MXR Phase 90 Mod

[blackdawg66]

Hello,

I recently performed a modification on my MXR Phase 90 with Revision F board. I removed the R28 feedback resister and then removed the C11 and C12 capacitors as the following mod recommended.
https://0f8f28a40dc58acc6203bfcbaf995d479b09a555.googledrive.com/host/0Bx_Iq64nvokMcXdCOUxsUF81RjA/MXRPhase90Mods.pdf

Now I get dry signal through the pedal but I don't get any phased signal when the pedal is kicked on. It's all dry signal.

What is the issue? Do I need to restore the capacitors?

Any help would be appreciated much.

Thanks!

[Rob Strand]

None of those mods should make the device stop phasing - even if you stuffed up the mods!

Maybe check for shorts or solder bridges in the areas you have soldered.

Make sure nothing is shorting to ground/case.

[blackdawg66]

Thanks Rob. I didn't really see anything that looked strange. I'll take a closer look after work today.

[MrStab]

"Re-adjusting the trimpot afterwards will help too" says the article you linked to.

Did you do that, by any chance? There's a very narrow range on that pot where phasing happens.

If you haven't touched it, DON'T!!! If you HAVE already touched it, and are able to safely turn the pedal on with the board exposed (ie. not straining wires or letting the board touch the metal enclosure etc.), you should turn the Rate pot up full (makes it easier) and adjust the trimpot until you hear phasing. If you can't safely tweak the board while the pedal's powered & plugged in, then you could keep making tiny adjustments and testing it, but that'd be a bit annoying.

[blackdawg66]

Thanks MrStab, I did work the trim but marked the setting position for factory. I haven't set the board back into the case as of yet so I'm not worried with case grounding. I will work the trim in smaller increments and see if I get any results. Tedious way to do it but oh well if it works I'd be happy.

[blackdawg66]

Tried all of it and still have no phasing. Any more ideas?

[mcknib]

I think your next step should be to take some voltage readings which may indicate where your problem area is

Here are some voltages I took from a working Phase 90 which wasn't phasing till after the repair (replacing U1 and U2):

U1

1. 4.51
2. 4.52
3. 4.09
4. 0
5. 4.08 - 4.85
6. 4.28 - 4.25
7. 3.98 - 5.25 these eventually settle similar to pin's 5 and 6
8. 9.10

U2
all pins read 4.52V except pin 4 - 9.11V power supply and pin 11 - 0V ground

Q1 2N4126
C. 4.52
B. 4.52
E. 2.74

Q2 to Q5 2N5952
All
G. 4.52
S. 4.52
D. 2.38 to 2.43