Cardinal Tremolo

[billstein]

Just finished a Cardinal Tremolo and have a couple questions.

1. The depth pot seems to do nothing until the last 1/4 turn. Everything is bunched up there. Any suggestions what this could be?

2. Biasing the Jfets.

The build doc states. "If you want to take things one step further, you can socket the drain resistors on both Q2 (R7) and Q3 (R9) to optimize the effect. This is much easier than matching FETs or plugging in several transistors to each slot searching for the one that sounds best. To do this, set the depth at max and remove Q2. Adjust the bias for maximum gain (you can also use a 10K pot to find the best value, then choose the closest resistor). Now insert Q2 and adjust its drain resistor, if necessary, to have a similar maximum output
level."

If I remove the J201's (Q2 and Q3) where would I be measuring the voltage at? On the socket where the transistors would be? At the drain?
When it says adjust the bias for the maximum gain. Is that the maximum gain stated earlier in the build doc? Where it states the following:

"Target bias voltages (acceptable range): Q1: ~8v (6v-8.5v); Q2: ~7v
(5v-7.5v); Q3: ~3.5v (2v-4.5v)."

I've read and re-read the build doc and am confused. Any help would be greatly appreciated.

Thank you

[midwayfair]

I'll get back to you on 1, but pictures might help.

For 2, I apologize that it was unclear.

You're just removing q2 while you work on q3 so you can hear the treble band in isolation. It should read 'set the gain on q3 for maximum gain.' That will be wherever it's loudest or just sounds best to you, not a specific voltage necessarily.

[gtr2]

What mode is the depth pot bunched at the last 1/4?  Does it do it for both modes?

Did you make any substitutions etc ?  (Just so we have a little more background info.)

You measure the jfets on the drain.

Josh

[midwayfair]

Now that I'm not on a mobile: Double check the values around the depth pot, especially that you've used 470R to the +9v connection and not 4.7K or 470K, and that you used a 1K for the depth pot. These are easy mix-ups that would both cause what you've described.

[billstein]

Thanks guys for answering.

I'll post some voltages later tonight. But just some quick observations.

1. I checked all the values of the resistors and caps and they are correct.

2. I reflowed all the solder joints and checked carefully for solder bridges.

3. The depth pot is still all bunched up at the end. After playing with it a little more it is far less than the last 1/4, more like the last 10th and this happens no matter where the mode switch is set.
When the depth pot is dimed it does work ok, though I think it's not as deep as it should be.
Plus, the LED does change rate like it should. Don't know if that is helpful but thought I'd throw it in anway.

4. Speaking of mode switch. This doesn't really do anything at all. I thought it might be because of a faulty switch so tried a new one. No joy.

5. In a brief check with the DMM, voltages on the IC and on the pots are not stable at all. They are jumping from the 2's to the 7's.

Like I said, later tonight I'll spend time to write down the voltage for the J201's.

Again thanks for the replies. I checked out your demo on this last night Jon and it's a beautiful pedal, really want to get this one working.

[midwayfair]

3. The depth pot is still all bunched up at the end. After playing with it a little more it is far less than the last 1/4, more like the last 10th and this happens no matter where the mode switch is set.

First major, major check: Did you connect P1 and P2? That would definitely mean your LEDs aren't working properly ... though I'm not sure how either vactrol would have any action except possibly via going backwards through the indicator LED ...

If you HAVE connected the pads ... this still sounds to me like what happens when the pot is the wrong value, or when something else is the incorrect value, but it's possible that the problem exists elsewhere. Anyway, take a look at the schematic. The depth pot works by panning between a stable current from +9v (through R15) at low depth settings (meaning the LED stays lit) and an unstable current at higher depth settings from Pin 7 (meaning the LED turns off). One thing that could go wrong here is if you have too much resistance between pins 2 and 3, it won't be until the legs connect together that you actually get any depth. This is why I suggested that it might be the wrong value pot.

Anyway, after you verify those two, I'd start checking with the multimeter around that area for bad connections (things that are connected where they shouldn't be ...), verifying that everything's connected to the next things, etc.

5. In a brief check with the DMM, voltages on the IC and on the pots are not stable at all. They are jumping from the 2's to the 7's.

The voltages on the TL072 won't be stable -- the voltage jumping around is the oscillation. The voltage swing is right. This is good news, because it means that the LFO IS working just fine.

1. I checked all the values of the resistors and caps and they are correct.

Post pics if you can anyway ... it's possible we'll spot something.

Plus, the LED does change rate like it should. Don't know if that is helpful but thought I'd throw it in anway.

1. What color LED have you used? A blue LED could make it malfunction.
2. Try jumpering the LED. If that fixes the problem (though to be honest, it should have the opposite effect), there's still a way to get a rate (but not +depth) indicator.

4. Speaking of mode switch. This doesn't really do anything at all. I thought it might be because of a faulty switch so tried a new one. No joy.

Check with your multimeter: Does it shunt the connection to ground when flipped at all? In either position, is the gate of Q3 grounded, and in the other position is one side of C6 grounded? If you pull either q2 or q3, do you still have sound (did you sockets? )? If it is not shunting the connection to ground, there may be a fault in the board. If it is breaking the connection to ground, the problem lies elsewhere. If the depth is super shallow, you very well might not hear an effect, because Q3 will be too close to ground to make sound (remember, at low depth settings, Q3 is "off"), and Q2 will just sound like it does in both modes. However, I would think you'd hear a slight tonal change.

[billstein]

Edit: N/M figured it out

[billstein]

Edit: N/M same as above. I figured it out.
Pedal still doesn't work but I figured out how to tell if the switch was shunting to ground.

I'm obviously just trying to up my posts count. 

[midwayfair]

I'm obviously just trying to up my posts count. 

No shame in that.

[billstein]

Haven't had any time to really spend on this, hope to in the next few days. I was able to take a few quick photos to see if anybody could see anything.

Thanks guys for your help.



[URL=http://imageshack.com/i/05xwtfj/]

[URL=http://imageshack.com/i/0nom17j/]

One other thing I noticed that might give a clue. When I turn the wave trimmer it doesn't seem to be changing anything.
Jon you asked about the depth pot. It is B1k. The rate pot is C100k.

[neve1272]

if you could point me in the right direction
my cardinal is behaving the same way all depth is in the last part of the control wise

[midwayfair]

To be honest, I was never able to guess out what could make Bill's act the way it did,and I don't know if he fixed it. Is your wave pot also no working? What are your voltages like on the op amp? Does the LED blink?

[neve1272]

every thing works perfectly that i can tell just the depth pot is bunch up at the end ill take some readings tonight

thank you for response

[midwayfair]

every thing works perfectly that i can tell just the depth pot is bunch up at the end ill take some readings tonight

thank you for response

This is normally what happens when a pot is wired backwards or something, but obviously that's not what's going on here. Is there any gradation at all in the depth, or is it like a switch? I'm trying to figure out if somehow a different taper would help in very rare cases, but it's not like this is an untested LFO.   

[rullywowr]

This is a long shot, but are the correct Vactrols installed?  There are many different flavors and they do behave differently.