What Does the Foxx Phase?

[anotherjim]

I'll bear all these possibilities in mind Larry. Another is increase C12 - maybe a lot.
The resistors around "IC3b) are probably high value so as not to load the preceding circuits. That said, if 2.2M was originally considered high enough for that - it probably ought to be the 1M feedback that is reduced, and that can be done with a trim pot. You could make holes in the bottom plate over the trimmer screws so it's easy for the user to tweak the sweep to their own taste.

Nick, I'm limiting myself to not pulling it all apart just yet - with the house move (hopefully) imminent, I've only got skeleton facilities available  for now. We've found the steering wheel and accelerator, there had to be a brake somewhere!

[nickbungus]

Sure Jim, I just wanted you to know that you can do whatever you like to it and not worry about breaking it.  'If he dies, he dies', Ivan Drago, 1985. 

[armdnrdy]

Here's the component values overlay:




and here's the component designation overlay:



Now that the overlays are finished...I can start work on routing the board.
Due to my not having access to an actual unit or trace side image...the designation overlay may not be 100% accurate. I'm sure that any inaccuracies will be revealed during the routing process.

[nickbungus]

Wow!  That really is comprehensive.  

I've tried a little cheeky one for you, theres a pedal reseller in the States selling a 70s one on eBay.  They've given it a bit of a refurb so they arent scared to take it apart, so I've offered to pay them if they take top and bottom shots of the circuit board.

I doubt it will come to anything but you never know.

[armdnrdy]

Yeah Nick...don't hold your breath!

That guy replied to a similar request about another pedal. He said he'd take some shots....and then never did.
He's a pedal manufacturer...but he also snipes older pedals off of ebay and then turns around and offers them for outrageous prices!

If you look at his description of the Foxx phaser...he states that it uses "Vactec Opto-Coupled Resistors." (Vactrols)

He is completely wrong! It uses JFETs as variable resistors in the phase stages. There is not one vactrol in this circuit!

[nickbungus]

Yeah I saw that, there also seems to be some discrepancy in the years as well, but trying doesn't hurt. 

[armdnrdy]

Here's my take on the Model 7.

I added a 1M pull down resistor at the input, and a .1µf cap between the feedback pot and the output.

I had a bit of a struggle around IC3 without trace side shots and limited view of the trace side from the component side shots.
I was down to three traces for the last three days. I would try different routing...and then end up with three different un-routed traces!
I finally got down to one today and was going to add a jumper but....there was no good place to add one.

Since the input/output is a bit different on my board than the original, I moved things down a little and snaked the last trace in.
There is a chance that the original had a jumper on the trace side. We may never know.

[nickbungus]

Outstanding.  Any chance of the PCB mask only and a size guide?  I will be building!!!  

Looks a lot less frantic than mine!

[armdnrdy]

I'm waiting on a few parts...as soon as the build is verified, I'll post a whole project including an overlay, component BOM, and a PDF of the trace art.

No word from MusicParts?

[nickbungus]

No, nothing.  Shame we couldn't get an original schematic but I reckon you've nailed it anyway.

Looking forward to building it!

[armdnrdy]

I also have to breadboard the LFO with the cap switching.

There are three different combinations that can work.

I can't see the trace side of the X, Y, and Z, pads to be certain where they are connected.

So...I have to breadboard all three combos to "see" which one makes the most sense.

[nickbungus]

While I'm really looking forward to Larry's version, its got me thinking of a question that I'm sure has been asked and answered a million times.  But I've had a quick Google and I'm still not sure.

Does the capacitor type really matter for a stompbox (especially the Foxx phaser )?  I know Larry is going for a 100% recreation, as will be my second go.  But on my MKI, I used ceramics and electrolytics only (because that's what I had). 

[armdnrdy]

It doesn't really matter for the most part. Film capacitors will sound better in the signal path but, I've seen phasers that use all ceramics.

I used to change every resistor to metal film, and ceramics in the audio path to film.
Depending on the circuit, I do not follow that "clean up everything" logic any longer.

I believe that sometimes the original components add a bit of "grit" to the audio path which could be partly responsible for a certain sound inherent in the original. I believe that this is called Mojo. 

Let take a phaser for example. A little bit of "grit" passing through the phase stages is probably a good thing. The sound will be a bit thicker.

Try the search engine on this site for more info and a lot more opinions!

[nickbungus]

Well MkII is well on its way.  I've made my board, drilled and picked the BOM.  The plan is to socket everything that's different between my original and Larrys parts.

Larry I'll do a quick video of with and without the R25,C12 section.

Jim, regarding the RF interference, did you get to try the following and is it worth it?

I think that a 47pF cap across R3 might do some good here, without affecting tone.

By across R3, do you mean in parallel?

I'll also add this:

I put a 100n in the wire to the feedback pot and tacked a 100k pull down between R51 and the ground end of R52. There was bypass popping before - there isn't now  That 100n value doesn't seem to alter the character of the resonance with high feedback - which can be nice and swampy.

[anotherjim]

Oh I think the RF protection is extra "in the case of", because this was designed before cell phones, switch mode power supplies, wifi etc....
No shielding in the plastic mk1 either  - wherever it might be now ;(
Shielded case is recommended!
...but yes, across = parallel.

I'll have to read back the thread, because I can't remember why I put 100n in the feedback or what I did to match the LFO sweep to the FET's control range - if anything.

[armdnrdy]

I'll have to read back the thread, because I can't remember why I put 100n in the feedback

The 100n in the feedback was suggested by Fender3D. (Federico)

He thought that DC would be present in the feedback circuit without it.

[Fender3D]

...He thought that DC would be present in the feedback circuit without it.

And I still do...
even better, I wonder what the bleeding resistor in the next pedal might do with stock circuit 

[nickbungus]

Well I'm chuffed to bits.  Just finished V2 and it works a treat.

I'll post a video soon but here's what I've currently got socketed:

R1   Sears = 270K, Studio 7 = 100k.  I'm not even sure what the purpose of this resistor is to be honest.  Because most of the Rs are either 100k or 10k, perhaps the 100k was preferred for keeping costs down.  I went 270k for initial testing.

R13  Sears 330k, Studio 7 = 300k.  I only had a 330k and I doubt there is any real difference here.

Optional R25/C12.  Sorry Larry, I screwed this up and put them in, forgetting to put in sockets.

C18  Sears = .01uf, Studio 7 = 10uf polarised.  I used the .01uf but will try the 10uf.  Again, I'm not sure what this will do but that is a big difference.

C16 and C21  Sears = .01uf, Studio 7 = .1uf.  I'll give this a swap tomorrow night and see what happens.

Larry I've yet to try the rotary switch but I'll do that tomorrow and Jim, I've got a loud pop when engaging the pedal so I'll try your pull-up resistor mod too.

Again the feedback pot still doesn't seem to do much except change the volume.  I need to try the 100n.

R34 is socketed and I put in 6M9 (model 7 value-ish) instead of the 2M2 in the Sears schematic.  Like you said Jim, I'm getting a very even smooth phase and the trimmer and the pot are much easier to control. 

Videos coming soon.

[nickbungus]

Jim, when you said you 'tacked a 100k pull down between R51 and the ground end of R52', which end of R51 did you use?

Thanks

[anotherjim]

OK, to be start with I think it needs the 100nF fitted in series with the feedback for the "anti-pop" resistor to work properly. This is to isolate DC on Z2 pin 13 from the extra resistors "pull-down" to ground. The 100nF cap can go anywhere in the feedback path from R51 to Z2 pin13 (these are original "mk1" schematic references) - but the feedback path must have to pass through the 100nF. I cut the wire to the Q pot, soldered the 100nF in between the 2 wires and covered with heat-shrink sleeve to get it in the Mk1.

BTW, if the feedback "Q" circuit is set up correctly, I can't think why you cannot have a resonance effect - you should be able to hear feedback if you adjust the Q controls enough, but it's usual to set the "Q adj" preset just short of uncontrolled feedback with the "Q Cont" at minimum resistance.
Mk1 did resonance ok for me - but, maybe adding the 100nF helped? Hmmmm...

The job of the pull-down resistor is to "pre-charge" the output capacitor. Pop is caused by the output capacitor charging or discharging when the effect is switched out of bypass and the capacitor is connected to whatever you have the pedal feeding via the bypass switch. So adding a resistor to ground on the output is the tried and trusted way of stopping the pop. The capacitor has already fully charged up via the pull-down when you activate the effect - so no pop!

So to answer the "where" question. The pull-down is from the negative end of the output capacitor (Mk1 C22) to ground. It happened to be convenient on the Mk1 to fit the pull down between the wires of R52 ground end and R51 at the C22 end.