Modding a 1979 Ross Phaser / Univibe

[RCZ53]

Anyone familiar with the original? I am wondering if I can get the Univibe mods into the original case. It appears there is room for spdt mini switches under the two pots. I thought i would use five, 1 for each of the caps and 1 for the vibrato/phaser switch. Does that cover the essentials? I seem to recall another pot in one of Mr. Hammers posts. BTW the current caps are 50uf which means I need four replacements (as opposed to leaving one in place.) I thought I would try .001  .0022 .003 and .0047
Thanks gents.

[Mark Hammer]

Whoa Nelly.

1) Extremely unlikely that those are 50uf caps.  Much more likely that they are 50nf (i.e., .05uf).  If so, then this is an earlier FET-based Ross unit, not one of the later ones using the LM13600.  You can confirm here: http://www.home-wrecker.com/phaserguide.html  

2) This model uses a 5th stage in the feedback loop.  Feedback/regen is not recommended for Univibe circuits.  It just plain sounds either bad or interferes with the effect.  So, to Univibe, you would need to install an additional switch to cancel the feedback path.

3) While the specific values used for an actual Univibe are not absolutely critical, something functionally equivalent probably IS.  Your suggested values are not even close.  JC - "Mystery Chord" - Maillet has some postings regarding the key cap-value relationships.

4) An additional control worth exploring is an effect intensity control.  This would essentially be a variable resistor in series with R28 (10k).  A reasonable value might be 50k or 100k.  The clean and phase-shifted signals are mixed in equal proportions at U4a via 10k resistors.  If R28 is increased in value then the phase-shifted signal moves a bit more to the background.  This is a mod obviously more in keeping with adapting the pedal to subtler uses such as with bass or rhythm guitar.  Since component tolerances do not always permit flawless matching of signal levels for max notch depth, you might want to consider replacing R28 with an 8k2 or 7k5 resistor and the suggested added pot.  This way, if the clean signal level should *happen* to be a bit less OR a bit more than the shifted signal, the pot/resistor combo will let you achieve better level matching.

If your unit works fine, though, you may want to consider leaving it alone.  Given that the critical chip in the first two versions of the Ross Phaser (AM9709/9711) is no longer available, whereas the LM13600 *is*, it follows that working Ross Phasers from the earlier period will become rarer and rarer as the critical chips get destroyed through one mishap or another.  If resale value is important (and there ARE a few quirks that make the early FET-based ones interesting and desirable), then you may want to monkey around with a $20 plastic-box phaser instead of this one.

[RCZ53]

Mark ....You are correct that this is the 2nd version with the AM97C11 JFET chip. I was debating the vintage aspect of doing this mod, and on your advice I will leave this unmolested.
Is 50nf the same as pf? The caps are actually marked 503Z  2Zv
In any case I am grateful you pointed out the 5th stage and its inherent complications. I have read with great interest your Ropez threads and was hoping to acheive something in that vain. There is also a phase 45 mod currently on the board that also sounds like a cool build. Of all the choices Neovibe, Easyvibe, and the others Which do you prefer? Thank you very much for expertise and your help with this.
regards, Roger

[Mark Hammer]

Mark ....You are correct that this is the 2nd version with the AM97C11 JFET chip. I was debating the vintage aspect of doing this mod, and on your advice I will leave this unmolested.
Is 50nf the same as pf? The caps are actually marked 503Z  2Zv
In any case I am grateful you pointed out the 5th stage and its inherent complications. I have read with great interest your Ropez threads and was hoping to acheive something in that vain. There is also a phase 45 mod currently on the board that also sounds like a cool build. Of all the choices Neovibe, Easyvibe, and the others Which do you prefer? Thank you very much for expertise and your help with this.
regards, Roger

Thanks for the props.  Much appreciated  Couldn't offer an opinion about the units you mention because I haven't built them.  I find Francisco's Ropez layout a pleasure to work with, though, and LM13600's are cheap and plentiful.  In addition to Univibing, the same board/design also permits the phasefilter option which the earlier Ross units do not.  As a result, it receives whatever priestly blessing I can offer.  The phasing sound is nice, and the other two options are HIGHLY desirable.

503Z implies "50" as the two significant digits, and 3 as the multiplier; i.e., 50,000 picofarads, which equals 50 nanofarads, which equals .05 microfarads.  The same thing often applies on 1% resistors, where the numbers indicate the significant digits and multipliers; e.g., "332" equals 3300 ohms.

[Torchy]

http://aronnelson.com/gallery/albums/album11/MXR_Phase_45.gif

[Torchy]

Double post (this board's getting worse...)

[RCZ53]

I apologize ...my original post seemed to be largly ignored.
Thanks again Mark..... Torchy, I love your work!! I plan to build the phase 45 soon.