SansAmp GT-2 problem

[Arno van der Heijden]

I finished my SansAmp GT-2 from Tonepad's pcb, but I have some problems.
I can't turn the drive control past 1-2 or it'll sound horrible, almost like misbiased transistors or something like that.
Is there anything specific I should look for?

[Joep]

It might be that the speaker sim in working incorrectly, you may check that.

It it on all "channels"? Are all the controls knobs working? If something is not working properly you might want to check the circuit around this control.

Hope this helps a bit....

Joep

[Arno van der Heijden]

Yes, it's on all channels. All other controls are working like they should.
I'll take a look at the speaker sim section.

BTW, is it normal that the volume level on the fender setting is much lower than on the two other settings?

[Joep]

no, there should not be a big difference, although is is lower gain

[Arno van der Heijden]

I just played a bit with it again. Unlike I said before, it's not the drive controls that's acting weird, but the volume control. If I turn it past 1 it'll generate some nasty crackling distortion on top of my notes. This gets more noticeble when the 'high' control is maxed out.
If I turn the volume further up, the sound even starts to cut out.

Anybody got an idea where to look for? I looked for solder bridges and cold joints, but I didn't find anything.

[Joep]

mmm, really hard to tell.

Check all the resistor values for errors, start around the volume control.

[Arno van der Heijden]

Checked all resistor and capacitor values. Everything looks okay to me.
Maybe I should check a second time to be sure...  :x

[Joep]

mmmm, maybe one of the TL072 is broken, try switching them, see if the problem changes.

[Arno van der Heijden]

Damn, I still haven't got it fixed.

mmmm, maybe one of the TL072 is broken, try switching them, see if the problem changes.

I didn't use sockets, so it's too much trouble switching the TL072's. I used new parts, so chances that one of them is broken are small anyway.
I did measure the in- and outputs on each chip. According to R.G.:

For an opamp to be working as a linear amplfier, both the inverting and noninverting inputs and the output pin must be within a few millivolts of the voltage on the noninverting input.

This is true for all of the ic's, except for ic1. Pin #3 (in+1) and #5 (in+2) of ic1, which are shorted BTW, are more than 1 volt (don't have the exact numbers here) apart from the other in- and outputs. Does this mean the ic is dead? Or does my DMM get loaded down or something? :?

[Arno van der Heijden]

I did some measurements on the ic's:

IC1:
out1: 4.66              
in-1: 4.65
in+1: 2.33
Vcc-: 0
in+2: 2.33
in-2: 4.64
out2: 4.64
Vcc+: 9.15

IC2:
out1: 4.63              
in-1: 5.04
in+1: 4.13
Vcc-: 0
in+2: 4.63
in-2: 4.82
out2: 4.50
Vcc+: 9.21

IC3:
out1: 4.43              
in-1: 4.43
in+1: 4.40
Vcc-: 0
in+2: 4.33
in-2: 4.65
out2: 4.65
Vcc+: 9.19

IC4:
out1: 4.62              
in-1: 4.62
in+1: 4.59
Vcc-: 0
in+2: 4.57
in-2: 4.59
out2: 4.59
Vcc+: 9.14

I think the voltages on ic1 look a little weird. Does that mean the ic is broken?

[Joep]

Can be...

Also check the 1 M resistor connected to in+1 and in+2 (of IC1) it's located right next to IC2.