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fuzzface
Bronze Member
United Kingdom
121 Posts |
 Posted - 06/23/2006 : 19:45:01
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Hi all,
Sorry for the off topic (non Boss) nature of the post but I just wondered if anyone could help me out with a problem I have with an MXR phase 90 (script model).
Basically mine doesn't work. There's a signal through it when just plugged in (I think these are true bypass) but nothing when the switch is pushed.
Does anyone have or know where I can download a schematic for this circuit? Also if anyone else owns one of these and could take some good close up photos of the circuit and wiring I would be eternally grateful!!
Thanks in advance for any help!  |
Edited by - fuzzface on 06/23/2006 23:52:40 |
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stahlhart
Platinum Member
1318 Posts |
Posted - 06/23/2006 : 21:00:52
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It's going to depend on which one you have -- there's the original one made by MXR, then the reissue from Dunlop, and I think that there are variants also (the ubiquitous "script logo" Bud case model associated with EVH, the reissue Dunlop with the switch for the resistor mod that may or may not more closely emulate the "script logo" version's sound, depending on which elitist type you ask, etc.
You might want to check, I think, at http://www.geofex.com -- I've seen the schematic(s) somewhere in my travels; as soon as it comes back to me, I'll check in again...
Original MXR units won't be true bypass, by the way -- they've got the old SPDT arrangement. You can verify which one you have by counting the solder lugs on the switch there will be six if it is, three if not.
edit: or nine (3PDT), if a separate circuit is used to switch an LED indicator on and off
edit: here's some more info... http://www.elixant.com/~stompbox/smfforum/index.php?topic=37893.0
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Edited by - stahlhart on 06/23/2006 21:06:47 |
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fuzzface
Bronze Member
United Kingdom
121 Posts |
Posted - 06/23/2006 : 22:16:08
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Thanks Stahlhart - some very useful info there.
The one I've got is one of the very first Phase 90's - Script Logo Bud Box. It only has the 3 connection lugs on the switch so it must be an SPDT without true bypass.
I'm kind of stumped with it to be honest as there are a few wires inside that have come off their solder joints so I'm not sure which one goes where. I've been trying to find a photo on the net of the wiring inside but not found one as yet.
I'm desperate to get it going as I love the sound of these!  |
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stahlhart
Platinum Member
1318 Posts |
Posted - 06/23/2006 : 22:21:32
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You've apparently got yourself the Holy Grail of phasers then. Kudos... 
I've seen photos somewhere -- give me a moment to noodle on this -- actually phostenix and I were having a private e-mail discussion about these, and I'm certain that I've got links to board photos in there somewhere. When I get home tonight I'll do some digging. Stay tuned...
You might also want to try and contact some of those guys in the forum link -- Aron Nelson would probably be the first one I'd try to reach. You might not get a response, as he probably gets a lot of mail from strangers, but you never know.
edit: how about this? http://www.panicobros.com/Strumenti/phase90script/p90script.htm
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Edited by - stahlhart on 06/23/2006 22:28:40 |
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fuzzface
Bronze Member
United Kingdom
121 Posts |
Posted - 06/23/2006 : 23:41:41
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Stahlhart you are genius! That website link was just the sort of thing I'm after, I also need a photo of the underside of the PCB to compare the wire connections but those photos are a great find - thanks man!
It's very useful to compare mine with those photos actually and rather spookily the pot code on that Phase 90 is exactly the same as mine so they were probably made in the same batch! 
I was very lucky to find this pedal yes. A friend of mine who owned a studio years ago was clearing out some things from his loft and came across a bunch of old broken equipment from the studio and the Phase 90 was in there! Couldn't believe it when I saw it - it's in mint condition (cosmetically) - just a shame it doesn't work at the moment! But I'm hoping to rectify that soon! When I opened it up and found out it was a Bud Box version I felt like I'd struck gold - I gave him a fiver for it (about $9)!! Sadly there weren't any other pedals in the collection (I was hoping he might have a SP-1 or a VB-2 stashed away or something!  )
I've heard these pedals before and I absolutely adore the phase sound of them so I'm really hoping I can get it going again! |
Edited by - fuzzface on 06/24/2006 00:03:56 |
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stahlhart
Platinum Member
1318 Posts |
Posted - 06/24/2006 : 00:17:29
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If you don't have a photo that traces out the individual wires, you should still be able to accomplish this by reading the schematic.
You've got the input and output jacks, the battery, the foot switch, and the speed control -- that's it.
Make note of all of the wires that are still connected, and that will leave the ones that aren't, and you should be able to see where they go by process of elimination.
I seem to recall from looking at the inside of my Micro Amp, which is an original MXR unit, that all of the external wires from the circuit board are along one side of the board. You follow the foil trace on the board from those contact points to the component on the other end. If you aren't sure what the components are, describe them to me and I should be able to help you get more specific. The resistors --\/\/\/-- will be cylindrical with coloured stripes that are a code for the resistance value and tolerance (the brown-black-orange-gold-yellow ones are 10K ohms 5% tolerance (or 10% if it's a silver stripe instead of gold), for example). The capacitors --|(-- will usually be flat discs, or rounded edge rectangles, possibly with a stripe or a (+) marking one of the leads (this is the outside foil of a "wrapped" capacitor or the positive lead of an electrolytic capacitor, respectively) and the numeric value printed either explicitly or coded... the caps are the green, red, and brown parts in the linked photo (greens and brown are ceramic discs, reds are tantalums). The integrated circuits -- the black plastic rectangles with eight leads -- have the pins read by looking down from the top with the notched or pitted side to the left, and then you read 1,2,3,4 from left to right along the bottom and 5,6,7,8 from right to left along the top (for an 8-pin IC). The transistors will usually be 3-dimensional "D" in shape with three leads (gate, drain and source in the case of the field-effect transistors in the Phase 90; you'll probably need the schematic to identify the leads exactly, but they'll be the same for all four FETs, so you should be able to see it.
I also see what looks like a couple of diodes -- that black cylinder in the middle, closer to the bottom edge in the photo, and the orange one in the upper left corner. The black/white flat round guy is a trimmer potentiomener, and will be the resistor with the arrow pointing to it (third lead) in the schematic.
I doubt that you have any bad components on the circuit board unless it was abused by an incorrectly applied supply or input/output signal voltage -- nothing on the Phase 90 board that I can see is particularly sensitive to static electricity. You probably just need to trace correctly the disconnected wires, correctly apply power, and can then start practicing the palm-muted intro to "Atomic Punk".
Good luck. If I find any other photos I'll post them here...
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stahlhart
Platinum Member
1318 Posts |
Posted - 06/24/2006 : 00:29:46
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A better schematic than the dumbed-down one on Geofex:
http://www.tonepad.com/getFile.asp?id=42
edit: you'll have to combine the two -- this one has been modified with the addition of an AC adaptor input and true bypass switching... but the essential phasing circuitry looks identical
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Edited by - stahlhart on 06/24/2006 00:45:07 |
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stahlhart
Platinum Member
1318 Posts |
Posted - 06/24/2006 : 02:16:21
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Here's that other link I had in my e-mail... unfortunately, it's the same photo from the Italian site (though it's larger and more detailed):
http://www.erikhansen.net/diy/phase90diff.htm
Apparently no one has yet thought of taking a photo of the underside of the board where all of the wires are soldered...
edit:
Take a close look -- the 10K resistors in the phase blocks are 5%, and in other less critical parts of the circuit, they're 10%... MXR made the decision to only put the tighter tolerance ones where it's critical (note that the FETs have to be "matched" with one another in order for the phasing to occur)... was there really that significant a difference in cost between 5% and 10% 1/2W resistors back in the 1970s, that it wouldn't have been cheaper just to order a larger quantity of 5% ones and use them throughout, as would undoubtedly be the case today? We've clearly gotten better at manufacturing them over the years.
Apparently so -- look at the block logo photo. 5% carbon film resistors everywhere, instead of the old (probably Ohmite or Allen-Bradley) carbon composition ones in the script logo -- we need phostenix here to tell us all about how much vintage "mojo" the carbon comps add to the circuit.
I wonder if those green disc caps needed to be matched also. (?) Those are probably 10-20% tolerance at best.
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Edited by - stahlhart on 06/24/2006 02:33:09 |
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diggum12
Silver Member
USA
282 Posts |
Posted - 07/07/2006 : 18:53:47
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I have personally had no less than 3 of those Phase 90's come across my path with the same problem. Try this fix first:
There are two diodes above the battery clip. The top is a Zener, the bottom is a 1N914. Replace them both. Make sure the line marking the cathode/anode matches what's on the board.
I'm 95% certain that will be your fix!
Jono |
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diggum12
Silver Member
USA
282 Posts |
Posted - 07/07/2006 : 18:55:49
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Here's a good schem.
Download Attachment:  p180.gif
22.54 KB |
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diggum12
Silver Member
USA
282 Posts |
Posted - 07/07/2006 : 18:57:18
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Uggh, that looks terrible in HTML. Save it to your desktop as a .gif.
Cheers! |
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fuzzface
Bronze Member
United Kingdom
121 Posts |
Posted - 09/13/2006 : 01:57:57
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Thanks everyone who's helped me out with this, I'm really grateful for all the advice and links
I've not looked at it for a while but tonight I thought I'd have a proper go at fixing my poorly Phase 90.
I'm afraid I'm a bit of a novice with electronics so here goes!:
The board in mine is virtually identical to the one in the photo on that link.
Following Diggum12's advice I tested the Zener Diode at the top of the board with the Diode tester on a Digital Multimeter. I'm getting a reading of '541' when I measure it and nothing when I test it in reverse. Does this sound ok? If I was to replace it, what value of Diode would I need as I can't see any numbers on the diode itself?
I may have to see if I can get someone else to repair it if I have no luck but I'm not sure who would be able to fix one of these pedals. I thought about contacting Analogman but it looks like he doesn't touch these pedals.
Thanks in advance for any help anyway! |
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stahlhart
Platinum Member
1318 Posts |
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Topic |
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MXR Phase 90
p180.gif