Phase 45 Debug

[Kearns892]

I'm a little embarrassed to post this, reason being it's my own layout  The layout checks out against the schematic I created of the phase 45. The circuit passes signal, but no phasing. I've tried a few different sets of matched  FETs and I have tried different pinouts by flipping and twisting the legs with no audible effect. Also have tried different ICs. For testing purposes I have replaced the pot with a single 249K fixed resistor, but didn't have luck earlier with the pot attached either.

I know exactly what is wrong with it - the LFO isn't oscillating, but for the life of me I can't determine why. My voltages on the gates of my FETs look high based on other voltages I have seen posted (though I have never seen working voltages posted).

I have (after removing all ICs and FETs from their sockets) reflown most all of the solder joints and spent a good deal of time probing around with the DMM lucking for shorts and breaks between and in the traces. I'll continue checking component values and traces, but does anyone see anything wrong with my layout, or have any idea what sort of problems would cause the LFO to not do its thing?

For now here is my layout and voltages.





IC1
1: 3.55
2: 3.55
3: 3.54
4: 0.00
5: 3.38
6: 3.55
7: 2.83
8: 9.02

IC2
1: 4.49
2: 4.53
3: 4.45
4: 0.00
5: 3.54
6: 3.55
7: 3.55
8: 9.02

D1
k: 3.55
a: 0.00

Q1 and Q2
G: 3.34
D: 3.55
S: 3.55

[Govmnt_Lacky]

Hi Kearns,

Right off the bat.... I believe the Phase 45 is supposed to use an ANTI-LOG pot (500KC)

Reference:

http://www.generalguitargadgets.com/pdf/ggg_p45_lo.pdf?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

http://www.tonepad.com/getFile.asp?id=70

Good Luck 

[Fender3D]

The LFO part looks ok to me (unless for shorts in the C10-R13-R14 area)
I had a similar issue with 5532 IC (they simply won't oscillate) maybe your 072 is the same

[Kearns892]

You're right Greg... I bought a reverse Log pot, but I think i ended up installing a log pot. Still though, I should be getting oscillation with the pot installed (just a weird sweep) and the 249K resistor. Thanks for the heads up though!

@ Fender

Thanks for looking over it for me. I am fairly confident that the issue is not with my layout skills but with my building skills. thanks for vindicating me

[Kearns892]

I'm looking over the parts Fender3D mentioned and don't see (or test) anything out of the ordinary. Will re flow the joints to be sure. Any other ideas of places to look?

[Govmnt_Lacky]

Just a quick though on this...

If you are "loosely" basing your layout from the tonepad layout, then you may want to refer to the build reports. I believe the FETs need to be installed backwards on the tonepad layout.

I would recommend the following for ya'

1) Verify ALL component values, orientation of electro caps, and look for solder bridges (if you have already done this, do it one more time  )

2) Double check your FET connections with a known good schematic. Make sure their orientation in the circuit is correct.

3) Since we discussed the 500KC pot as being the correct value, and if you incorporated that change... make sure you have it installed correctly.

Good Luck  

EDIT: After re-reading your posts... exactly which pot did you replace with the 249K fixed resistor? If it was the trim pot, then that might be another issue. You need to use that trim pot to dial in your voltage on the FETs. If this is what you did, I HIGHLY RECOMMEND you replace it with the recommended 250K trimmer.

[Kearns892]

The FET connections are very bizarre. It seems most are completely different from each other. I designed my layout with the pads oriented left to right as Gate Drain Source. According to the schematic I have the FET goes Gate Source Drain, but clearly notes the Source and Drain are interchangeable on this device http://www.datasheetcatalog.org/datasheet/fairchild/2N5457.pdf. I'll also try twisting the legs, but is there something I don't understand going on here?

[Fender3D]

Lift R11, if you  have other issues on FETs or surrounding doing so it should start oscillating.
Hook an LED + 2~10k res at pin 1 for an easier check up...
Sometimes this LFO fails start if the pot value is 500k or more (MXR put a 200k-300k res in parallel on their Micros, Flanger and Chorus)

[haroldjenkins]

In my experience in dealing with FETs, which isn't a whole lot but, I've found it useful to check the data sheet by the manufacturer who made yours.

It turns out I used several as small and smelly FIRECRACKERS on my last build. I only searched Google for a picture of a "FET pinout" and believed the first one I saw. Turns out it had two leads reversed. I then found Fairchild's datasheet (the ones who made mine) and found out the real pinout.