EVH Phase 90-- Script Mode Failure?

[Cruton]

Hello DIYSB,

I have an MXR EVH Phase 90 in for the usual repair-- the owner thought any old PSU would do and blew the 5.1V Zener, took out the TLC2264 that handles the 4 phase shift stages in the process. I've replaced both, and while it now functions as it should with the Script switch in normal mode (switch "out"), when I put it in Script mode it just wails like a police siren. More specifically, it just produces a very, very loud tone that oscillates in pitch with the LFO. You can hear it even if the effect is bypassed (though it's much quieter).

The Script switch appears to be working correctly, and based on what I can gather, it just swaps the 24K feedback resistor with a non-polarized cap of unknown value (it's SMD). I know that people often just clip out the feedback resistor for the script mod, so (1) I'm kind of confused about how swapping it with a cap compares and (2) I have no idea what could be wrong to cause it to squeal when the cap is in the feedback loop (Script mode), but work fine with the 24K resistor (normal mode).

I know at least a few folks on here are Phase 90 gurus-- any pointers on where to look next for the problem are greatly appreciated.

[Mark Hammer]

Feedback in a phaser can't be too high or else it starts to oscillate.  Keep in mind that each of the phase shift stages is supposed to be unity-gain, but even small tolerances in the input and feedback resistors in each stage can give it a hint of gain (e.g., 1.05x).  Once you take such error into account across 4 stages, more than just a bit of feedback can be risky.

The difference between Script and Block was really the amount of feedback resistance, and not a changeover from resistor to cap.  One of them was set for a little wider sweep and a little more feedback - optimized for slower sweeps - while the other had a bit less feedback and slightly narrower sweep - optimized for faster speeds.

Possible that the Script switch results in some sort of short or over-the-limit amount of feedback?

[Cruton]

Thanks for the help (yet again!), Mark. It makes sense to me that excessive feedback could cause the oscillation if the gain is not < 1 across all 4 stages. I wonder if a resistor was damaged in the original incident so that once I replaced the TLC2264 the feedback is just over the top in Script mode. I can't imagine it's going to be very fun to track that down...

I hear what you're saying about the difference between Script and Block-- everything I've ever read suggested that it really is just the amount of feedback resistance. It's still odd to me that the common mod is "clip feedback resistor", which to me would mean no feedback (well, at least along that path... I'm no EE), whereas the EVH "Script mode", which switches in a cap in place of the resistor, would be like all the feedback (with perhaps some filtering I suppose). Perhaps I just understand feedback less than I thought, or perhaps the script/block switch in the EVH is doing something a little different than the common DIY script mod. Not sure... and in any case, not sure what to do to remedy it. Maybe just solder in some series resistance with the Script mode cap and see what happens? Maybe run leads to a pot wired as a VR and crank it up til the oscillation dies and measure how much it took to get there...

[Cruton]

This thing has been sitting on my desk, taunting me, and today I picked it back up with the faint hope that something might jump out to me that I'd missed before. Recalling Mark's wisdom, I was looking yet again at the feedback path, at the 24k feedback resistor and the unknown non-polarized cap that the "Script" switch selects between, and I just happened to notice-- the spot with the cap in it was labeled "R6"!! You can see this just below the bias trimpot in the photo below.



So I took to the web looking for info on R5 and R6 on these boards. The EVH, block, and script versions are, as is well known, built on the same boards. I read that on the block version, R5 is 24k and R6 is empty, with the switch omitted. Both R5 and R6 are not populated on the Script version (again with no switch), which matches up with what I had expected a "script" version of the circuit to look like-- no feedback at all. Finally, without any supporting photo evidence, it was suggested that the Script switch on the EVH model selects between 24k R5 and empty R6.

So, I just desoldered the cap that was in R6 and now it sounds great, and the script switch seems to accomplish what I remember from the EVH Phase 90 I had a while back. Unfortunately that one is long gone-- I'd really like to verify that R6 should be empty on the EVH version.

I recalled then that when I got this thing, someone else had already attempted to replace the power supply diodes, but had swapped the zener and the 1n4148 and burnt it out again. Perhaps they also added that cap for some reason? Who knows.

Thanks again for your help, Mark, you pointed me right where I needed to look!

[nooneknows]

be very careful with that pot, I broke the one in mine just bending it twice, and I was very careful too. they are made with the lowest quality I've seen, they lack a metal cover too, just a thin slice of board material

[Cruton]

Funny you should mention it, I too found it very brittle and it snapped off after I moved it a couple times gently. I soldered it back on the leads for troubleshooting purposes. Now that it's sorted out it's getting a new alpha pot. Hopefully that one will hold up better!