GGG MGV Debug.

[sweetwilly]

Hi all.  Just completed my second (semi) successful build of the GGG layout of a Guv'or clone -

http://www.generalguitargadgets.com/index.php?option=com_content&task=view&id=99&Itemid=26

I put it all together last night and it didn't work at all.  Scratched my head for a bit then put it down.  Checked all the wiring this afternoon and noticed that I neglected to connect the ground from the volume pot to the output jack. 

Sweet, a working pedal. 

Sounds OK, but not as good as my mate's build of the same pedal that I tried in my setup a month or so ago.  Could be that month of building up expectation, but I checked the voltages on the IC pins to be sure.

Batt - 9.09
Pin 1 - 4.56
Pin 2 - 4.55
Pin 3 - 2.25
Pin 4 - 0.00
Pin 5 - 4.45
Pin 6 - 4.55
Pin 7 - 4.55
Pin 8 - 9.09

All are pretty good apart from Pin 3.  Half what it should be. 

If anyone knows why this is so I'd really appreciate it!

Cheers...

[sweetwilly]

I forgot to mention, I had to use 43k resistors for R4 & R5 instead of 47k.  Don't know if this could be the problem though?

Is it possible I've damaged the IC when I powered it up without the ground connection from Volume pot to the output jack?  I measured the voltages at that point and IIRC the voltages on most, if not all the pins was around 9v. 

Thanks in advance!

[sweetwilly]

I've checked the voltages from the 9v in on the board through various components.  Everything seems fine ie - 9v in, 4.5 at the centre of R4(43k) & R5(43k) - 2 equal resistors so voltage halved. 4.5 where R3(1M) meets C7(10uf - polarity correct). The side of R3 that goes to Pin 3 on the IC is only 2.25.  Pretty sure this should be 4.5 also, but can't figure out why it's half what it should be!!!

Any suggestions would be amazing!!!  I've come to the limit of my skills!!!

Oh, it does work pretty well, but with the volume pot maxed it is at unity gain.  The other GGG build I've tried was much, MUCH louder!!!

[jsleep]

The 43k subs will make no difference, as you have noted.

Are you sue the value of R3 is 1M?

JD

[sweetwilly]

JD!!!

Cheers mate.

Yeah, R3 deffo 1M.  On the "right" side (looking at the top of the board) I get 4.5v and on the left I'm getting 2.25.  My DMM only measures up to 2M, so can't check the 2M2 resistor properly. It measures over 1900 before it goes to unreadable though.

I've triple checked the component values and all correct.  Totally stumped. 

Thanks again!

[Ice-9]

As said the 43k resistors in the voltage divider will work fine. Have you build this on a  ggg pcb or have you made your own layout. (just in casethere is a layout mistake)

[sweetwilly]

It's a GGG pcb so that should be fine.  The fault lies somewhere on my side!

I understand how the 9v is divided by R4 and R5, but how is it being divided again between there and R3?  R3 reads pretty much dead on 1M, I tested resistance of each resistor as I soldered it in (before soldering and after using adjacent pcb holes to test the joints) and I've checked the value of each cap at least 3 times now!

[slacker]

Don't worry about the 2.25volts on pin3, as all the other voltages look ok it's probably really 4.5 volts, it's just that your meter can't measure it properly.
This is a common problem caused by the impedance of your meter interacting with the 1 meg resistor, there's a slightly better explanation here http://www.diystompboxes.com/smfforum/index.php?topic=67213.msg535660#msg535660.

The fact that pins 1 and 2 have 4.5volts on them also means that pin 3 really has 4.5 volts on it. Pins 1 and 2 are isolated from the rest of the circuit by capacitors C1 and C3 so the only place they can get their DC voltage from is via the DC voltage on pin 3. If pin 3 really was 2.25 volts then pins 1 and 2 would have a similar voltage.

[sweetwilly]

Cheers for that.  Thought I was missing something blatantly obvious!  My volume issue could be down to poor craftsmanship and after playing around with it a bit this evening I realise it is pretty sweet nonetheless. 

Thanks Ian and JD for the help!