Great Cheddar debug

[Jose Aparicio]

Hi folks!

This is my first post in this forum. I've been reading it for a while and it's a very good reference for those who are getting into the DIY world.

I'm building a Great Cheddar, all the components are in place and I manage to get some fuzzy sounds out of it. However I have some problems biasing the transistors. With the Fuzz pot at full range I get the correct voltages but as I turn it down I loose these values and the sound dissapears gradually. The Fuzz pot is 1K and if I add a serial 1K resistor I manage to get sound at almost the full Fuzz range pot, but the amount of fuzz in the signal is constant.

Is this a normal behaviour? If not, what should I look for?

[Jose Aparicio]

Nobody?

[MartyMart]

Hi Jose and welcome
I think you need to read the "what to do when it doesn't work" stickie
More detail is required :
Schematic link
Build details/mods ?
Transistor voltages,  etc etc
Then hopefully you will get more help
Yes, you have a bias problem, but without voltage readings
its just pointless guess work .... OK ?

MM.

[Jose Aparicio]

Hi Marty!

Thanks for the response 

I used the 1.6 version linked here http://www.geofex.com/Article_Folders/grtchedrschm6.PDF

The transistor voltages are:
Q1:
E:0,03V
B:0,59V
C:1,23V

Q2:
E:0,6V
B:1,23V
C:2,8V

I only get this values with the fuzz pot at full range. They decrease when i turn it down and then the fuzz fades and no sound comes out the board.

Should this values vary when you turn the Fuzz pot?

Hope this can clarify something.

[MartyMart]

Your voltages are spot on, going by the schematic guideline voltages.
I think that its working properly, the "fuzz' pot will decrease fuzz as its tiurned down ( normal )
and may well turn signal "off" at minimum.
It will change the "bias" as that's partly what its doing.
If all else seems to work, then you dont have any problems ??

MM.

[Jose Aparicio]

Thanks again Marty

As you said, it looks like is working properly. But this only happens when the fuzz knob goes from 3 o'clock to maximum range, if you have it at 12 o'clock it doesn't come any sound at all. The biasing trim pot doesn't have any effect with the switch in the fourth possition. Again, I'm suspecting something is wrong. Should I modify any registor value to be able to move between bias values?

Best regards.

[MartyMart]

I see, you loose all sound under "12 o'clock" position !
Check that you're 1k pot is working with a DMM, out of circuit with
wiper and one outside lug connected to DMM red/black leads.
Perhaps at 12 o'clock it's value is zero ....
Also go over the circuit around Q1/Q2 and check part values and any
"solder bridges"   shorts etc ...

MM.

[Jose Aparicio]

Thanks a lot for your support Marty!

It works now. There was a short between P1 and the fuzz pot. I still have to test it a bit more to get the ugly sounds I'm looking for.

Thanks again Marty

[MartyMart]

GREAT !! 

Glad i could help .... in a small way !

MM.