NEOVIBE detective work...would someone mind helping???

[yunger]

hello,
i've been going up and down my neovibe circuit for the past few weeks trying to figure out why it is so dull. i do see some clues based on voltages but i'm getting in over my head as to causes. my pedal seems to be fine in amplifying. i have to cut back on the volume pot to be at unity gain. based on other peoples posts, the voltages across Q1,Q2,Q3 seem normal.

the lamp driver part of the circuit seems pretty normal also. the bulb flashes just fine and is completely controlable via the dual rev. speed pot.

the problem seems to be in the circuit around the LDR's. voltages read from Q5,Q7,Q9 seem good. i'll list the problem transistors.

Q4-B-4.3
      C-15.16
      E-4.36

Q6-B-4.29
      C-15.17
      E-4.32

Q8-B-4.33
      C-15.17
      E-4.36

Q10-B-5.4
        C-15.17
        E-5.42

these are all 2n3904's.

another oddity i noticed was actually in one of the LDR's. check these DC voltages out.....

from R28 to LDR(A) i read 5.37/ from that LDR(A) to C14=1.45

from R27--->LDR(B)=4.31/ from LDR(B)--->C11=1.2

from R16--->LDR(C)=4.27/ from LDR(C)--->C8=1.0

from R15--->LDR(D)=4.28/ from LDR(D)--->C6=3.12

to my untrained eye, it appears that the LDR i named (D) is toast. i do remember spending a little extra time with the soldering iron on that particular one.

if anyone here would help me, i would really appreciate it. hopefully someone here has dealt with the same problem before. i can supply all my voltage readings if that would help. thanks,

david

[R.G.]

Is this the GGG board, or one of my earlier layouts?

All of the transistors you list seem to be having problems. The base and emitter can't be helping much if they're all at the same voltage.

Take the stage with Q4 and Q5. Measure the DC voltages across every component around that stage, including the capcitors and LDR. Post those back here.

[yunger]

disregard my previous LDR readings. they have changed. i don't know how i got those numbers the first time.

i got the layout from the current GGG and cross referenced it with the current Geofex.

here are my Vdc results for the stage around Q4 & Q5:

Q4-B-4.3--->R15--->4.28--->LDR--->4.3--->C6--->5.74(Q3 E)
         " "--->C5--->9.06(Q3 C)
     C-15.17
     E-4.36

Q5-B-4.36
     C-11.45--->C9--->4.29(Q6 B)
            " "--->R18--->15.17(supply)
     E-3.68--->C8--->4.24(LDR 2nd stage)
           " "--->R17--->0(ground)
           " "--->C7--->4.71--->R13--->4.3(Q4 B)
                                 " "--->R12--->0(ground)
                                 " "--->R14--->15.17(supply)

sorry for the redundancy. i wanted to be thorough. for the record: i used 3.9k for R9 and 560r for R8. maybe this is my problem.

hope this helps and thanks for the reply R.G.                                                                                                         

[yunger]

i see..... from Q3 E to the negative pin of C6 is 5.74 dcV
      ...... from the positive pin of C6 to the LDR is 4.3 dcV.............positive side smaller than negative?

maybe my trouble is coming from the preamp stage around Q3?

just speculating, but i'll go back and re-test and measure the preamp stage again just in case.
any comments are definately welcome and appreciated.

[yunger]

more info: i'm using "dc-brick" 18v power, i hooked up the ground lug of the output jack to pad O. here are my dc voltages....hopefully something stands out to someone.
Q1- C-1.63
       B-.76
       E-.71

Q2- C-6.27
       B-1.63
       E-1.21

Q3- C-9.08
       B-6.27
       E-5.74

Q4- C-15.17                                                             4.3--->(+)C-6(-)--->5.74 !!!                               
       B-4.3
       E-4.36

Q5- C-11.45
       B-4.36
       E-3.68

Q6- C-15.17
       B-4.29
       E-4.32

Q7- C-11.44
       B-4.32
       E-3.66

Q8- C-15.17
       B-4.33
       E-4.36

Q9- C-11.43
       B-4.36
       E-3.69

Q10- C-15.17
         B-5.4
         E-5.42

Q11- C-16.96
         B-3.12
         E-(8.3-10.2)fluctuation

Q12- C-16.96
         B-(8.3-10.2)fluctuation
         E-(7.5-9.7)fluctuation

Q13- C-14.95
         B-3.14
         E-2.5

D1- A-8.21                                                                        3.12--->(+)C-19(-)--->8.21 !!!
       K-8.21

D2- A-8.21
       K-8.21

[R.G.]

Q1- C-1.63
       B-.76
       E-.71

NPN transistors don't work when their base is less than 0.5 to 0.7V more positive than their emitters. There is a problem here.

Q4- C-15.17                                                             4.3--->(+)C-6(-)--->5.74 !!!                               
       B-4.3
       E-4.36

NPN transistors don't work when their base is less than 0.5 to 0.7V more positive than their emitters. There is a problem here.
On top of that, electro caps don't work right when their + lead is higher than their - lead. Is the cap inserted backwards or is something else forcing it to be backwards?

Q6- C-15.17
       B-4.29
       E-4.32

NPN transistors don't work when their base is less than 0.5 to 0.7V more positive than their emitters. There is a problem here.
On top of that, electro caps don't work right when their + lead is higher than their - lead. Is the cap inserted backwards or is something else forcing it to be backwards?

Q8- C-15.17
       B-4.33
       E-4.36

NPN transistors don't work when their base is less than 0.5 to 0.7V more positive than their emitters. There is a problem here.
On top of that, electro caps don't work right when their + lead is higher than their - lead. Is the cap inserted backwards or is something else forcing it to be backwards?

Q10- C-15.17
         B-5.4
         E-5.42

NPN transistors don't work when their base is less than 0.5 to 0.7V more positive than their emitters. There is a problem here.
On top of that, electro caps don't work right when their + lead is higher than their - lead. Is the cap inserted backwards or is something else forcing it to be backwards?

3.12--->(+)C-19(-)--->8.21 !!!

Electro caps don't work right when their + lead is higher than their - lead. Is the cap inserted backwards or is something else forcing it to be backwards?

Check those places for the usual: shorted traces, soldering problems, reversed components, transistor leads incorrect.

[yunger]

well....i rechecked my board again. traced and documented every component and compared with both the layout and every schematic i could find on the vibe. i cut in between tight traces with an xacto knife. touched the ground trace with 9v and tested multiple points around it to verify that it wasn't broken at some point. twisted the board to pop loose any potential bad solders.

i must admit, i am stumped. before i go back through the components for the 1000th time, i'm going to check the pots and switches. i'm tempted to just reverse the caps that are out of wack polarity-wise and living with it. aside from the fact that q1,q4,q6,q8,q10 arent working, my neovibe vibes.

one thing i noticed.....when i touch either the depth or speed pots, the speed doubles or even triples. then when i let go of the pot things slow down again, gets intensely choppy and the bulb flashes brighter. maybe that is a clue to someone. you can hear this in the clip i'm providing. btw....i am not the best guitarist in the world and i recorded the clip using the sound recorder on my computer and a cheap mini-tape recorder mic.

https://filebox.vt.edu/users/dalambe2/sample/

[John Lyons]

The Neovibe is a difficult build and especially since the board is very dense and the transistor pads are IMO too close together.

Here are somethings to note.
The square pads (for the electrolytic caps) are positve
The speed pot is shown wired wrong on the layout. Can you please fix this RG?
Twisting the board is a sure way to break traces and or soldered pads.
Make sure you have no solder traces. Get out a fresh print out of the layout and a colored marker and start at the input and trace the whole board to make sure you have the transistors and caps in the correct orientation. Also make sure they are the correct transistors..

Make a print out of the PCB trace and reverse it so it looks like the bottom of the board. CHeck the print out aagainst the soldered board to make sure you have no solder bridges. They are easy to get on this board!!!

Are you getting the correct voltages at the points noted on the Neovibe pdf schematic?

The problem is something simple, always seems to be.
When you find it you will feel really stupid to have made such a mistake, we all do it.

John

[R.G.]

The speed pot is shown wired wrong on the layout. Can you please fix this RG?

I did, long ago. See Geofex, 6/24/2006:

6/24/06      Updated - there was a bug in the wiring diagram for the Neovibe.  That's corrected in this write up. Notice that the write up at General Guitar Gadgets will still have this error until it gets updated as well. If the resolution is still not good enough, check this schematic and wiring diagram.

What I can't do is get all the local copies updated all over the internet, so we're stuck with this one forever, just like I still get notes about the reversed electro cap in the input of the FIRST Neovibe layout, from before the time when there was a WWW, only usenet groups.

On a positive note, his LFO is oscillating. That's not the problem in this case. Something is wrong in a systemic sense with the first transistors of his darlington pairs. They all have Vbe~0. Could be solder, could be reversed cap, could be etching, could be he's offended some minor deity.

But I agree - it's going to be something simple. It nearly always is.

[John Lyons]

Oh, ok RG.. I assumed the GGG Neovibe PDF was a link to the GEO documents.
Thanks for the clarification.

John

[yunger]

just out of curiosity.....what differences will i notice when i get q1,q4,q6,q8,q10 working? i'm assuming that ac signal is passing through regardless or else i would get no sound at all.

i noticed something strange on the schematic's recommended voltages. the base of q4 is called out as 4.1v as is the emitter of q5. if the emitter has to be .5v lower than the base, one of these transistors won't work according to the schematic. was this a misprint?

[R.G.]

the base of q4 is called out as 4.1v as is the emitter of q5. if the emitter has to be .5v lower than the base, one of these transistors won't work according to the schematic. was this a misprint?

Yes - sorry! - it's a misprint. The base of Q4 (and Q6 and Q8) should be 4.6-4.8V, depending on your meter loading. I'll fix the schemo. I think this was right in earlier schemos and I introduced the error in updating. Thanks for catching that.

On your original question. Your 'vibe is dull because the phase stages are missing the input darlington transistor to buffer the signal coming in. This causes loss of treble and a general dullness, I think.

Let's cut to the chase on this one. Set your meter to its ohms range and test for resistance between the base and emitter of Q4, 6, 8 and 10. Are any of them shorted together? Measure on the transistor leads, not the PCB traces.

[yunger]

R.G.,i hope you don't regret the day you released neovibe info. i can see how much time and effort went into laying it out and of course helping us diy'ers get it working. it is appreciated and i'm inspired to be able to do something similar one day in my own profession.

i will check those transistors when i get home from work.

[yunger]

Rbe= resistance at b & e

Q1-Rbe=1177k

Q2-Rbe=877k

Q3-Rbe=59k

Q4,Q6,Q8-Rbe=(681k-683k)

Q5,Q7,Q9-Rbe=(555k-560k)

Q10-Rbe=148k

Q11-Rbe=not stable enough to measure, higher than 2000k? my meter apparently only reads up to 2000k

Q12-Rbe=580k

[yunger]

could be he's offended some minor deity.

this one's right on the money.

i spent 4 hours last night checking continuity. according to my buzzing dmm, everything is as it should be. no shorts or breaks.

i think i'll step away from this before i'm tempted to bring out the hammer. i'm going to build another board. only this time i will pre-measure all components and make a checklist to verify placement for each.

[R.G.]

Your measurements seem to indicate that the board is OK, and that shorts, solder or otherwise are not the cause for Q4,6 and 8 being at the same voltage on base and emitter. Just to be sure - did you verify that you have the correct transistor types, and the correct pinouts for those transistors in the board? That's all I can guess from what you have posted.

A properly functioning NPN transistor correctly inserted into the board would not give those DC voltages. So either
(a) it's not an NPN
(b) it's not inserted correctly
( c ) it's not functioning properly
(d) the board is not correct
or
(e) the collected data is incorrect.

(d) is by far the most common problem, but you seem to have eliminated it. (a) and (b) would the next most likely, but you have functioning transistors at other places, so that makes them less likely. (e) is random, and happens to all of us at times, but generally measuring a couple more times uncovers the mistake since making random mistakes the same way several times is not random. ( c ) is by far the least likely, but it does happen.

[yunger]

i think i got those 2n3904's from mouser. they were oddly spaced. the base actually stuck out on the flat side. i had to bend it back past the curved side to fit the board. perhaps i broke leads inside a transistor itself in doing so.
        e                                               e
        l                                                l
   b --D                                               D-- b
        l                                                l
        c                                               c
the way they came                          had to bend base