Problem with DIY GGG Rodent - VERY low output

[djaveh]

Hey all,

I am trying to build my own Rodent based upon the General Guitar Gadgets PCB layout, I etched my own tracks and checked continuity and shorts. The problem I am having is that the output is extremely quiet, in that I can barely hear it after I have turned the gain and volume up max. The signal absolutely fine when bypassed. I have checked that the components are oriented correctly and the correct values are in the correct places.

The source of the error I think is with the voltages on the IC. Comparing the voltages from the ggg build guide and my own:
  ggg (9V supply)   me (9.2V)
1 8.4                     8.7
2 4.5                     4.37
3 4.1                     3.97
4 0                        0
5 0                        0
6 4.5                     4.37
7 9                        8.76
8 4.6                     8.7 (!)

The actual IC I am using is http://uk.farnell.com/texas-instruments/op07cp/ic-op-amp-low-offset-dip8/dp/1106130

My understanding reaches its limit at this point. I am not sure what the offsett null pins actually do, but the fact that 8 should be reading 4.6 not 8.7 looks like a major error. Could this account for the low volume of the output? as the signal is being offset by too large a voltage?

Any help you guys could offer would be greatly appreciated.

Cheers,

James

[djaveh]

Hey All,

I used an oscilloscope to chase the signal and found a problem with the JFET and the circuit now seems to work! sorry I hadnt checked this before I posted 

However the difference in the offset pins are still incorrect, does anyone know why this would be? Or explain what the point of the capacitor between the pins is?

Or perhaps I should be happy I got it working and generally get on with my life!

Cheers

[DavenPaget]

Hey All,

I used an oscilloscope to chase the signal and found a problem with the JFET and the circuit now seems to work! sorry I hadnt checked this before I posted  

However the difference in the offset pins are still incorrect, does anyone know why this would be? Or explain what the point of the capacitor between the pins is?

Or perhaps I should be happy I got it working and generally get on with my life!

Cheers

You actually have a oscilloscope   I can't afford even a DSO20X .
I only know that the offset pins cancel the offset on the input by connecting directly to the supply via a 20k trim ( pin 8 goes to trim leg 3 , leg 2 goes to Vcc , pin 1 goes to leg 1 ) at least for the OP07 .
I don't know why GGG used a 30pfcapacitor .

[djaveh]

Dont worry, I could never afford one either!

I have an NI USB DAQ which i have borrowed  from work over the xmas period which I used to output a sine wave and use as an oscilloscope. Its pretty cool.

I feel stupid for not checking the contacts earlier, what a mug! All good learning I guess.

I guess they use the 30pf cap as thats what is listed on the lm308 datasheet, no explanation as to why tho. DC blocking mebbe?

[DavenPaget]

Dont worry, I could never afford one either!

I have an NI USB DAQ which i have borrowed  from work over the xmas period which I used to output a sine wave and use as an oscilloscope. Its pretty cool.

I feel stupid for not checking the contacts earlier, what a mug! All good learning I guess.

I guess they use the 30pf cap as thats what is listed on the lm308 datasheet, no explanation as to why tho. DC blocking mebbe?

I don't know . But the LM308 and the OP07 are different animals 
Given the chance , i would rather buy a ATTEN ADS1022C at just slightly more money .
I was referring to those DSO203 quad-channel ARM based portable DSO's ... Useless . Fair enough , might as well settle on sparkfun's digital oscilloscope kit .
At the same time , i might actually buy one of these ( but i suddenly think LCR meters are kind of a waste . )
http://www.ebay.com.sg/itm/DM4070-LCR-Meter-Multimeter-Inductance-Capacitance-Ohm-/290539831419?pt=LH_DefaultDomain_0&hash=item43a586007b#ht_4174wt_905

[stringsthings]

Or explain what the point of the capacitor between the pins is?

C4, the 30pF capacitor is a frequency compensation capacitor ... the LM308 is an uncompensated op-amp ... may op-amps, like the standard 741 are internally compensated ... uncompensated op-amps allow the designer to trade off frequency response, gain, and stability, IIRC ....

the OP07 is internally compensated, so you can probably leave out C4

... i just checked my GGG Rodent and i read 8.6V on pin 8