Phase 90 Debug HELP!!!

[will_raymo2000]

Have been struggling on this one for a week or two now hoping something very obvious is going to jump out at me but its just not happening.

Here are some voltages for you to muse over:

       Q1     2        3        4
G   0.23   0.23   0.23   0.23
S   0.00   0.00   0.00   0.00
D   0.00   0.00   0.00   0.00

Q5
C 1.02
B 3.76
E 0.00

IC1

1 1.44
2 1.44
3 0.87
4 0
5 0.82
6 1.2
7 1.35
8 9.63

IC2

1 8.85
2 5.13
3 0
4 0
5 0
6 8.94
7 9.03
8 9.63

IC3

1 9.03
2 9.02
3 0
4 0
5 0
6 8.99
7 8.96
8 9.63

D1
A 9.63
K 0

D2
A 0
K 0

Its the Tonepad Phase90 and the parts I have changed are as follows:
Using a 470k log pot instead of 500k rev log
Using 10uf + 4.7uf in parallel to create 14.7uf instead of 15uf cap
Using 47nf instead of 50nf
Using 220k trim instead of 250k trim


Clearly something is very, very wrong here but I can't work it out at all! 
Any suggestions would be much appreciated 

[Govmnt_Lacky]

1) Ensure that you have your FETs oriented properly. Did you use 5952s? If not, you may need to check their orientation.
2) Did you install ALL of the jumpers? Especially the ones going to your ICs?
3) Double check ALL of your component placement and values.

Im gonna bet your problems lies with one of the above.

[Govmnt_Lacky]

Also, if you matched your FETS, did you remember to jumper over the 22K feedback resistor?

[will_raymo2000]

The FETs are oriented correctly, and yes I am using the 5952s.

I have installed all the jumpers and have double checked more or less everything with the continuity setting on my DMM. Equally I have checked component values a ridiculous amount of times!

Something strange I am measuring is a short to ground off the 10k resistor that's underneath D1. I'm pretty sure thats not meant to be grounded as its the signal path however I have traced it and can find no shorts or solder bridges...

[Govmnt_Lacky]

Check to see if there is a short between the Ground side of D1 (can never remember which side is Anode and which is Cathode) and the non-power side of the 10K. You might have a solder bridge.

[will_raymo2000]

I have just desoldered the ground side of D1 and there is still a short from the non power side of the 10k to ground. I have also added the jumper over the 22k to no avail. I shall carry on looking for this short!

[Govmnt_Lacky]

The only thing that sticks out imediately is the 10uF electrolytic coming off of IC1 Pin 4. Check for solder bridge and/or bad cap.

[will_raymo2000]

Have checked the 10uf cap and it was fine but replaced it anyway. Still that short to ground! Argh!!

Off to bed now but will attack it in the morning with a fresh mind. Cheers for your suggestions anyway though

[Govmnt_Lacky]

Happy to try to get you through this  . I know what it is like to try to debug this circuit.

It might help to know where EXACTLY on the circuit that you initially probed this "short to ground." BE SPECIFIC. Also, where else on the board do you get the short.

[will_raymo2000]

It turned out it was the Zener that was causing the short. I cut it off the board and now the circuit works fine.

Thanks for the help debugging!

[Govmnt_Lacky]

Bad component. I have had a rash of bad components in the last few builds. Try socketing the Zener as I have heard that the value put in there (i.e. 5.1V, 4.7V, etc.) can make a difference with the circuit.

[Scruffie]

As i'm sure Mark would have chimed in with, the point of that Zener is to keep the effect biased as the battery runs down, if you're using a battery, you'll want to replace it.

[will_raymo2000]

I didn't know that, the unit is only going to be used with DC so I dont think I need to replace it.

Thanks

[R.G.]

The JFETs in the P90 have to have their sources held a fixed voltage above ground, and their gates a little lower than that.

The zener in the original was about 3.1V.

The swing on the LFO from peak to peak has to be less than that, and is about 1.7V roughly in most of the LFOs in the stock version.

The zener voltage needs to be approximately equal to or larger than the cutoff voltage for the JFETs.

The original JFETs had a gate cutoff voltage of about 3V.

You can make the circuit work with JFETs with larger cutoff voltage JFETs by increasing the zener voltage.

If you get the zener too big, it will fall out of regulation on low battery voltages.

4.7 or 5.1V zeners are fine. Above that, you get into oddities with low batteries.

The zener will in general not change the tone of the pedal, only the setting of the bias offset pot.

Unless your zener was itself defective/shorted, you had a wiring or solder short across the zener, and any new zener you put in will have the same problem.

Ordinary NPN transistor base-emitters have a "zener" voltage when broken over backwards of 5 to 8 volts if you are ever in deep need of a low voltage zener.

If you ever use a base-emitter for a zener, it permanently makes the transistor somewhat noisier than it used to be, so don't then use a zenered transistor for a low noise transistor again.