sunking -no gain - need help

[joshcamp]

so, i built the sunking a while back and it has worked great until recently there has been no gain.   in fact, it seems as if the gain knob when maxed just decreases vol or tone a little. 

i checked the diodes - i replaced them with new diodes (bat41) and same problem occurs - checked orientation, etc..

i have an audio probe but since i get audio throughout the board, i'm not how i can use this to check for gain loss...

any advise ?

[jkokura]

Josh, so you're saying this a problem that developed?

What we need to be looking at here would be what changed. Perhaps some pictures would be helpful. Also, maybe taking some readings using a multimeter on all the IC's.

Jacob

[joshcamp]

yes, it is a problem that developed.   though, i noticed that the problem wasn't all of a sudden - the gain would intermittently come and go.

i just checked with the audio probe and i get a dirty tone when i touch the probe to lug 3 of the volume pot... 

[joshcamp]

i'll check the ic's soon

thanks for your help !

[joshcamp]

so here are my IC measurements:

IC1 (tL072), pin 8 = 7.3v
IC2 (tl072), pin 8 = 12.6v
IC3 (max1044), pin 8 = 7.4v

[jkokura]

That's a good start. We need the voltages from all the pins. You're just telling us the input voltages.

I will also note that your voltages are VERY low. You want to be seeing 8V or higher. 9V is best of course. IC2 should be seeing about 16-17V.

Jacob

[joshcamp]

ahh, for some reason i thought i was giving the output voltage thinking that would be sufficient. yeah, im new to this.

ive since changed the battery, here's what i get:

all pins:
    IC1    /  IC2   /  IC3
1) 4.23  /  4.2   /  8.22
2) 4.21  /  4.2   /  4.08
3) 3.23  /  4.2   /  0
4) 0      / -8.13 /  -4.04
5) 4.2   /  4.18  /  -8.02
6) 4.2   /  4.18  /  3.57
7) 4.2   / 4.16  /  5.94
8.4   / 14.95 /  8.28

[joshcamp]

not sure how the 'cool' smiley got there.. pls disregard it !

[madbean]

Your supply voltage is still low. How fresh is that battery? If it's new, then you have something leeching the voltage. Are any of the ICs really hot?

[joshcamp]

since my last post, i switched to a 9v power supply and these are the new numbers:

IC1      IC2       IC3
4.49     4.54     9.02
4.5      4.49      4.46
3.45    4.49       0
0       -8.67     -4.42
4.48    4.45     -8.77
4.46    4.48      4.28
4.47    4.46      6.65
9        16.27     9

[madbean]

K, your voltages are correct. That's good. The problem MOST LIKELY is a solder bridge / loose connection / cold solder joint.

[jkokura]

seconded.

Jacob

[joshcamp]

so, is this the job of an audio probe ?

if yes, what's the best approach to isolate the problem ?

[madbean]

The easiest thing to do first is gently remelt all the solder joints. If that does not fix the problem, then you should check pins 3, 1, 5 and 7 of IC1 and IC2 to see if the problem is isolated to any of those areas.

[joshcamp]

yep - remelted every solder joint.   that did it !

Thanks for the help !!