XC Phase (Phase 90) Issues

[equinox]

I just finalized an XC Phase pedal. I have one question and one issue. The thing is functioning appropriately and engaging/disengaging.

The issue I have is that there is a loud audible "POP" when engaged (not when disengaging)! I have an actual MXR CSP026 74 Vintage MXR Phase 90 I purchased in 2020. That thing sounds killer and sweet and there is absolutely no popping when engaging or disengaging, so wondering what's up?!? I've built literally hundreds of pedals and was a tech and sr. tech when i started my career in engineering almost 30 years ago, so it's built right and has to do with something else.

This must be a Rev. 1 board, as yes, Q1-Q4 are installed backward from the original silk screen, fyi.

The second thing and question is how to set the trimmer? I know a lot of information just says set it in the middle of the phasing, but I'm wondering if there is something a little more definitive, like....if 9.00VDC input, set drain at 4.50VDC exactly OR to via pinX and monitor on oscilloscope until just before clipping on the screen....both would be pretty definitive.

Since I have an actual one, I'm wondering if there is anything I can look for on it, like a voltage at a certain pin or something on the oscilloscope?? Then I can copy the same thing I'm seeing on the actual unit, to the clone??!?

Also, when making these modulate components like Phasers or Choruses, the JFETs (2N5952) need to be matched. To ensure they are, what is the best method to test them??

PLEASE ADVISE. Thanks in advance.

 

[jimilee]

That’s great news, glad you got it figured out. Phasers are finicky for some reason, I guess Chorus is too:

 

[Laundryroom David]

That’s great news, glad you got it figured out. Phasers are finicky for some reason, I guess Chorus is too:

I have a ehx small clone. Pops like hell. I power it up and stomp it a few times before I turn on my amp. Works great.

 

[equinox]

Hey guys, I know it's expensive, but think I'm going to pull the trigger on the Peak DCA75. I think it's really cool and will supplement by 3 other Peak/Atlas units.

Second thing thou....WHY does the '74 Phase 90 Reissue (as the vintage ones) have 6 ICs and 5 Transistors, but the Pedal PCB (clone) only has 3 ICs and 5 Transistors??

 

[Nic]

WHY does the '74 Phase 90 Reissue (as the vintage ones) have 6 ICs and 5 Transistors, but the Pedal PCB (clone) only has 3

I would guess the vintage ones used single opamp vs dual in the ppcb.

 

[equinox]

I would guess the vintage ones used single opamp vs dual in the ppcb.

Yep, I see. The original had six UA741CP's in it. The clone has three TL072's...single v. dual Op Amp.
I see the Slew Rate for the YA741 is only .5V/us and the TL072 is 20V/us!!
I wonder what the difference is in TONE?!??

 

[equinox]

A transistor tester that measures the gate-source voltage needed to turn the device off (vgs-off).

Some folks like the R.G. Keen method, which is a simple DIY measurement comparison circuit.

http://geofex.com/Article_Folders/fetmatch/fetmatch.htm

I use a Peak DCAPro.

Hope this helps.

Ok, so the DCA75 meter came in today!

I tried out 3 2N5952s just to see how it worked.

Below are the results of 4. I have a bunch of them and will test all! I want to look specifically for the VGS (0ff) only?...and ones then match the closet together?? What is the maximum difference in percentage (I think I seen this somewhere but cant remember where seen it)?

I seen on a search some tester were reading Vp and IdSS. Why and if only VGS (0ff) is necessary?? Thanks in advance.

VGS (off) -2.46 -2.15 -2.17 -2.33V
@ID 4.8 5.3 4.8 5.2uA
VGS (on) -0.33 -0.19 -0.23 -0.14V
@ID 5.00 5.01 5.01 5.00mA
gfs 3.8 3.8 3.9 3.4mA/V
@ID ----------3.0 to 5.0mA----------
IDSS 6.44 5.86 6.02 5.54mA
@VDS 2.99 2.99 3.00 2.99V
RDS 163.0 188.1 184.1 225.3ohm
@ID 5.0 5.0 5.0 5.0mA
and VGS 0.0 0.0 0.0 0.0V

 

[equinox]

Ok, found this while searching around. In summary, #1 you want Vgs (off) as #1 prime requisite and Idss as secondary #2, but #1 is biggest thing to match.

merlineb - "In general, Vgs(off) takes priority. That's the thing you should match first. The bias trim can be adjusted to suit whatever Vgs(off) batch you have selected (within reason). With Vgs(off) matched, all the JFETs will be equally modulated. This gives smooth phasing, avoiding thumps. It's nice if there is also an LFO amplitude trimmer."

"Of course, it's nice if you can match Idss as well, but do that as a secondary consideration. Matching Idss means all the JFETs will sweep down to the same minimum resistance, meaning you get the largest range of 'swoosh'."

"Thumps are more annoying than losing a bit of swoosh that you never knew was possible anyway. That's why Vgs(off) takes priority."

ref - JFET Matching Link

I'm going to get readings on all 2N5952s I have, number then 1 thru xx and take note of ALL measurements with the DCA75.
I'm also going to REMOVE the JFETS from the 1974 Phase 90 Reissue (as it sounds incredible) and read what values they actually are and let you guys know and in what socket position each is located!

I'll report back later today and we can put this baby to bed!!

 

[equinox]

Ok guys, here is the results...

The MXR 1974 Phase 90 Reissue has BOTH Vgs (off) and Idss values around 3% (2.88% and 3.37% respectively)!

When I tried to test by other means and put in a couple of clones, I was actually at over 10% on one and over 18% on another, soo WAY off and probably why was not getting the Swoosh and good Phasing I was looking for.

With the ~50pc 1N5952 I tested, I was able to get about 7 matched sets, where BOTH were between 1% and 3% and should produce the same output as the MXR 74 Reissue. This will be my task today, to install the "Matched" set(s), trim it, and see how it sounds! I also have a matched set of 6pc that I need for the BYOC unit which you can select between 4 and 6 2N5952s! Previously it was over 18% matched, soo anxious to see how lush this is going to sound now!

Attached is also a photo of the PCB board and enumberated 1, 2, 3, 4 to represent each specific FET in the readings for the 74 Reissue pedal..

Wish me luck! Attached are the actual figures using the Atlas Peak DCA75. Enjoy!

 

[equinox]

This is the unit I picked up for testing components if anyone is interested.

https://www.peakelec.co.uk/acatalog/dca75-dca-pro-semiconductor-analyser.html