rapper). Neat. I wonder if all circuit designs during this era we as freeform and organic.

After sitting in awe for a moment, I bundled everything back up, tested it a couple of times and called it fixed. I did a little research on this unit and came up with this site, which has plenty of history on the PS-1, and other effects, not to mention Maestro and Tom Oberheim.

Lessons learned

  • Don’t assume there is only one problem
  • Do something stupid every so often, it might work.

And here is an unnecessary shot of my test setup:

maestrops-1b-13

10 comments

  1. Hello… Just above the 470 cap… there are 4 small caps?…. I have a ps-1a with one broken… it has 1N4 002 and GI on it…. Would you please tell me what it is, and how I might get one? Thanks, Jimmy

    1. The 1N4002 is a ubiquitous diode actually, not a capacitor. I think you can even get these at Radio Shack for like $0.15, but Mouser and any of the online bad boys carry them too. To make sure you’re looking at a diode, peek at it again and make sure that you see a solid bar on one side, this is an indicator of polarity. You might take a picture and send it to me so I can be sure we’re talking about the same thing…

      While this is an easy thing to replace it’s usually an indicator of a different problem. I wouldn’t be surprised if I put a new one in just to see it blow out again. It’s worth the money though to at least give it a try, if it blows again you’re only out the 15 cents or so, and you know that the problem is “upstream” of that (which I might be able to help you with.

      Let me know,

  2. Hello Grant, Thanks for the information and quick reply. Yes, that is it. I looked up a photo and you are 100% correct. The part looks as if it has dried out and shattered with age. It does not look burnt. I will keep you updated on this. Thanks ! Jimmy

  3. Hello Again, Problem Solved ! Radio Shack had the 4001,4003 and 4004 but not the 4002. I found some NOS 4002’s, on eBay. Works like a charm. Thank you for your help. I owe you one ! Thanks, Jimmy

  4. Hello Grant, I have a Sunn 1200S 100 watt tube amp. The amp is quiet. The volume level at max seems about 1/2 or less of what it should be. Does this sound like a transformer? The components look very clean. Thanks, Jimmy

  5. Hello my friend,

    I’m going to ask what you might find a stupid question, but does your pedal really work ??? According to the first picture that you provide, it seems that your circuit is missing one trim pot and a total of six resistors, when comparing with pictures of the interior of PS-1As that I collected from eBay listings. I also have the original schematic of the PS-1A/1B, and it really looks like your pedal is incomplete. Did you make modifications to the pedal ?

    I have a defective PS-1B, and it is missing the exact same components as yours. I don’t know if I should add the missing components…

    Any clues ?

    Thanks !

  6. Grant, You may be the life saver I’ve been looking for. I bought a Maestro Phase Shifter back in ’72. At the time, I thought it was the greatest effect pedal available. About 5 years later I installed it into my Acoustic amp thinking I would buy a foot pedal for it and have no need to take up floor space. Not to mention the fear of the plastic buttons breaking during performance. Anyway, I found the foot pedal was not available anymore and, the amp went into storage. Now, for old times sake, I want to build a foot pedal with three switches and bring the amp/pedal back. I know I need a molex connector which is no problem but I’m not sure of the wiring configuration. Do the switched wire across the top to the bottom row in pairs from the left to right ? Thanks in advance for any information you may provide. Jimmy Hicks

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