Help me with Big Cheese power + biasing

[Dylan]

I just finished putting together a Great Cheddar using the Geofex layout. I plugged it in to my power supply and nothing works (not even the LED, I wired it using a 3PDT switch). The bypass is fine though. All the other pedals I have connected to the power supply (it is one of those small black supplies with 5 outputs) don't work when I plug in the pedal. What are the most likely causes. For the record, my soldering was good and I wired all the pots up as shown in the diagram. Help me please.  :cry:

[mikeb]

Short from V+ to ground. What resistance value do you get from V+ to ground (power off, obviously)?

edit: also...
http://www.diystompboxes.com/pedals/debug.html

faster than asking on HC I'd say .....

Mike

[Dylan]

I got the LED working, and I may have found the problem. The circuit is set up so that when the input ring connects to the sleeve it powers on (although I'm using a DC adapter not a battery). The power cuts out whenever I connect the ring to the sleeve. Is this the short?

[mikeb]

Did you do the debugging listed above? Have you measured resistance from various pins on the input jack to V+ and ground? The answer is there probably there right in front of you!

Uh .... you do have a multimeter, right?

Mike

[Dylan]

I usually use the multimeter at my school, but I'm at home on holiday at the moment so that's a no go. I'm gonna try directly wiring up the -ve power to the PCB instead of the jack and see if it works. Failing that I'll have to buy a multimeter.

[R.G.]

I got the LED working, and I may have found the problem. The circuit is set up so that when the input ring connects to the sleeve it powers on (although I'm using a DC adapter not a battery). The power cuts out whenever I connect the ring to the sleeve. Is this the short?

I believe you have found the problem - I think it's likely that you have the power connection instead of the ground connection connected to the second signal lug on your input jack. That would short out the power supply through the signal wires in cords to the other pedals.

Be sure you know which wire is which when you're using an adapter. If you connect your power adapter backwards to the PCB, you're likely to fry the opamps.

[Dylan]

The plot thickens.. I actually wired the adaptor correctly.

Next thing to look for is something touching the enclosure I guess? It's possible.

[mikeb]

Trying to debug without a multimeter is like trying to drive home blindfolded ... it's possible, but likely will take a long time and take out some innocent bystanders along the way ...

Mike

[R.G.]

You're right Mike. I thought I detected a pony in the description. It's multimeter time.

[Dylan]

The thing shorts out when I connect up the negative, I guess this means there is something on the PCB touching the enclosure right?

I'll have to wait before I get a multimeter because I'm going on holiday on Saturday for a couple of weeks.

[Dylan]

Got a multimeter today and cured the problems. I now have a working, enclosed Great Cheddar and it sounds great. Any recommended settings? I heard some octaves can be heard at some settings?

[mikeb]

Congratulations - multimeters rock!   I have three - one 'good' one that is used all the time, a smaller one that is used when monitoring of a couple of voltages is necessary, and one that is used for measuring hFE (this function came 'broken' on both of the other two meters  :? )

Can't help you out with the settings - there's only a handful of controls so it can't be too hard to find 'that' sound. You'll need to adjust someone else's settings to allow for the difference between their axe and pickups anyway.

Mike

[Dylan]

Mine was only a cheap one but it did the job well.

I'm just interested in the octave setting really, since that's dependent more on the pedal than the guitar/amp.

Thanks for your help, by the way.

[Dylan]

My Great Cheddar transistor voltages; (voltage in = 9.03V)
Q1 C=1.25V, B=0.51V, E=7mV. Correct values for Q1 are C=1.25V (mine is fine), B=0.6V (mine is slightly too low), E=34mV (mine way too low)
Q2 C=2V, B=0.62V. Correct values for Q2 are C=2.81V (mine is too low), B=0.62V (mine is perfect).

Do I need to change resistors to get the bias perfect?

[markusw]

Also have a look at this one: http://diystompboxes.com/sboxforum/viewtopic.php?t=11359&highlight=cheese  

There are some slightly different values (also measured by R.G.), but I dont know what the difference was in the setup compared to the values in the schemo.

I have used two BC549Cs with about the gain values R.G. mentioned in the thread highlighted above. However, if I got it right the end-to-end value of the fuzz pot is also important for Q2 collector voltage. Mine has about 1020 ohm and I get more or less exactly the same voltages as in the link above.