LTD debugging - drive knob does nothing.

[BoxOfSnoo]

Hi, it doesn't seem like anyone reads the "happy holidays..." LTD thread anymore, so allow me to repost here.

I have built the LTD, with only slight modifications - I used the 1k and .22uF resistor/capacitor mod as suggested - mainly because I can't find the original spec parts around here!  I also had no success with a 91 ohm resistor, I assumed the error rate on a 100 ohm resistor would get me in the neighborhood...

I am using the layout from here: http://www.diystompboxes.com/smfforum/index.php?topic=72917.msg721834#msg721834

I get a quieter-than-unity (though distorted) signal, and the gain knob doesn't vary the drive at all, it sounds like it's at max.

I have tested the IC and it gives me these figures:

Battery: 8.98v

1: 1.491
2: 1.49
3: 1.423
4: 0
5: 1.431
6: 1.485
7: 1.455
8: 8.52

I know the pin 1 number doesn't look right, I can't figure out why it's dropping the voltage so low?

I'd appreciate if someone could give me some pointers of what to look for...  Thanks in advance.

[petemoore]

  Test for 1/2v at the voltage divider [2 resistor string between V+ and Gnd. where they meet].
  Gnd. 0v and the supply V looks ok, all of the inputs and outputs are well away from 1/2v, they have the bias voltage in common.

[BoxOfSnoo]

Yes, there's a problem there.  It's only 1.5v instead of ~4.5.  What do you suggest?

Thanks, by the way

P.S. Theory is good, I like to learn how things work and why things work that way.

[petemoore]

  Lift the supply
  Check from divider center to ground:
  Test R value of each divider resistor [between Vbias/Gnd.  then Vbias/V+.
  Test R value of resistors [or anything/everything else] that connects where the 2 divider R's meet, follow to the opamp pin.

[BoxOfSnoo]

Ok...  divider center to ground and between vbias and gnd are the same thing, right?  That one measures 3k, which I don't understand - I verified that it is a 10k resistor, and even replaced it, and it still measures 3k when in the circuit. 

Does that mean there's another unwanted resistance source to ground from vbias?

I presume more of the voltage is being shunted to ground than I want, which results in the low voltage, is that correct?  Do you think this is related to the gain knob issues?

It seemed like such a simple circuit.

[BoxOfSnoo]

Still looking for hints on this one.  I have no idea why the vbias is so low...  I've traced and retraced this circuit many times, I just don't know what to look for.

[Quackzed]

maybee one of the caps at the bias point 47u or 100u are bad... diode oriented the wrong way?, flux on the board is creating a path btw vbias and ground or 9v and ground... seems like something is dropping the vbias down to 1.5v ?!? dead battery? maybee one of your tone pots is touching the box and grounding out through that 3k to vb... that would make sense if your getting 3k resistance when measuring the 10k from vb to g. yep, i believe one of your tone pots (mid? or bass?) is getting grounded somewhere...
c'mon lucky 7...

[BoxOfSnoo]

maybee one of the caps at the bias point 47u or 100u are bad... diode oriented the wrong way?, flux on the board is creating a path btw vbias and ground or 9v and ground... seems like something is dropping the vbias down to 1.5v ?!? dead battery? maybee one of your tone pots is touching the box and grounding out through that 3k to vb... that would make sense if your getting 3k resistance when measuring the 10k from vb to g. yep, i believe one of your tone pots (mid? or bass?) is getting grounded somewhere...
c'mon lucky 7...

Aha, you are the doctor!  I removed the ground jumper from the tone section, and boom, 4.5v.  I rewired it slightly, (i.e. more neatly) and when I reconnected the tone again it changed again (this time it's actually HIGHER - in the neighborhood of 6v) so I think I may be able to narrow down on one or two parts now...

Still no effect on the drive knob though; I think that's a second problem.

Thanks!

[Quackzed]

 
nailed it!

[BoxOfSnoo]

nailed it!

Almost!  I'm gonna give you full credit for reading the post and pointing me in the right direction, of course.

The bug was a subtle yet pretty important error in the layout I made.  Hint: it has everything to do with vbias - and it's DEFINITELY related to the gain knob.  I'm gonna blame DIYLC with its sticky points feature for this one .  I'll post the corrected one tomorrow morning, it's too late right now even to test it properly...

Thanks again!

[Quackzed]

so, you figured out what was wrong with the gain pot as well? cool. I just surprised myself that i came up with a plausible reason for the vb issue and was right. rarely happens to me.  glad you found the issue. i hate to see alot of hard work go to waste. ive had plenty of issues that went unresolved, and consequently many boards go unboxed and in the bin. 

[BoxOfSnoo]

Yup.  Though it wasn't quite right, it was in the right direction - something was pulling voltage away from the vbias.  I started desoldering and puling things away from the vbias line, there was no change... I had a couple of things that needed to be grounded in the tone circuit and those were connected by a jumper to the ground wire along the bottom of my layout, when I disconnected that it got better.  I figured like you did, that the tone circuit was causing it.

It turns out that was 100% a side-effect though  Probably because in so doing I disconnected the second source to ground from the chip...  wait a minute, why is there a second source of ground to the chip??  (ding)

The real issue that solved everything all at once was that my vbias that went under the IC was accidentally drawn as connecting to pin 1 of the IC.  Like I said, DIYLC sticky points are the real culprit here  

I will post the updated layout in the "Happy Holidays..." thread.

My poor solder sucker gave its life for this project.  A moment of silence, please.  That's enough... play overdriven guitars now.