Schertler Stat-pre

[Zanius]

Not really a stompbox, its a preamp for Schertler electrostatic pick-ups. That thing.

I use it for my upright and I am very pleased with the quality. I wanted to rehouse it and put some other things in the box too, so I finally went ahead and took it out of the box. So while I have it out why not clone it? I need some help though.

The back side is pretty straightforward, with all the smd components and the lines clearly shown. The front side has a few connections that go under the jacks and caps so I can't really see what goes where. I don't want to desolder the jacks, it is pretty difficult and I don't want to destroy anything, since that little guy costs about 200 euros.

Here is an album of pictures. http://imgur.com/a/UBGG8
and the front: http://imgur.com/TAKqzq1

I highlighted the circuit on the back, put some names on the components (not everything was marked so I went ahead and marked everything kinda randomly).

Another thing to know and not give much attention to is the led circuit. When you plug the "out", led lights up to test the battery. When you plug "in", the circuit fires up and led turns off.

Now, where I need your help:
1. There is a white-ish smd component behind the pot (marked "?"). There is no writing on it, what can it be?
2. I haven't learned enough to know what to92 trans will be needed to replace the smd ones. I marked all their information on the picture.
3. The front side. I can't see clearly what connects where due to the jacks and caps hiding the lines. I numbered them so we can identify them. Here are my guesses.
 1- Lug 3 and Input sleeve go to ground?
 2- Seems like it goes to C4's outer-side, but can't really tell where it comes from.
 3- Lugs 1&2 to Input? Is that a valid thing? Never saw that before, killing the signal before it hits the circuit.
 4- Part of the Led circuit, ignore.
 5- Can't see the connection, but it is the input going to C1 "input cap"
 6- Also part of the led, also ignore.

The 4 electolytics are 100uf, C1 is 47n, C4&5 are 15n, Pot is A20K.

Any input would be appreciated here, sorry I can't get a better view, as I said I really don't want to desolder the jacks. They have a ton of solder on the front side of the board I can't really reach. The only way would be to snip them. Then again, I am going to rehouse it, so I may have to snip them anyway, I just want to have everything else ready (which I don't) before doing so.

-Man do I write long posts-

[Ciaran Haslett]

1. Thats an SMD ceramic capacitor.  You can try measuring it in circuit but that usually throws off the results.
2. Where they live in a schematic will help identify workalikes.  Not getting much with those codes.
3. Get out the DMM in continuity mode and start probing the component "legs" until you find what goes to what.  You'll get it eventually.

3-1. Does the pot completely kill the input signal when turned fully in any direction?  If so, then it's wired as your usual voltage divider.
3-2. There's a few vias under those jacks. Looks like it's coming from the middle pin of the transistor marked WB.  Use your DMM to confirm.
3-3. This would contradict the voltage divider theory.  Confirm these with a DMM or see notes below.

NOTES:

Set all pots to 12 o clock.  That way the wiper isn't connected to lug 1 or 3 and confusing things when tracing.
Put jack plugs into the sockets.  Those are switched jacks that may ground signal when nothing is plugged in.  Again confusing things when tracing.
Get a pencil, sharpener and eraser.  The schematic will change 10 times before it's correct.

Good luck.  Post back if you need any help/reassurance.

[Zanius]

Omg, I've been trying to see where those tracks go by looking at it from any angle possible and not once It occured to me to check them with the meter....
 Thanx Ciaran, I will post any progress!

[Zanius]

Well, after a few tries it came down to this, it's the best I can do. I hope it is readable enough.

Now I really need help because I don't know what all these transistors are (some seem to be more like switches). I wrote down all the markings on the transistors, hope someone can figure out what they mean.  I would try to vero it myself but I don't know if the schematic is right to begin with. C8 is smd capacitor with no marking on it, so I don't know what it is. Hope someone has the time to take a look. Any input welcome. If I can do anything else to help, please ask me!

Full res: http://imgur.com/8fvKDla

[fx-fidden]

Hi.
Did you have any news about it? I'm really curious about the schematic.
I find very iinteresting the trick that the battery is on when you connect the jack cable not only on the box but in the amp (or something else), too. How?