EA Tremolo ... oscillator debugging

[cmdrfun]

Hey all,

I built an EA Tremolo on perfboard, based on the GGG schematic. The only change I made was to leave out the 10 uF cap that is in series with the 0.22 uF cap, because it just looks like the 0.22 uF cap.

When switched on, it works like a clean boost. No distortion or anything, and it's louder. So I assume my change above is not a problem, since it's between the input buffer and the amplifier stage. But there's no modulation, so the oscillator's not working. As a test, I just connected the oscillator out (center lug of the depth pot) to my amp, and get nada.

My real question is - what's a good way to debug the oscillator without an oscilloscope? All the voltages at the transistors look OK, but since a bunch of the stuff is AC coupled that doesn't mean too much. I've done the usual first debug steps, look for solder bridges, cold joints, etc. and it looks pretty good.

Thanks...

[armdnrdy]

Posting a schematic or a link to one is very helpful in helping you.  

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

So we're on the same page....what component number did you leave out?

[armdnrdy]

Put your depth control all the way clockwise and the speed control counter clockwise and see if your voltage at the R4 side of R8 moves up and down.

Also, check you transistor orientation with data sheets on the net. 

[cmdrfun]

Yes, that's the schematic I used. C9 was left out. (The 10 uF.)

Transistor orientation is correct. (Checked it when I did the layout, and again while troubleshooting.)

The point you're talking about is fixed at 9V (or in my case, about 9.5V). It won't oscillate no matter what.

I'll check the voltage on the other side of R8 though and see if it varies with changing the pot values.

Thanks...

[armdnrdy]

Hold the phone!!!!

I told you the wrong point to check. I had to scroll over to see the rest of the schematic.

Check for oscillation at the depth pot wiper (lug 2)

[cmdrfun]

Yep, did that... see original post. 

[cmdrfun]

I will check to see what the voltages are at the extremes of the depth pot. If there's a loose connection or I laid it out wrong, maybe that will be a hint.

[cmdrfun]

Well, at the moment, no matter what the depth pot is set at, my meter reads 0 V. Again, it's AC coupled so I don't know if that's right or not.

[armdnrdy]

Did you ever try the Q3 side of R8?

If you don't get any oscillation, and you're sure that the transistors are oriented correctly, I would look for solder bridges or a bad solder joint.

Also go through all of the components and check them for proper values. A misplaced resistor can ruin your whole day!

[cmdrfun]

Yes, it comes in at 7.8 V, and doesn't oscillate.

I just built the oscillator section alone on my breadboard, and it also doesn't work. I wonder if there may be a schematic issue.... I'll check some of the other schematics online and see if I can find a difference. I know that sometimes an error may be fixed on a PCB without the schematic getting updated.

Thanks for the suggestions...

[armdnrdy]

This is the schematic that I punched into Eagle when I built one of these a few years back.



It looks like the same thing!
R8 on my drawing is a 10K for the "rate" indicator mod.

[cmdrfun]

I built this schematic in my circuit modeling app and it worked fine as well. 

I guess it's time to take a break, do something else and come back. I'm not seeing anything that could be the problem.

[Davelectro]

Pay attention to C5, C6 and C7 orientation (unless you are using non-polarized caps).

Also, make sure C3 is not leaky. For this very reason the original EA tremolo article recommends a tantalum cap.

[cmdrfun]

OK, came back to it this morning, got the oscillator working, and it's modulating.

However, the modulation is extremely weak and barely detectable. I'm not sure how to tell if the 22 uF cap is leaky or not.

I went ahead and ordered a PCB from GGG... everything I've ever had from JD has worked well. He specifies a 22 uF electrolytic cap so I have faith that works fine.

[Davelectro]

Yes, a healthy electrolytic should work fine.

[duck_arse]

using your amp to test the output of an lfo is a very bad idea. the oscillator may well be working, and feeding a large signal that you can't hear to your amp, which will then try and destroy the speaker by driving the cone across the room. repeatedly.

if the osc is oscing, it comes down to the amplifying section. you may need to vary the resistors above and below the depth pot to get the fet gate voltage where the fet is happy. a leaky 22uF will feed dc to the emitter of the amplifier, producing thumps you won't want.

do you know what the osc prob was?