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Ward
Copper Member
Australia
28 Posts |
 Posted - 06/18/2009 : 11:50:52
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I bought this thing off ebay not too long ago. It was advertised as the guts of the pedal but it came in a block logo box. I have had plenty of dramas with this thing, most of them are now fixed except for::::
The phasing seems to have it's own lungs, ah, that is to say, it will phase for a few minutes then it gets weaker until there's no phasing. Leave it alone for an hour and it's back working again but only for a few minutes. If I adjust the trimmer which is the bias pot I can make it phase nicely but then it gets weak after a couple of minutes, so then I adjust the trimmer again ( move it a bees dick to the right each time)to get strong phasing but it soon fades to jack again. This goes on until I get to the end of the trimmers range ( full clockwise) and then there's no phasing to be had at all no matter where the trimmer is set. I leave it alone for an hour and it all starts again. I have swapped out all the caps, no change, then all the 741 chips for TL071s, no change. I'm trying to find equivalents for the 2N5952 transistors to change next if I can get a matched set of them, how the hell you do that is beyong me. Am I missing something here, is the pedal rooted beyond repair seeing as I have tampered with the trimmer or is it indeed one or more of the trannys. The power diode tests OK so I'm not sure if it's had a power reversal somewhere. I'm very close to hurling this thing onto the road so the next truck may put it and me to rest. Anyone got any ideas as to why this thing is ruining my life. |
Edited by - Ward on 06/18/2009 11:52:54 |
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FRANZONI
Double Platinum Member
Ireland
3543 Posts |
Posted - 06/18/2009 : 12:40:46
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I've felt this way with certain pieces of gear over the years(recently with my musicman amp which gave me a belt of DC voltage which nearly sent me across the room during a repair..that hurt like a b*****d.... )... i find leaving something like this for a couple of days and coming back to it with a fresh approach can work better sometimes...... that said...i'm no techie but to me it sounds like somethings draining the power,it sounds like a battery dying type of thing so i think your on the right track with the power supply....are you running it off a battery or an adapter....? i think your best bet is to repost this the tech area ..some good guys over there who could probably help you with this...good luck and let us know if you solve the prob...... |
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Ward
Copper Member
Australia
28 Posts |
Posted - 06/18/2009 : 13:39:08
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| Thanks Franzoni, the pedal operates on battery only. I'll see how we go here before posting in the tech dept. |
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FRANZONI
Double Platinum Member
Ireland
3543 Posts |
Posted - 06/18/2009 : 14:37:56
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| I had some strange problems with a Dod chorus pedal i used to own which i ran off batteries at the time.... it turned out to be a loose 'rose' connector on the battery clip not making a tight connection... the pedal was coming on,but no chorus effect....i can't remember if it was draining batteries but something in your original post brought it to mind so it's possible it was as well.... as it was about 20 years ago my memory is a bit vague........ it was my first 'repair' on my gear..... |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 06/18/2009 : 15:10:22
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This type of fault is usually either heat related, or "charge" related. "Charge related" would be something like a very high impedance part of the circuit accumulating charge until the bias of something goes out of whack, then when you turn it off, it slowly discharges back into specification.
It's unlikely to be heat related because it comes back to life with an incremental turn of the trim pot.
FETs circuits can do that sometimes because they are high impedance, but I'm blowed if I know where to suggest to start looking for your problem. Helping with something like this remotely is pretty hard 
Replacing the FETs is pretty simple. You have to buy a lot of them (maybe 20?) and then match them as described here:
http://www.geofex.com/Article_Folders/fetmatch/fetmatch.htm
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Ward
Copper Member
Australia
28 Posts |
Posted - 06/19/2009 : 03:07:30
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Thanks Laurie, ignorance being my best friend, can you tell me where the 9V- goes please mate ( the triangle at bottom ? ) and just to be sure, can you point out which leads on the JFET are the S D and G? I think: D is the top one, S is lower left and G is lower right, yes?thanks/sorry. If I can find an equivalent for the 2n5952 I'll be in biz
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Edited by - Ward on 06/19/2009 03:17:27 |
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Dr. Bob
Moderator
Australia
6593 Posts |
Posted - 06/20/2009 : 03:36:49
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Hi Ward
Thanks for your PM.
It's usually quicker & better, to post these type of questions & requests on the forum.
As there are many other Tech's/Eng's that could possibly help you out as well.
I have edited you PM to reflect the main question/s.
quote:
Can I buy equivalent replacement trannys for 2N5952?
I'm still having dramas with my MXR script phase 90.
I'm awaiting the arrival in the mail of some TL071s.
I've replaced all the caps,
next will be the 741s, then if it still isn't right
I'll only have the 2N5952s plus one other (a 2N4126) which I have found an equ for at Jaycar,
I'm hoping you can find an equivalent to the 2N5952 that are available at Jaycar also. I hope this is no drama for you, thanks in advance for any help mate. Cheers, Nic.
First of all, Laurie has made you aware that you will have to match at least four of the FET's.
This can end up costing a few $$$ as you will most likely have to buy a small batch of them for the testing & matching.
Secondly, (initially) I would not be replacing the the 741's, with TL071's.
They are a different beasts, with different input impedance.
I would get the thing working & then socket the the opamps & replace them.
I'm guessing you will change the characteristics of the unit, but also admit, That I have never attempted such a mod.
Thirdly - This is not meant to be deeming Ward; But are you sure you replaced the components with the same vales & in the case of electro's with the same polarity?
From what I have read, you have all but rebuilt this from the PCB up.
Have you checked the value of all the old carbon composition resistors, may times they go out of spec, or just fail at the brass cap connections to the resistor's body.
Can you also refresh my memory, as to why you have undergone such a drastic rebuild?
I might have missed something in the thread.
Regards Dr. Bob 
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MXR script phase 90 pos
