MXR Script phase 90 noisy.

[Ward]

Hey dudes, my first post here, been lurking for eons so anyway, hope someone can help me out here.
I have just bought an old 76 scrip phase 90, the thing works but when you don't play there is a pretty loud load of rubbish going on with it, sounds like a mini thunderstorm goin on. Also the ability to phase seems to come and go, I'll have the back off it and adjust the trim pot so it works good, then I go to it later and it don't phase or if it does only a little, then I adjust the pot again and get it going again and then etc etc.
I have sprayed contact cleaner on the pot so I think it's good there and besides, sometimes I can't get it going at all so I think that pretty much rules out the trimmer? Then like magic, it comes good again, bad chip maybe or,,,,,,,
The noise sounds very much like when I put a tant cap is ass about once in another pedal so I was thinking it might be a bad cap? Could this also explain the coming and going of the phase ability. I only have a DMM and although I've built a few tube amps, I'm no techy type.
Edit:
Just came back from playing through this thing, this is the first time I have turned the phaser on and as usual it is not working but I just kept playing, after a couple of minutes it starts to work, little by little until it is working fully, now if I unplug and then plug in again, it is not working, then after a few minutes it works again, so something is leaking in there and requires some kind of power build up or charge up once the power has been turned off. I really like/prefer the tone of the pedal when it is only half working, sort of like a real shimmery chorus but really suble, then when the thing starts phasing properly I wish it just stays on half so to speak, anyway, who knows, once it's fixed the phasing might not be so full on, is there a mod where you can hook up a pot to control the phase volume, that would be great. The noise is ever present no matter what as long as the thing's engaged.

[Ward]

Fix it. It was one of the Tants. As for getting the sound I like out of this thing, it's all in the bias adjustment, I think i'll need to change the trimmer value though because it's so hard getting it where I want which is much more subtle than having an over the top phase thing going on, maybe I'm not a phase type of guy but setting the bias so you can only just here that the pedal is doing something sounds awesome to my ears.

[Mark Hammer]

As near as I can tell, the tantalums were used on the early MXR pedals simply because they were smaller than the same value of electrolytic, and they could be laid on their sides to make a low-profile board. These days, you can get electrolytics of the same value in a package every bit as small as the original tants.

[R.G.]

... I think i'll need to change the trimmer value though because it's so hard getting it where I want which is much more subtle than having an over the top phase thing going on, maybe I'm not a phase type of guy but setting the bias so you can only just here that the pedal is doing something sounds awesome to my ears.

If you can get the sound you want by messing with the trimmer, great.

But there is a more direct way of just lessening the phasing. The P90 has a final PNP transistor to mix the dry and phase-delayed sound. For deepest phasing, there are two resistors which must get the signals to cancel almost perfectly. If you deliberately mismatch the mixing resistors, you can dial the phasing down to zero or up to as full as you get with it stock.

As near as I can tell, the tantalums were used on the early MXR pedals simply because they were smaller than the same value of electrolytic, and they could be laid on their sides to make a low-profile board. These days, you can get electrolytics of the same value in a package every bit as small as the original tants.

I think that's right Mark.

I also believe this to be an illustration of what I was talking about on the other forum - tantalums go bad over time too.

Ward-dude: if you like this phaser, replace **every** electro cap in it. They're due for failure.

[Ward]

RG, thanks for the reply, can you please identify those two resistors you're talking about mate. Also, by adjusting the trimmer I have this thing sounding more like a chorus than a phaser but in saying that, it's not really like a chorus but just adds some shimmer on top with a dash of modulation if I can use that word, different to any chorus I've heard but I really like the sound of it. Anyway, if you can show me which resistors you're talking about I'll have a go at it. Thanks again and I will swap out the other tant as well.
[IMG][/IMG]

[Ward]

Oh I guess it must a secret.

[D-Man]

I would like to know too

[Ward]

Ok, have brain will do not a lot. Just call me slow, I had a look at the board again and there are two 150ks next to each other. I decided to just pull one out (I guessed of course but got lucky first time) and solder in a couple of wires then exp with different values. I don't have a mini pot handy so I decided for the time being to just put in a DPDT mini switch with two different values, I ended up with 100k for a slightly less than stock volume of the phase and a 68k for even less again. I hope to find a pot soon, I think 100k will work OK seeing as I'm happy with the 100k resistor for the loudest phase, I can find a 100k pot but not 150 or 250, the next one up is 1meg in those little green pcb mount pots so I suppose 100k will have to do. See the pic for which resistor it is. Cheers amd thanks to RG and also Mark Hammer for the info here Phase 90 mods

[Ward]

Ok, while I now have the volume of the phase sorted, the original problem is still present, ie, the phasing is good and strong but only for a minute or two, then if I move the trimmer clockwise just a tad I get good phasing again, then it dies out again, then I move the trimmer and get it back, then it dies out again etc etc. This goes on until I reach the end of the trimmers range, now I can get NO phasing at all no matter where the trimmer is set. So now I leave it for an hour and then it works again and so the cycle continues. I have replaced all the caps, all the 741s. The only thing left to do now is change the fets and or the pnp tranny. Can someone please advise if this is the next step, well obviously I suppose it is, not obvious at all really that's why I'm asking. I'm having trouble finding an equivalent replacement for the 2n5952 Jfets. I have a pile of J102s but I'm not sure if they are a good choice. I can source a PN200 in place of the 2n4126 but need to find a replacement for the 5952s and also matching them is not something I'm jumping out of my skin looking forward do. I won't be able to get into town for a while so I can't swap out the 2n4126 just yet. Is it more likely to be the Jfets? and if so, can someone pleeeeaaaasssseeee tell me what is a good/common easy to get replacement. Thanks.
edit: I just discovered that I can source 2N5458, will they do the job of the 5952?