Zvex SHO troubleshooting

Started by Lost_soul, October 27, 2024, 01:55:27 PM

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Lost_soul

Quote from: R.G. on October 28, 2024, 02:36:53 PMThanks for the voltage measurements.

What they tell me is that the gate is probably not shorted to the channel.

Another thing I guessed might be happening is that the gate-to-drain 10M might have accidentally been substituted for a 1M. The voltages from gate to source were very close to this, but ultimately the numbers did not work out for this guess.

This means that it is possible that a capacitor is leaking and messing things up, as mentioned earlier. Try this: disconnect the input and output caps. Just unsolder them and pull them out. Then try the voltages again, with the source resistor potentiometer turned to both extremes. If the DC voltages change, then the capacitors (or something like the solder joints, wiring, board contamination, etc. that I'm ignoring right now) are messing up the MOSFET biasing. If they don't change, the caps were not the problem.

I go to testing capacitor influence because the bias resistors are so big. It's the equivalent of a 5M bias resistor, and 5M is up in the range where capacitors can leak that much. Hmmm. Are all the parts new, or have you re-used parts from another project or salvaged?



I will try to unsolder the input and output caps and post the voltages here again :D

Yes, all the parts are new. Nothing is used or salvaged.

Also i have a question, is it normal to have the source grounded  when the pot is fully clockwise?

Btw i did't substitute the 10M for a 1M by mistake. You can also see the colors of its strips in the pictures i posted.

I am sorry but i am still a beginner and don't understand most of this but i will try to do what you say and understand it along the way :)

antonis

Quote from: Lost_soul on October 29, 2024, 07:07:03 AMis it normal to have the source grounded  when the pot is fully clockwise?

In theory(*), yes..!! :icon_wink:
(that pot sets stage's gain, which is maximun when Source is grounded..)

(*) theory stands for "ideal" pot, meaning wiper can travel all down(up) to zero(full resistance)..
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

Lost_soul

Quote from: antonis on October 29, 2024, 07:47:57 AM
Quote from: Lost_soul on October 29, 2024, 07:07:03 AMis it normal to have the source grounded  when the pot is fully clockwise?

In theory(*), yes..!! :icon_wink:
(that pot sets stage's gain, which is maximun when Source is grounded..)

(*) theory stands for "ideal" pot, meaning wiper can travel all down(up) to zero(full resistance)..

but why does it interact with the LED despite the LED having a circuit of its own? like when it's fully clockwise LED is OFF. when it's fully anti clockwise, LED is fully ON

antonis

Quote from: Lost_soul on October 29, 2024, 08:08:31 AMbut why does it interact with the LED despite the LED having a circuit of its own?

Probably due to circuits unitentional interaction.. :icon_wink:

Could you plz post pictures of your build..??
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

Lost_soul

Quote from: antonis on October 29, 2024, 08:36:11 AMCould you plz post pictures of your build..??

I posted some pictures in the 1st page. Do you need more pictures or a specific part maybe?

antonis

Quote from: Lost_soul on October 29, 2024, 09:06:24 AMI posted some pictures in the 1st page.

My bad.. :icon_redface:

But self-coloured wires don't help much for tracing..
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

Lost_soul

Quote from: antonis on October 29, 2024, 09:10:01 AMBut self-coloured wires don't help much for tracing..

i am sorry but i just used what i got. as this kind of wire is a little expensive here :-\

duck_arse

find. steal. reuse. recycle. find yourself an old printer cable, from before usb. or a scsi cable. cut the ends off, many many colours inside, correct sized wire for pedal use.
" I will say no more "

R.G.

Quote from: Lost_soul on October 29, 2024, 08:08:31 AMbut why does it interact with the LED despite the LED having a circuit of its own? like when it's fully clockwise LED is OFF. when it's fully anti clockwise, LED is fully ON
There are several possible reasons. That's why I was steering you to test the LED separately, and only later connect it to the signal circuit. Your testing seemed to show that the LED circuit worked in isolation. Once the signal circuit works, we can work on marrying the two. It could be that the output capacitor is leaky, it could be that the signal circuit is oscillating at radio frequencies, it could be that the wiring to the bypass switch is not correct, other things.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

Lost_soul

Quote from: R.G. on October 29, 2024, 11:05:17 AM
Quote from: Lost_soul on October 29, 2024, 08:08:31 AMbut why does it interact with the LED despite the LED having a circuit of its own? like when it's fully clockwise LED is OFF. when it's fully anti clockwise, LED is fully ON
There are several possible reasons. That's why I was steering you to test the LED separately, and only later connect it to the signal circuit. Your testing seemed to show that the LED circuit worked in isolation. Once the signal circuit works, we can work on marrying the two. It could be that the output capacitor is leaky, it could be that the signal circuit is oscillating at radio frequencies, it could be that the wiring to the bypass switch is not correct, other things.

I removed the input and output caps and the volts measurements are the same on both extremes.

Only change happened that the LED doesn't work any more. (And the gate volt was the same as drain when the pot was anti clockwise)...

R.G.

Quote from: Lost_soul on October 29, 2024, 04:15:41 PMI removed the input and output caps and the volts measurements are the same on both extremes.[...](And the gate volt was the same as drain when the pot was anti clockwise)...
I have to be picky, because debugging over the net means that the exact voltages are all I have to work from.
Does this mean that
QuotePot is fully clockwise...
Source: 0v
Gate: 1.79v
Drain: 1.89v

Pot is fully anti-clockwise
Source: 3.47v
Gate:  5.13v
Drain: 5.36v
Except that for the last set of measurements, the gate is 5.36V, not 5.13V? Oh, and is the battery voltage still 9.28?


QuoteOnly change happened that the LED doesn't work any more.
We expected that to happen after we disconnected it. Effectively, this is another successful test of the LED circuit working correctly.


R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

Lost_soul

Quote from: R.G. on October 30, 2024, 11:02:00 AMExcept that for the last set of measurements, the gate is 5.36V, not 5.13V? Oh, and is the battery voltage still 9.28?


I am sorry i should've posted the numbers  :(

Here they are:

Battery at: 9.22v

Pot is fully clockwise

Source: 0v
Gate: 1.78v
Drain: 1.83v

Pot is anti clockwise

Source:  3.6v
Gate: 5.4v
Drain: 5.4v

They were not the same, but they were close so i thought they didn't matter. I am sorry R.G

R.G.

No problem. I'm squinting at millivolts, so it makes me picky.  :icon_lol:

Here is the problem I'm wrestling with in my mind. The MOSFET does not appear to be shorted, either drain to gate, drain to source or gate to source. The voltage from drain to source is not approximately 0.85V, as it would be if the drain and source pins were reversed. The Vgs voltage appears to be about 1.8-ish volts, within the specified range of 0.5V to 3.0V on the 2N7000 datasheet.

What is not checking out is that the gate always appears to be at or slightly below the drain voltage.
This is not where it's supposed to be, because the two 10M resistors are supposed to be making it 1/2 of the drain voltage, and that is not happening.

The gate voltage is hard to measure, because the two bias/feedback resistors are 10M each, and the gate has an input impedance up in the giga-ohms. That's why I was supposing maybe the top "10M" was really much lower. An alternate theory is that the bottom "10M" might be open, or not well soldered to either the gate or to ground. That would make the MOSFET into an odd version of the "amplified MOSFET diode" circuit.

In all of these cases, it is quite difficult to measure the gate-ground voltage because the bias resistors are so very large, and meter input impedance is so variable from meter to meter. The meter itself could be mucking up the reading.

Needs more thought. Mean time, you might try lifting one end of the bottom "10M" resistor and seeing if it measures 10M in fact.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

Lost_soul

Quote from: R.G. on October 30, 2024, 04:46:32 PMNo problem. I'm squinting at millivolts, so it makes me picky.  :icon_lol:

Here is the problem I'm wrestling with in my mind. The MOSFET does not appear to be shorted, either drain to gate, drain to source or gate to source. The voltage from drain to source is not approximately 0.85V, as it would be if the drain and source pins were reversed. The Vgs voltage appears to be about 1.8-ish volts, within the specified range of 0.5V to 3.0V on the 2N7000 datasheet.

What is not checking out is that the gate always appears to be at or slightly below the drain voltage.
This is not where it's supposed to be, because the two 10M resistors are supposed to be making it 1/2 of the drain voltage, and that is not happening.

The gate voltage is hard to measure, because the two bias/feedback resistors are 10M each, and the gate has an input impedance up in the giga-ohms. That's why I was supposing maybe the top "10M" was really much lower. An alternate theory is that the bottom "10M" might be open, or not well soldered to either the gate or to ground. That would make the MOSFET into an odd version of the "amplified MOSFET diode" circuit.

In all of these cases, it is quite difficult to measure the gate-ground voltage because the bias resistors are so very large, and meter input impedance is so variable from meter to meter. The meter itself could be mucking up the reading.

Needs more thought. Mean time, you might try lifting one end of the bottom "10M" resistor and seeing if it measures 10M in fact.


I removed the 10M ohms resistor and measured it and it was around 10-11M (i think because it has 1% tolerance)

I don't know what's wrong with this stupid circuit. I did everything to check where the problem is. I reflowed every solder joint, i made sure there are no bridges, i changed the Mosfet with 10 diffrent ones and it still didn't work.

So i decided to make one from the start and i didn't connect the switch. Just input, output,9v and ground and i tested it with the Mosfet that i used in the LED circuit and it worked!
The pot did change the level of boost (with its old man's crackles haha), it was quiet with no hum or noise and it sounded great!

I also made the LED and signal circuit on the same board and i made cuts along the line between them so i can do it all on one board but i am still to install the LED circuit. Now if something went wrong i would know it's from the switch or something. I will post the final thing when i install the LED. But here it is


I will tag you if i faced any problems with the new one and i will also try to troubleshoot the old one ;D
Thank you so much man.

These are the voltages for the new one.

Fully clockwise

S: 0v
G: 1.6v
D: 4.4v

Full anti-clockwise

S: 1v
G: 2.2v
D: 8.35v

Idk if gate should be half of drain but that's not happening here.

antonis

You can't get precise Gate voltage measurements due to DMM impedance loading high value bias resistor(s)..

Both FCW & FCCW measurements look fine..
At max gain (FCW) Drain is set at half-supply and at min gain (FCCW) you have proportionally equal voltage drops across Source and Drain resistors..
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

Lost_soul

#35
Quote from: antonis on October 31, 2024, 08:52:01 AMYou can't get precise Gate voltage measurements due to DMM impedance loading high value bias resistor(s)..

Both FCW & FCCW measurements look fine..
At max gain (FCW) Drain is set at half-supply and at min gain (FCCW) you have proportionally equal voltage drops across Source and Drain resistors..

excuse me but i don't understand most of this as i am not experienced yet. till i do, from what you said does this mean that everything is fine?  :icon_wink:

antonis

#36
Quote from: Lost_soul on October 31, 2024, 09:03:33 AMexcuse me but i don't understand most of this as i am not experienced yet. till i do, from what you said does this mean that everything is fine?  :icon_wink:

I'll let you judge.. :icon_wink:



So, does MosFet (and relevant circuit) behave well..?? :icon_wink:
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

duck_arse

if no smoke, if sounds good, if some volts look about right [and Ohm's Law checks out good] - don't worry. circuit works.

you will need to test that switch you pulled out, and the wires attached to it. good work!
" I will say no more "

R.G.

Quote from: Lost_soul on October 31, 2024, 08:22:42 AMSo i decided to make one from the start and i didn't connect the switch. Just input, output,9v and ground and i tested it with the Mosfet that i used in the LED circuit and it worked!
The pot did change the level of boost (with its old man's crackles haha), it was quiet with no hum or noise and it sounded great!

I also made the LED and signal circuit on the same board and i made cuts along the line between them so i can do it all on one board but i am still to install the LED circuit. Now if something went wrong i would know it's from the switch or something. I will post the final thing when i install the LED.
Good work! I have many times had to start over on something. When nothing makes sense after hammering on it for a while, a fresh start can be the best choice.

Do check out your switch and wiring to it, as already mentioned. That might have been the issue all along.

Now go make LOUD noises!!   :icon_lol:
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

Lost_soul

Quote from: antonis on October 31, 2024, 09:51:51 AMSo, does MosFet (and relevant circuit) behave well..?? :icon_wink:

that does make sense! Thank you so much for sticking out until the thing was completed :icon_wink:
but where did you get the 900 micro amps and 200 micro amps from? and you used V=IR to calculate the Vr right? but how did you know the I