Zener not measuring at rated voltage

[Aentons]

I built a tractor beam phaser and am working thru some bias concerns.

The 4.3v bias zener in circuit measured low so I took it out and with a 10k resistor in a test circuit it measures at 2.5v which is about the same as in circuit. I measured another zener of the same batch and got about the same as the other? The voltage changes with the resistor value. Shouldn't this be, and stay pretty close to 4.3v? Did I get a bad batch?  Btw, these are from Mouser


I've been posting some troubleshooting steps I've on this thread. I thought the distortion was something with the JFets at first, but ( I think) it may be that the bias voltage is too low
https://www.madbeanpedals.com/forum/index.php?topic=31196.0

[EBK]

Sounds like your circuit is running well below the reverse breakdown voltage of that zener.  It will only be nominally 4.3V if it is conducting.  If the circuit measures 2.5V without the zener, then that is what you'll measure with any zener with a reverse breakdown of 2.5V or up.

It ia possible I misunderstood what you posted, of course.

[Aentons]

Sounds like your circuit is running well below the reverse breakdown voltage of that zener.  It will only be nominally 4.3V if it is conducting.  If the circuit measures 2.5V without the zener, then that is what you'll measure with any zener with a reverse breakdown of 2.5V or up.

It ia possible I misunderstood what you posted, of course.

I added a pic of the test circuit using a 9v battery. The tractor beam circuit has about 7-8v on it

[EBK]

Ok, it looks like I definitely misunderstood your original post.  Sorry about that.
Have you tried flipping the zener around and measuring it?

[Aentons]

I did at some point but I don't remember the reading. It was not correct near 4.3v. (Edit: it's 0.7v)

In the other thread, I have a pic that shows my build and a production vfe build side by side and the zeners are in the same orientation.

What is strange is that the VFE prod build only measures about 3.5v. The tractor beam calls for a 4.3v zener and I have verified that is what is in the prod build. The phase 90 has a 5.1v and  always measures right around that. So not sure why the tractor beam is lower to begin with. It even uses split rail power and so there is something like 15-17v available. The schematic in the build doc is hard to look at so I'm not sure what is different. I am speculating that the TB uses a 4.3v to accommodate a potentially narrower negative voltage range from the charge pump

TLDR: I'm missing at least 1V from my zener

Voltages:
OPA2134
1 = 0 mV
2 = 0.1 mV
3 = 0.1 mV
4 = -7.53 V
5 = 0.1 mV
6 = 0 mV
7 = 19.6 mV
8 = 8.25 V

LM358
1 = ---
2 = ---
3 = ---
4 = -7.53 V
5 = ---
6 = ---
7 = ---
8 = 8.26 V

TLE2074
1 = -6.1 mV
2 = -1.7 mV
3 = 0.1 mV
4 = 8.25 V
5 = 0.1 mV
6 = 0.3 mV
7 = 1.5 mV
8 = -15.4 mV
9 = -2 mV
10 = 0 mV
11 = -7.53 mv
12 = 0.1 mV
13 = 2.6 mV
14 = 11.3 mV

J113 = G,S,D (removed)
Q1 = ---,0.1mv,0.1mv
Q2 = ---,0.1mv,0.1mv
Q3 = ---,0.1mv,0.1mv
Q4 = ---,0.1mv,0.1mv

[madbean]

I did at some point but I don't remember the reading. It was not correct near 4.3v. (Edit: it's 0.7v)

Measured for forward bias, you should read b/w .5-.8v on a silicon diode like the zener, if I've read your testing method correctly.

How I understand it in the Tractor Beam: the zener is there to prevent the bias voltage on the FET gates from swinging too far to the negative end of the rail, with optimal bias being within a volt or two of 0v (since it's a split rail design). I don't think the Zener diode value is that critical here - only that it clamps the voltage in a predictable way to give that result.

Your testing method included putting a 10k resistor in series with the 4.3v zener and you got about -2.6v. So, I did the same test (just with everything in reverse for +9) and got basically the same reading on a 4.3v Zener. With 100R instead of 10k, I got 4.13v. So, I think your Zener is fine. It's even a little more complicated in the TB circuit since you also have basically a 250k resistor in parallel with the Zener (200k trimmer + 47k resistor).

[Aentons]

Thanks for testing, that helps.

I think understand what you're saying about the zener value. I tried to draw it out how I understand it. It sound like that as long as the zener value is larger than the 2n5952 Vgs(off) max value of 3.5v then all is well.

What I still don't quite get is where/what the jfet distortion is and why (if it's just a variable resistor) can have, or needs headroom.

(Edit: I did some searching and found some answers in the link below, but Im going to revert back over to my other thread I posted above for some more stream of consciousness type troubleshooting. )

https://www.diystompboxes.com/smfforum/index.php?topic=39904.0