DEFX Pompei JFET question

[Zoot]

Continuing...
With a quick question.
I observe that on resistor R104 I get -9.9V in one side and 10.1V in the other. Obviously, the whole thing of the LFO section is about the oscillation from positive to negative, but as this point is somewhere where I start finding anomalies... thought was worth to ask.
Any further suggestion on how I continue is very welcome. I'm getting short of ideas now...
Thanks
Zoot

[BetterOffShred]

Not sure if this is the unofficial Pompei build thread but I figured I'd ask here..   on the BOM for upper board  - R31 is listed as (SOT) 390K.. I got concerned because I didn't see a note in what SOT stands for.. I checked the schematic and it also has this note..   anyone know?

[Zoot]

SOT = suit [your] own taste
That's what I found when I had the same question...
Cheers

[BetterOffShred]

You're the man. Thanks.  Did you get yours running? I'm about to start picking out Jfets

[Zoot]

I'm still on it.
I had some power issues first time it went on that might have caused further issues.
Still fighting with the LFO, which seems not to oscillate properly. But the different functions all work, and if I run signal through it I can hear the different treatments, but no oscillation.
This is not a difficult build if you get it right in the first time. Debugging is quite challenging though.
I also ordered another board, sometimes for me an easier way to go about it is building it again and checking than 2 builds side by side...
Wish you success with it.
Cheers

[duck_arse]

s.o.t. = select on test. a standard circuit notation.

[BetterOffShred]

Nice.  I have never seen it specified as such.  But I only build guitar stuff.
Thanks everyone.  I'm just waiting on a few parts before I start it up! 

[BetterOffShred]

Well I got it all together and fired up.  I need to build the jfet tester recommended in the build guide, but used the one I had and tried a bunch of 3819's.. then I tried some J113's and everything seems operational.  I had to watch Dino's video to make sure it was all working as it should.  The buzz switch doesn't seem to do anything in my build that I can tell so I'll need to see what the issue is there.

I was hoping there was a description of what the 6 specifies property Jfets did in the circuit that someone might have at hand. If not I was going to read through the EMS document on the dead end site and try and match parts to their schematic.

Does anyone have any info on the duties of the 6 jfets? 
Thanks for looking

[Govmnt_Lacky]

The buzz switch doesn't seem to do anything in my build that I can tell so I'll need to see what the issue is there.

"The sub-octave signal is a square wave. The buzz switch allows the option of converting this to a triangle wave which adds different harmonics providing a more 'buzzy/raspy' sound effect."

To be honest, the sub-octave is probably my favorite part of this pedal. When mixed with some of the phasing effects it makes for an awesome and unique sound

[BetterOffShred]

So I've been troubleshooting my Pompei which has a few issues, I think some related to the Jfets and some not.. I have ordered some 2N5163's to hopefully fix that because I also cannot get any 3819s or anything else to test in the recommended specs.

But I think I have found a possible error in the schematic.. my slow LFO setting doesn't work unless at the fastest setting and the continuous setting [fast] only works from about 11:00 and up on the knob so I started comparing the OG schematic and the DEFX one and there seems to be a disparity as follows:
R118 DEFX is 4.7M, and it seems to be 100k(?) On the OG.  See pics

The BOM lists this part as 4.7M




The end of the 100 on the OG schematic looks blury - think it's a 100k or R? It seems the rest of the tens or hundreds resistors do not use the "r" nomenclature and are labeled like "150" etc. So I'm leaning towards the 100k, but a couple orders of magnitude is a lot to guess

[Zoot]

I check the drawings as well and it seems to me you did spot an inconsistency between the 2 schematics, and the two resistors are quite different. Why would it be there, and if it's a mistake, that's the question. Some members of this forum have built it with the available BOM, and they seem to work... I guess only Dino can validate if there's a mistake.
I have a (non/mal-)functioning Pompei around the LFO (am building a second one). I just changed this resistor to check and it didn't make any difference.
Best of lucks!
Zoot

[digi2t]

I check the drawings as well and it seems to me you did spot an inconsistency between the 2 schematics, and the two resistors are quite different. Why would it be there, and if it's a mistake, that's the question. Some members of this forum have built it with the available BOM, and they seem to work... I guess only Dino can validate if there's a mistake.

The entire trace was done by comparing the original schematic, to an original unit. While we did not have the actual physical unit next to us (it is in France), Brent did provide us dozens (no, seriously... a lot!) of extremely high resolution pictures, of both sides of the boards, taken from every conceivable angle, and overlapping, to trace by. I painstakingly blew up, cropped, and taped the images together to rebuild the boards on paper. I then used the original schematic, the printed images, and could simultaneously zoom in on each component on the computer, to validate, trace by trace. The original unit also had the later Mk.2 Growl mod, which I had to draw into the original schematic which represents the Mk.1 version.

I back traced every inch of those boards, and triple checked everything. Whatever values that are stated in the DEFX schematic, are what I found to be physically on the boards. The only toss ups (if memory serves me) are the SOT resistors, and the Depth resistor. Both of which are referenced in the scan of the original service notes.

In other words, any discrepancies that you may find in the original schematic, are because it does not jive with what was loaded into the original unit that we traced.

EDIT;

I've gone back through the files, and traced down R102. Here is a close up of R102 from the original board. As you can see, not only was it called out wrong on the original schemo, but the board calls for a 10M. A 4M7 sits in this spot, as per the board, and as indicated in the DEFX schemo.



This was just one of several discrepancies that we discovered when comparing the original schemo to the boards. The DEFX schematic matches this unit.

[BetterOffShred]

Very interesting. Thanks for looking at that, I thought the difference between 100k and 4.7m was enough to ask 

I've gone over all my components and the unit mostly works so I'm thinking my problems at this point are JFETs .. I've ordered a handful of 2N5613 and hopefully they will work in the circuit. 

I'll keep going on it