Debugging EA Tremolo...

[audiolife]

I'm taking a crack at my 3rd build or so...the Beavis EA Tremolo

http://www.beavisaudio.com/bboard/projects/bbp_EATremolo.pdf

I am not getting any signal. I have traced through my breadboard layout many times and everything is totally correct. I'm using all the exact parts as on the link. I have jumpered the power ground rails from one side to the other, and I test continuity on the grounds with the mm but no signal.

Any suggestions? I'm a noob but I know everything is in the right place.

Thanks~

[audiolife]

I feel I should mention that all transistors are oriented correctly. I checked the data sheets to make sure.

[GGBB]

Do you have an audio probe?  Use it to trace the input signal through the circuit until you find where it stops.

[audiolife]

No probe, can the same end be accomplished using the continuity setting on the DMM? Hold the black lead to neg. of the battery and touch the leads of the components? If so, I've tried that and I get nothing. Everything is connected to the power and ground rails correctly.

[Pyr0]

some breadboards do not have a continous ground and power rails - the horizontal rows top and bottom, instead they are broken into isolated segments. So make sure you have continuity on those rails with your Dmm.

[audiolife]

I know that my rails are continuous down one side. I jumpered them to the other side of the breadboard as well.

[hannibal827]

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

Post your voltage readings.

You will probably discover the problem when you take the readings.

[audiolife]

Just measured the trannies out:

Battery: 7.99v

Q1: 2n5457

D - 7.98
S - 5.1
G - 2.61

Q2 2n3904

E - .66
B - 1.22
C - 5.6

Q3 2n5457

D - 0
S - 0
G - .62

Q4 2n5088

E - -.01
B - .58
C 4.4

Some of these measurements fluctuated a bit but hovered around the above numbers. Thanks for the help everyone!

[hannibal827]

Well, I looked for voltage readings in other EA Tremolo threads where Q3 *wasn't* the problem, just to make sure, but I couldn't find any.

But I am guessing Q3 is your problem.  It can't be good that the drain pin is grounded.

Try turning up the speed all the way, to minimize the fluctuation of the LFO voltage, and see if the drain pin stays at 0v.

[hannibal827]

Re. audio probing: it's easy do do a makeshift one.

In the case of this circuit, since you have it on the breadboard:

break the connection between C3 and the collector of Q2.  Connect a wire to the end of C3 that isn't connected to Volume lug 3.  The other end of the wire is your probe.  Keep lug 2 of the Volume pot connected to output, and listen as normal.

Stick the wire in the breadboard at the following points:
1) Input
2) Q1 gate
3) Q1 source
4) Q2 base
5) well, the next test point would be Q2's collector, but that's your existing circuit, and you aren't getting audio there.

So at which of these first four points do you lose the signal?

[audiolife]

I'll have to try the audio probe approach tomorrow. When i hold the neg. lead of my DMM to the neg. of the battery and poke around with the positive on continuity, I don't get anything. Not even on either lead of the first capacitor after the input.

Everything's in the right place and all parts of brand new. I can post a gut shot but I doubt it will help...

I was just going over the GGG schematic and noticed that I had the transistor numbers wrong, but the part and readings were correct.

Q4: 2n5457

D - 7.98
S - 5.1
G - 2.61

Q1 2n3904

E - .66
B - 1.22
C - 5.6

Q2 2n5457

D - 0
S - 0
G - .62

Q3 2n5088

E - -.01
B - .58
C 4.4

I compared the voltages to what the GGG EA manual says and they're all spot on to what thye should be.

[GGBB]

I'll have to try the audio probe approach tomorrow. When i hold the neg. lead of my DMM to the neg. of the battery and poke around with the positive on continuity, I don't get anything. Not even on either lead of the first capacitor after the input.

That's good.  The audio probe does not check for continuity, it checks for an audio signal.

Another quick check is to connect the gate of the 2nd 2n5457 (which is also lug 2 of the depth pot) to ground.  This defeats the tremolo section and turns the pedal into a boost.

[audiolife]

Tried replacing it as you said GGBB and had the same results

[PRR]

> can't be good that the drain pin is grounded.

In this, as in several other FET trems, the FET is not an active amplifier. It is a variable resistor. In this case, source goes to ground, drain has only a cap to the signal path, the DC voltage on drain is zero.

Anyway, complaint is "not getting any signal". To "get signal", only Q1 and Q2 are needed. Q3 changes the gain of Q2, but can't turn-down to zero. Q4 wobbles Q3 for tremolo effect, which may explain "fluctuated a bit"; for debugging basic signal-passing you could short Q4 Collector to ground to stop the wobble and incidental fluctuations.

Your Q1 Q2 voltages look correct. (Q1 Gate measures lower than expected 4V because of meter-loading in this high-impedance point; source a volt higher than 4V suggests the gate really is 4V when you are not poking it.)

Despite your protests, the prime suspect is still a miswiring. Possibly in the signal-only path. Sometimes as simple (and hard to self-spot) as getting jacks wrong.

Skip VR1 volume pot. Take the bare end from output jack tip-contact and touch it. Should make your amp buzz LOUD. Now connect that end to C3, where you disconnected VR1. Does that pass signal? Put your finger where C3 and R8 connect together. That should buzz (less because R8 loads-down your finger-buzz). Now touch each end of C1 and C2. That should buzz. OK so far? Lift the grounded end of R4 and stick it to Q1 gate. Touching either end of R4, or C1, or input jack, should buzz.

[audiolife]

Thanks for all of the help guys! I've tested all the connections to ground rails and they're good. I've tested all connections to the power rails, and they measure approx. 7.7v, which is what my battery measures (it's drained a bit).

@PRR thanks for the insight! I'll get back in the shop tomorrow and tinker some more. I'm 100% that everything is hooked up according to the diagram, so my problem could be in one of the jumpers. I'm at the point where I just want to tear it down and start over. By "problem in the signal only path" do you mean it's likely somewhere on the left side of the breadboard as per the diagram?

I've gone through all of the DIY and RG Keen debugging pages and tried it. I'm really learning a lot. I've had to troubleshoot before but nothing like this, really enjoying the experience.

My next question would be could there be an error on the Beavis diagram? Doubtful, as the problem is usually me!  

[GGBB]

My next question would be could there be an error on the Beavis diagram? Doubtful, as the problem is usually me!  

Actually ...

I noticed earlier that the schematic showed 2 Q2, so I looked some more and ...

Schematic also shows 2 C1.  But the big problem is ...

On the breadboard layout, in the upper right corner, the 68K resistor is shown connected to power, but it should go to ground.

[audiolife]

I did notice that on the schematic about the mislabeling of C1 and Q2. I have them connected as per the diagram with the correct parts.

I didn't notice the 68k resistor. I switched it to ground and tested the same results with the meter. Don't have too much time today, but I'll crack at it tomorrow.

Maybe Beavis is on the board here and can chime in? His site is amazing and I would love to get this baby working for a tour I have coming up.

[PRR]

Q2 Q2.... hmmm, missed that.

The 68K may be wrong, but it would max-out both trem and gain, not "no signal".

> "problem in the signal only path" do you mean

Basic signal-flow:



> everything is totally correct
> I'm 100% that everything is hooked up

Nevertheless, something's not right. Could be bad transistor, but the key transistors show good DC voltages and thus are not dead (they don't know DC fron signal). Could be a bad jack or pot or joint. Could be dodgy poke-board. But almost 99&44/100% of the time, it is a wrong connection the builder can't self-spot.

[audiolife]

Hello again all!

So I took the project apart and started from square one. I did another project in the meantime and it worked perfectly, so I know my breadboard is fine.

Hooked up the Beavis EA Tremolo again, meticulously making sure that everything was correct. Test it out...

I get some signal now, but it just sounds like a really bad distortion. No tremolo at all and just not right.

Transistors all measure the same. I really wanna get this working for a show, don't want to abandon the project.

Thanks for all of your help! 

[GGBB]

What happens when you remove R12 and C4?