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ranjam
Bronze Member
Canada
82 Posts |
 Posted - 02/03/2010 : 04:01:35
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| Hello! I am looking for a proper MXR Phase 90 schematic. This one is pretty old, and has six 741 IC's, four 2N5952's, and a 2N4126. Any leads would be greatly appreciated. Thanks in advance. |
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Dr. Bob
Moderator
Australia
6593 Posts |
Posted - 02/03/2010 : 07:27:07
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Hi ranjam
My cupboard is bare on this one. 
Regards Dr. Bob  |
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jack
Platinum Member
USA
1418 Posts |
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Pedal Dan
Silver Member
USA
419 Posts |
Posted - 02/03/2010 : 16:50:36
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quote:
Originally posted by ranjam
Hello! I am looking for a proper MXR Phase 90 schematic. This one is pretty old, and has six 741 IC's, four 2N5952's, and a 2N4126. Any leads would be greatly appreciated. Thanks in advance.
Hi ranjam, how old? Bud box, script? I understand you can aproximate the date by the # on the pots. Mine dates around sep. 74 w/ Bud box. |
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ranjam
Bronze Member
Canada
82 Posts |
Posted - 02/04/2010 : 01:09:06
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I am horrible at bragging about what I have, so I am bad at guessing the age of this pedal. It is so beat to crap it could be from the Mesazoic period for all I know. What is left of the logo on the case appears to be 'block' (so it ain't that old), it's single knob, with a pot code of '056244' and the 500K, and then '020'. The circuit board has '101' and '102' as a solder trace.
You now know as much as I do about this Phase 90. I was trying to fake my way through it, and I could trace a signal through all but one IC (input at pin #3 and output at pin #6), so I changed one 741. It didn't do anything to alleviate the problem. You get bypassed signal, and in effect mode you get a plain signal with a slight boost. No phasing.
I'll find a schematic, or try changing the closest FET, or the tantalum capacitor. Something has to make sense, and eventually make this sucker work. |
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verivorax
Platinum Member
Canada
1185 Posts |
Posted - 02/04/2010 : 02:56:35
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oh man.. a guy came to me today to show me a killer '75 Script Phase 90 he got off craigslist - the seller was selling it as a reissue, not knowing the actual value of a real "Bud Box" vintage MXR product, so the guy I know got an insane deal (something under $100).
I've never played a real vintage Phase 90 before, but it was just about the nicest phaser I've heard (maybe with the exception of my favourite, the PH-1r  ).. |
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Cobra94
Copper Member
Canada
2 Posts |
Posted - 02/04/2010 : 06:12:49
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ranjam
The fact you are getting unphased sound is a good thing (no burnt out components). Sounds like you may need to adjust the trim pot that controls the jfet bias. Move it in very small increments (starting from the half way position) either CW or CCW. It has a very small sweet spot, miss it and the pedal doesn't phase. |
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ranjam
Bronze Member
Canada
82 Posts |
Posted - 02/05/2010 : 01:00:45
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Weird, but I'll try anything once. Twice if it doesn't hurt  . Maybe the track got really dirty. But....... no, I was going to say 'how did the original owner misadjust it?', but if it got gummed up from being in smoky bars or having a beer spilled on it, it all makes some sense. I'll clean the beejezus out of the pot, try adjusting it while I am playing, and let you know. Thanks!  |
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ranjam
Bronze Member
Canada
82 Posts |
Posted - 02/05/2010 : 04:49:28
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Yee - freakin' - haw! Although, I shouldn't brag just yet. Cleaning the pot, and adjusting it as I played through it didn't seem to work at first, so I scoped it out as I adjusted it. There were very small spots that made the sine wave 'jump', so next I took two decade boxes and added them in parallel to the pot, with a common center. There is only one combination that works, but it does work.
So, it's clean, clean, clean, curse, clean, clean, clean, curse, and clean some more. I may just buy another trim pot, but for tonight it's a huge step. It seems to work now, but breathing on the trim pot makes it 'die' again, and it takes literally a few minutes of caaarefuuuuuully nudging the trim pot to get it back. So, maybe a new trim pot is all it cost me for this. Well, plus that one 741 IC that turned out to not be the problem. I feel pretty proud right about now. |
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Pedal Dan
Silver Member
USA
419 Posts |
Posted - 02/05/2010 : 05:26:52
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Congratulations Ranjam!!! Glad to hear you got the old girl hummin'! Percistance pays off and you'll have the best phaser of all time to enjoy! |
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Dr. Bob
Moderator
Australia
6593 Posts |
Posted - 02/05/2010 : 13:10:03
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Hi ranjam
Good work.
Maybe post a couple of pics in the Pedal Internals Thread for us?
Regards Dr. Bob |
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Cobra94
Copper Member
Canada
2 Posts |
Posted - 02/05/2010 : 20:47:22
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You're welcome!
Depending on what vintage this pedal is the trimpot may have been soldered in position. MXR used to do that with all metal type trimpots so maybe that is why it is jumping. Replacing it with a multi-turn pot will allow you to dial in the right value with more stability.
Good Luck. |
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ranjam
Bronze Member
Canada
82 Posts |
Posted - 02/06/2010 : 01:13:00
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Keep in mind I have never bragged about my photographic skills. This is because I have none to speak of. But, here I go trying to balance the pedal in one hand, take a picture with the other hand, try and peak over to the view finder, and get a focused photo. Yeesh.
While not great, maybe you can make out the one lighter grey colored 741, which is the one I changed. Right below that is a tantalum capacitor I had envisioned changing as a possible fix, and right in front of that is the trim pot.
That's all I know. It still seems to work, knock wood. I might just go ahead and order a trim pot, and have it handy. Now I have to think about what to do with the case. Transplant the board? Repaint the case? Hmm, choices, choices. |
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Pedal Dan
Silver Member
USA
419 Posts |
Posted - 02/06/2010 : 02:39:33
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| Yo ranjam, if it was mine I wouldn't change the orig. Box as my 74 looks new.Other than being a Bud box it looks like the 74 reissue. My 79 line driver/gate however is completly beat to f**k, it gets way more comments! Imagine the places your Phase 90 has been! If it could talk the stories it would tell! Mojo ranjam! |
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'Proper' Vintage MXR Phase 90 Schematic?
