Improved EA Tremolo - Problem with voltage on BS170 drain

[cab42]

Hi All and happy new year.

Earlier today I built an (ROG) Improved EA Tremolo on vero. And it worked beautifully. Sounded really good, just as I hoped.

However, as I did not have a 100K trimpot, I used a 100K lin potentiometer, and then replaced the pot with two fixed resistors (56K and 33K) when everything was working (with app. 4.7v on bs170 drain).

But after installing the resistors, the BS170 drain voltage was way off! I took out the resistors and put the pot back in, but no matter how I adjusted it, the drain voltage remained constant.  So I have obviously done something (bad) to the circuit.

The voltage divider is working. The voltage changes as expected when I turn the pot, but it does not seem to affect the BS170.

The LFO part seems to work fine. At least the LED is happily pulsating

I have reflowed solder joints, checked for continuity across and along rows, and even tried a few other BS170's and I am running out of ideas.

Voltages:
Q1 BS170   
D 5.6v (This is constant regardless of the position of the trim pot)
G ~1.1v
S ~0.9v

Q2 MPF102
0
-2v-2v (fluctuating)
0

Q3 2n5088
C 0v-6v (fluctuating)
B ~0.6.
E 0v

Schematic: http://www.home-wrecker.com/eatremolo.html
Layout: https://tagboardeffects.blogspot.com/2012/02/ea-tremolo-modified-rog.html

[Rob Strand]

Measure the voltage on the trimpot wiper and make sure it is varying with the trim pot setting, Make sure it gets to 0V.  Next measure the gate voltage and make sure that can also be varied.   You should be able to set the gate to 0V.

It's possible the MOSFET is fried but you need to follow through the debugging process before drawing that conclusion.

For example if the trimpot wiper is at 0V but the gate doesn't go to 0V that would be weird.  You might even pull the MOSFET and check the gate point in the circuit can go to 0V with the MOSFET removed.   If the presence of the MOSFET is the cause then maybe it's fried and leaking current through to the gate.

[cab42]

Thanks, Rob

I checked the trimpot as one of the first things, and the voltage varies on the wiper from 0v to supply voltage. So that works fine.

However, the voltage on the gate does not go to 0v. When I adjust the trimpot to 0v on the wiper I get just above 1v on the gate, fluctuating on the second decimal.

When I pull the transistor, the voltage fluctuates between -2 and 2 volts on the gate position with the trimpot wiper on 0v. When I turn the trimpot all the way up (8.89v on the wiper) the voltage on the gate position fluctuates between app 1v and 5v.

I don't think it is the transistor. I tried a few others, and while the specific voltages varies, the pattern is the same. Unless off course, I have built a Mosfet Fryer©, that kills any transistor I put in

[antonis]

Q1 BS170   
D 5.6v (This is constant regardless of the position of the trim pot)
G ~1.1v
S ~0.9v

I don't think a Vgs of 200mV could cause a Drain-Source current of 700μA..

Could you plz post some photos of your build..??

[Rob Strand]

When I pull the transistor, the voltage fluctuates between -2 and 2 volts on the gate position with the trimpot wiper on 0v. When I turn the trimpot all the way up (8.89v on the wiper) the voltage on the gate position fluctuates between app 1v and 5v.

I don't think it is the transistor. I tried a few others, and while the specific voltages varies, the pattern is the same. Unless off course, I have built a Mosfet Fryer©, that kills any transistor I put in

The point you used for ground (0V) in those measurements is somehow raise positively by 2V to 3V.   The trimpot 0V point looks like the a true ground (0V).   If you trace the ground from the battery to the trimpot you should find it connects normally.  Trace tracks from the measurement ground point and you will find it doesn't make its way to ground.   That's going to show something up for sure.

The MOSFET drain voltage not going above 5.6V has probably go something to do with the 2V to 3 offset between the grounds as well.   In fact if you measure the drain voltage relative to the trimpot 0V I suspect it will be going up to 8V or 9V.

The problem is more than the MOSFET, so the MOSFET is probably OK.

So you need to trace through your grounds.   Use you eyes and the multimeter continuity checker.  It's a major problem on the ground/0V so it shouldn't be hard to find what is broken.

If you can't see anything post pics as per antonis's recommendation.


I don't think things are quite like I outlined, nonetheless it should put you on the right path.

[cab42]

I have spent most of my evening debugging, what should be a pretty easy build. At a point where a) I found myself trying things that I had already had done (unsuccesfully)  and b) my board looked like a disaster after soldering/desoldering, I threw the towel in the ring and built a new one in a little more than a hour.

I could conclude that my first build HAD fried the mosfets!!! None of the ones I used, worked in my new build. I pulled a BS170 from a known working effect and used that instead.

I am also pretty sure that there were a grounding issue. I had some pretty weird continuity measurements and they were not even consistent.

Only thing is that the max depth on the new build is a bit weak compared to sound samples. I'll probably look into that.

Thanks for your help.

Carsten

[Rob Strand]

The important thing is you have something working.

If you find the unit misbehaves in the future perhaps read-up on protection diodes on the gates of the MOSFET.   Bad cases of static electricity (ESD) can damage the MOSFET.   It's more likely you just had some problems with your build which fried the MOSFET and the story ends there on your new build.

[anotherjim]

I seem to remember an old post where the MOSFET's were being zapped by an ungrounded soldering iron with AC leakage to its tip.