Small Stone Voltages Please

[Jaicen_solo]

Hey guys, i've recently decided to resurrect one of my (very) old projects, the Small Stone from GGG. I built it back when it was JD's Guitar Effects page, something like 5 years ago. Unfortunately, I never got it to work properly. I can get a subtle phase sound from it, but it's buried under a load of hiss and hum, like a bad earth. It's really quiet too and only works with one position on the color switch. Turning down the speed stops the LFO as well (I think, the phasing is hard to hear at slow speeds).
I've been looking for pin voltages, but I can't find any for the CA3094 version i'm building. I suspect it's got something to do with Q3 & Q4 and IC5, but I can't think what it is. I have no experience with OTA's so i'm looking for any help you guys can give me!

The voltages for IC's 1-4 are within a few millivolts:

1: 7.4v
2: 7.3v
3: 3.2v
4: 0.65v
5: 0v
6: 2.84
7: 2.84
8: 3.7

IC5:
1: 2.7v
2: 7.2v
3: 0.7v
4: 0.65v
5: 6.85v
6: 2.4v
7: 2.84v
8: 1.9v


Q1:
E:2.55v
B:2.0v
C:7.79

Q2:
E:2.5
B:7.83v
C:7.9v

Q3:
E:5.62v
B:5.4v
C:4.5v

Q4:
E:6.3v
B:6.2v
C:5.6

[johngreene]

Maybe all your IC's are wired reversed?

Pins 7&8 are connected together in the schematic and connected to V+, But it looks like you have pins 1 & 2 wired together and connected to the supply.

--john

[Jaicen_solo]

I just went back to re-check all the voltages on the pins, it seems I was numbering them wrong. I'm using metal can CA3094's, so I had to do a bit of bending to get them into the layout. Anyway, the correct pins and voltages are as follows:

1: 3.9v
2: 2.84v
3: 2.84v
4: 0v
5: 0.65v
6: 2.8v
7: 7.23v
8: 7.23v

[Jaicen_solo]

Bump..

[johngreene]

I'm not sure what you are looking for Jaicen.

Your measurements cannot be right.
Q1 shows the Emitter voltage higher than the base. This would normally mean it is biased off. However,
Q2 has it's emitter connected to V+ but you show the emitter as being 2.5V.
Q3 and Q4 are supposed to have their emitters tied together but you show 2 different voltages at these pins.

Either something is way wrong or you have your pinouts wrong.

The help can only be as accurate as your measurements. 

--john

[Jaicen_solo]

I'm really looking for pin voltages, to give me an idea of where I should be debugging.
It's pretty obvious that the transistors are mis-biased somehow.
I'm getting the following voltages:

Q1
E: 7.4v
B: 7.3v
C: 2.4

Q2
E: 2.4v
B: 1.9v
C: 7.3v

Is it normal for me to have 7.4v at V+??? All the component values are correct so there must be a bad component in there somewhere. I've got a soundsample of the unit in action, but I don't have anywhere to host it, so I guess it's not much help either.
I'l go check Q3 & 4 now, see what voltages I get.


EDIT:  I'm fairly sure these are the correct pin voltages that i'm reading. Some fluctuate a little, so i've averaged them.

Q3
E:5.1v
B:4.8v
C:4.9v

Q4
E:5.10v
B:4.8v
C:4.8v

IC5:
1: 1.9v
2: 2.84v
3: 2.4v
4: 6.85v
5: 0.65v
6: 0.7v
7: 7.2v
8: 2.7v


Also, I'm only getting a clean sound out of the pedal now. No throbbing, vibrato or hiss/noise. I remember why I shelved this project now!

[johngreene]

I would start with Q2, it's emitter is supposed to be connected directly to V+ but the collector is according to your measurements. Q1 is also looking like it is installed backwards because the emitter connects to ground through the resistor and should be .6V lower than the base voltage but the voltages you measured are opposite of what they should be. The base of Q1 should be around 3.2V with a V+ of 7.6V. What voltage is Vbias?

--john

[Jaicen_solo]

Vbias is measuring 1.7v on R1. Does that seem a little low? I'd been expecting around 3-4v with a V+ of 8.7v. I'm fairly sure this has a lot to do with why the pedal is not working. If the B of Q1 is biased to 3.2v with a 470K resistor to Vbias, that has to be way higher than 1.7v.
The transistors are definitely oriented the correct way, unless the pinout (EBC) has changed and nobody told me! I'm using 2N3906's and one 2N3904 so the trannies are oriented in the same direction as on the parts layout.

I don't get this pedal at all. If it didn't have sentimental value it'd be shelved for good already, but I really want to get it working.

[johngreene]

At this point I would recommend removing the transistors and measuring the pins. Q2 is measuring V+ on it's collector and it should be on it's emitter. Try tracing out the connections to the transistors to verify that the connections are actually going to the pins you think they are.

Is this a printed board?

--john

[Jaicen_solo]

The board is a proper etched board made using the layout on JD's site. I don't know if it's been updated, but it all looks right to me. I've gone over it a number of times, so i'm pretty sure it's error free. I've gone over looking for solder bridges and shorts too but to no avail.
I think the problem with the transistors lies with the voltage at Vbias, but I don't know why that's so low, or even if it should be that low.
If I had the voltages from a working unit, I suspect it would be obvious but alas. The transistors are definitely in the correct way, no doubt about that.

[Tony Forestiere]

Jacien solo

The tabs sticking out of the IC cans should point to pin 1. It looks like they are pointing to pins 4 or 8. Of course...I'm old and the peepers are goin' on the fritz

Good Luck

[Jaicen_solo]

Yeah, I originally thought this when I got them, but if you check the datasheet for some reason the tab denotes pin 8, which is also connected to the case. Ergo, the case is connected to +v in this circuit for some reason. 

[johngreene]

For now I'm going to assume that you have the measurements for Q1 and Q2 exchanged, it's the only way they could be installed correctly and have your measurements make sense. That being said....

Your problem is definitely with Vbias. Are you sure R1 is a 10K and not a 1K?

--john

[Jaicen_solo]

The measurements are definitely correct. Since Q2 has its collector connected to the base of Q1, I think it's entirely reasonable for that to be too high, especially if the bias is off.
I also took R1 off the board and measured it (10K) and replaced it with another 10K resistor with no effect.

[johngreene]

Uh, according to the schematic (and parts layout) on GGG Q1 has it's collector connected to the base of Q2. And from your pic of the board, this is how it's marked there as well....



--john