Script logo Phase 90 not phasing very well

[Jasonvilla]

Hi all. I recently cleaned up and installed a true bypass on my '75 phase 90. I cleaned out 40 years of melted foam in the process. Well something I did pissed it off because it isn't phasing very well anymore. Everything works fine except the depth of the phase doesn't seem to match what it used to. And when its on the slowest setting I can't even hear a difference in tone. Highest speed seems to at least tremolo with minimal phase shifting. The volume is the same with the effect on or off, so I'm not noticing any volume loss. Theres 9.5v going into it. The diode is working fine. I checked c15 to see if it was open and its fine. The bias trimpot has 5v going into it and 2.5v going out which seems good. I've also tried adjusting the trimpot. I checked all of the voltages that are listed on the http://www.generalguitargadgets.com/...0-script-logo/ instructions. Most everything looked about normal except:
Pin 6 on IC2-IC5 is 5v on my unit and apparently it should be 2.1v.
Pin 6 on IC6 goes from 1.8v to 8v on mine. Instructions says 4v. Not sure if this is normal.
The only other thing is the collector on Q5 is 3.2v on mine and instructions says 4.3v.
Sorry for the long post but I wanted to try and cover everything I could. I really appreciate any help!! Thank you!

[52 Bill]

Welcome to the place. Until Mark Hammer checks in, I'll post a thought.

Measure the dc voltage at the gates of the fets. You should see a fluctuating dc changing at a rate set by the speed control. The least amount of voltage will be set by the bias trimmer.

Some MXR pedals had a trim pot that was soldered to keep you from adjusting it at after it left the factory. I have see some of the trim pots get fouled up from the melting foam.

[Mark Hammer]

You made me chuckle.

Bill pretty much covers it. Unless you or anyone else has actually changed some parts, your difficulty most likely lies in the trimpot setting. The recommended voltages are certainly not to be avoided, but neither do they necessarily represent "best".

T'wer I, I'd set the thing for a medium fast throb - fast enough that you'll be able to easily hear how wide it is sweeping, but slow enough to be able to hear where the sweep range is - and play with the trimpot until you hear something you like.

I will note that the trimmer has a couple of functions. Its first function is to bias the FETs into a range where they all change drain-source resistance with the LFO, but its second function is to replicate a characteristic sweep range (i.e., a signature P90 sound). You can actually move the range where it sweeps up and down with the trimmer, although if you move it up or down too far, you get little or no sweep.

My point is that you should fee free to dicker with the trimmer in search of something you like the sound of. It won't hurt anything, and you can always reverse it.

[Jasonvilla]

Hey guys, thanks for the replies. I actually registered to this forum because you two helped someone out a few years ago on his phase 90. Haha
Bill, am I measuring across the gates or relative to ground?
I've already played with the trim pot. If it moves much too much to the left or right the tremolo effect will stop. Its set for full term effect right now - which is actually where it was soldered from the factory.
I have some 741s on ordered... incase I have to replace any.
Thanks for the help!

[52 Bill]

Bill, am I measuring across the gates or relative to ground?

Either way. If you measure from gate to source you will read the voltage from the LFO. If you read gate to ground, you will read the output from the LFO and the voltage from the bias trimmer together. You just want to see that there is in fact output from the LFO and about how much voltage.

If you have already adjusted the trimmer for the deepest effect, just leave it there for now. Seeing that the unit passes signal, you can assume that IC1 and Q5 are working and that the problem lies in the phase shift sections IC2, IC3, IC4, IC5 and IC6. IC6 is oscillating or you would not here any effect at all, so that leaves the last 4 chips.

Because all of these stages are essentially direct coupled, the voltages will all influence each other. Try reading the voltage to ground at each input and output pins (2-3 and 6). Compare the readings and see if any one of the chips is different from the others. The difference may be subtle.

[Mark Hammer]

Based on the sonic result you describe, there is a relatively short list of things that could be wrong, as I see it:
1) The bias is not set right so the FETs are not changing their drain-source resistance very much as the LFO sweeps.
2) The LFO output is limited resulting in little sweep to be had.
3) The phase-shifted signal is being mixed with clean at a much lower level than it should be, resulting in faint phasing sound.
4) One or more of the JFETs is shot and not tielding much of a change (or any) in phase-shift, during the sweep cycle.

I'm confident that isn't likely to be everything, but it sure covers the most likely possibilities. It appears we can cross #1 off the list, so far.

I'm a big fan of cleaning the flux residue off the copper side of the board so one can clearly see any cracks, cold joints, or solder bridges. There are a few things one can use to do this, but I prefer (rightly or wrongly) methyl hydrate.

[Jasonvilla]

Well I just changed out IC2 - IC5 with the same results That was a lot of work for nothing. I guess those weren't the problem! It still is doing the tremolo effect without the phase. IC3 had SLIGHTLY different voltage at pin 3 than the others. Could that be a problem? Thanks guys!

[52 Bill]

Well I just changed out IC2 - IC5 with the same results That was a lot of work for nothing. I guess those weren't the problem! It still is doing the tremolo effect without the phase. IC3 had SLIGHTLY different voltage at pin 3 than the others. Could that be a problem? Thanks guys!

And there's the reason why shotgunning circuits is not the best way to approach troubleshooting. Back to square one.