Chunk Chunk Biasing

[dfine22]

Just built a Chunk Chunk.  Works OK with the exception of I cannot bias T2. No matter how I turn the trimmer, I still get the same reading (about 9V) on the drain.  Thinking it may be a bad trimmer but unlikely.  I need some help. Thanks, David

[stecykmi]

test for connectivity between 1 and 3 legs of the trimmer. you might have a solder bridge or as you suggested a broken trim pot.

[jkokura]

First, double check you're reading the right transistor. If you were reading Q1, you would find yourself unable to bias, so double check you're reading the correct Q.

Second, double check the connection between the transistor and the trim pot. You need to be sure you haven't got a broken trace or problem like that.

Third, if both of those problems aren't the issue, it may be that there's either a problem with your soldering on the trimmer, or that the trimmer itself is faulty. You can check the trimmer if you take it out. If you have another trimmer available, replace it.

Jacob

[dfine22]

Well, I switched out the T2 and there was no change in the biasing.  It stays around 9V regardless of what position the trimmer is in.  Is there a component that may be bad.  I am fairly new to building and circuits etc.  Any additional help would be greatly appreciated. 
Oh yeah, I can sucessfully bias the other trannies .

[jkokura]

There are no other parts involved, just the trimmer and the transistor.

It sounds like you have a bridge between your 9V supply and the biasing leg of the trimmer. Leg 3 of T2 is the biasing leg, so you need to make sure that leg 3 doesn't connect with your 9V, or legs 1 & 2 of the trimmer. Take a look at the PCB in the build documents, or the schematic found there also. You'll see that there's almost a straight connection between T1's power connection and T2's power connection, and that both of them use leg 3 to connect to their respective transistors.

No worries about being new, we've all been there and we're happy to help. Just try and follow all the steps that we give you to try out, and report back on all of them. Clear communication will save us all headaches.

Good luck, and let us know what happens. Also, if you can, take some pictures if you can, nice clear ones of the solder side, and that might help us spot a problem for you also.

Jacob

[dfine22]

Thanks Jacob,  I looked at all the components and found a mistake.  I used a 470 ohm in
R7 when it should be a 470k.  I do not have the correct resistor and will have to order it.  Could this be the the cause of the issue that I am having. R7 follows C7 which follows Q2. I do not know enough about circuits yet to make that connection.

[jkokura]

That will create problems for you, and definately make the circuit not work correctly, however I'm not sure that's the only source of your problems. You should be getting Q2 to bias, but ultimately you have to put this aside until you get the right part in. Can you not find one locally? It sucks to have to order one resistor - that's very expensive in terms of those parts.

Jacob

[mgwhit]

C7 should be blocking DC, so I wouldn't think that R7 would affect bias on Q2.  Have you tested that trimmer yet?  You definitely want to do that before you order that resistor!   Good luck!

Heading downstairs now to work on my own Chunk Chunk board....