GGG EA Tremolo Trouble Shooting

[Luc_antonio]

Hey

So ive put together the EA Tremolo by General Guitar Gadgets, upon powering up the pedal the sound is completely blown out and distorted with the tremolo effect pulsing vaguely in the background.

Here are my pin read outs

Q1 C =3.7
     B= 8.7
     E =2.5

Q2 D = 0.0
     S = 0.0
     G = 0.0

Q3 C = variable
     B = 0.6
     E = 0.0

Q4 D = OL
     S = OL
     G = OL

They dont match up to the GGG readouts. Im hoping someone out there can give me some insight on how to debug this pedal.

Here are the build details:
http://www.generalguitargadgets.com/pdf/ggg_eat_lo_dual_fs.pdf?phpMyAdmin=4a28f86a515b7883e7bc35a68d4e7b6d&phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

http://www.generalguitargadgets.com/pdf/ggg_eat_instruct.pdf?phpMyAdmin=4a28f86a515b7883e7bc35a68d4e7b6d&phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

[PRR]

Schematic?

[duck_arse]

at the very least, your q1 voltages are all over the place. I'd suggest checking the transistor pinout AGAIN, and the values you have in R2 and R3.

most people (and there are plenty) have oscillator problems, so you seem lucky there. you might try reading the ac voltage at the gate of q2, see what it says at max and min settings.

what does "OL" mean?

[Luc_antonio]

This is the schematic

http://www.generalguitargadgets.com/pdf/ggg_eat_sc_improved.pdf?phpMyAdmin=4a28f86a515b7883e7bc35a68d4e7b6d&phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

OL stands for over limit. Not sure if that's good or bad..
I'll retry the pin outs this weekend.
When I plug in the pedal it has the tremolo effect but it's coming through all blown out and distorted. Something is pushing it over the edge

[PRR]

Thanks.

The Q1 B= 8.7  E =2.5 suggests the pinout (or part!) is wrong, because that should should be 0.6V not 6V. And 6V is the reverse breakdown voltage.

Not at all sure how "OL", assuming your power (not stated) is 9V and your meter will read to 10V. It is *not* simply confused by the wildly swinging voltage, because there should be no trem-swing at Q4.

[Luc_antonio]

The pedal is supposed to be run at 9v and I just verified that the Q1 transistor is the right kind according to the kit (2N3904). I just retested Q4 at a higher setting on my DMM. Here are my results

Q4 E=8.7
      B=5.3
      C=2.9

I've been talking to JD at General guitar gadgets and he seems to think that the problem is around Q3 but I can't seem to find anything wrong unless the transistor itself is defective. Q3 is a 2N5088

[PRR]

> Q4       C{S}=2.9

R6 is really 10K?

> sound is completely blown out and distorted
> think that the problem is around Q3

You can throw Q3 away and you will still get straight (un-wobbled) signal.

So Q3 is nothing to do with "blown out and distorted".

[duck_arse]

if you disconnect C3 (22uF) from R5/R6, do you get clean audio out?

if you then reconnect C3 between R5/R6 and ground (bypassing the trem fet), do you get louder/distorted output?

if so, then it may be you need to tailor R19 and R7 to your particular Q2. or maybe check your Q2 pinout again.

[Luc_antonio]

So i removed the 22uF capacitor from the circuit part surrounding the tremolo part of the circuit and it got even more distorted with no tremolo effect.  Although its starting to sound like a really cool lofi fuzz pedal haha 

[duck_arse]

well, that narrows it down to either Q1 or Q4, but leaves me stumped. you need to go round and measure/post their voltages, and check your resistor values  again.

[Luc_antonio]

Alright Duck Arse here are the latest voltage readings

Q1 E= 0.25V
     B= 0.85V
     C= 0.37V

Q4 E=8.67V
     B=5.28V
     C=2.85V

As per the resistors, they match up with the schematics.

[duck_arse]

well, you should be reading q4 as gate (to C8), source (to R17) and drain (to the positive supply), not as E,B,C. the gate should be half the supply, and none of your voltages is close enuff so something is wrong there (R15//R16).

q1 should have about 1V8 at its base, and its emitter should be ~0V7 lower, so 1V1 or so. this produces 1V1 / (1k2 + 180) = ~800 uA. the collector should  be 8V67 - (800uA * 4k7) = 4V9, so there is problems there as well. as prr says, wrong pinout or part. as mr ohm says, V = IR.

can you post photos of your board, top and bottom?

[Luc_antonio]

The board

(Top)

http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/photo_007.JPG.html?g2_imageViewsIndex=1

(Bottom)

http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/photo_1_.JPG.html?g2_imageViewsIndex=1

[GGBB]

The only problem I can see is that C3 is backwards.  The schematic and the layout diagram disagree with each other, although according to their build guide where they say some older PCBs have a screen print error "all the diagrams show the correct orientation."  (You've really got to be careful with GGG stuff.)  In any case yours is backwards in comparison with the layout diagram and the photo of their build in the guide.  However I don't think this would cause the problem you are having.  My guess is that Q1 is damaged and needs to be replaced.

[duck_arse]

I can see a scratch on the track connecting R17 (10k) to Q4 /C9. is that track continuos or open?

other than that, I'm stumped if it's not wrong or ded transistors.

[Luc_antonio]

Well guys, i hate to further the mystery but i just changed Q1 with a brand new part and still no change in the blown out sound. Also i looked into what may be the scratch on the board between R17 and C9 but it was only some grime left from my soldering iron but otherwise the track has no break in the line.

Perhaps maybe a fault in the wiring layout? should i just order a new PCB and components all together?

[duck_arse]

have you read this page:

http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

if not, read/repost. don't buy new board, the problems are with Q4 and Q1, but I'm still not sure of your meter readings. I tried those two sections on my breadboard today, and couldn't get voltages anywhere near yours.

[Luc_antonio]

Okay, well im pretty sure im doing this right. DMM on 20v, Black point on the Ground, Red point on the pin of the transistor. My readings are always the same. Should i be taking measurements from the components in series from the pins of the transistor?

[GGBB]

It's the voltage readings at the pins that matter, but it wouldn't hurt to check continuity/resistance across components etc.

Try bypassing Q4 - you can do that by connecting the input to C1 - jumper from the C1-C9 junction to the signal input.  Do this with C3 removed.  If you get clear sound that way then it must be Q4 that is the problem.  Otherwise its Q1.  (Or related components.)

After that it would be time for an audio probe starting at the input.

[Luc_antonio]

So i tried the jumper to bypass Q4 with C3 removed and the result was the sound became much more gated while being distorted with no tremolo effect.