BSIAB II not quite right

[dosmun]

I just built the BSIAB II .  It is built on a GGG board.  It is kind of sputtery like a Fuzz that isn't biased correctly.  I used my audio probe on it and it seems like it is getting sputtery at Q5.

Any suggestions?

[RDV]

Pinout?
Trimpot?
Bad JFET?

RDV

[sir_modulus]

RDV's nailed it. Now one other thing to check is to see if any resistors that are biasing ones are off value, or fried. That can also cause this sound.

Could you describe the sound a bit better? does it change the sound if you wiggle parts?

Cheers,

Nish

[pyrotek]

I did experienced similar problem with my BSIAB build recently.  sir_modulus is right check those biasing resistors and also check the drive pot and trimpot. Check your FETs voltages.

Ed G. gave me the FETs voltages and it helped me a lot.

From: Ed G.

Tip on voltages: The 2 "top" fets should be approx. equal in voltage, give or take a few tenths of a volt. Same for the "bottom" fets. The source of the top fet and the drain of the bottom fet are connected, they should be the same and should be reading about half of your supply voltage, e.g. 4.5v if you have a 9v battery.
The final fet should give voltages similar to the 'bottom' fets.

[jsleep]

Sometimes you can get bad JFETs as well, try replacing the fet after you audio probe the circuit to find the sputtering stage.

JD

[dosmun]

So far I replaced the trim pot.  Checked for loose parts.  Changed Jfets.

When I said sputtery I meant it sounds similar to a Fuzz face that is mis biased in that it is almost cutting out.

I will check my voltages on the Jfets.  Does anyone know what it should read on each pin of the Jfets?

I may even try a few more Jfets to see if that may be the problem.

[Ed G.]

Dosmun,
The drain on the 5th fet should bias at about 4.5v, roughly half of your 9V supply. On the first and second gain stage, the minibooster configuration does this automatically. Due to the variation in fets, sometimes even using the trimmer won't bias the fet, so you might just have a bad fet.
Hope this helps, if not, I might have some voltages written down somewhere to dig up.

[dosmun]

Thanks Ed.  I will check that out tomorrow.  I hve a feeling that the FET is not biasing correctly.  I have checked everything else multiple times.

[dosmun]

SUCCESS

I checked all of the voltages last night and everything was fine so I started going through the circuit again with my audio probe and found the problem around Q4 and when I started checking votages again with my meter it started cutting in and out when I touched the probes on the 1M resistor on the gate of Q4.  Turns out it was a bad solder joint on the 1M gate resistor. :roll:   So I got to play a bit with it last night but only at low volume because the wife was sleeping.  So far it sounds Great.

I am kinda surprised I missed it early on because I went over the circuit multiple times with my magnifying glass to check for solder bridges bad solder joints etc.

Thanks again guys for the tips.

[Ed G.]

Hey Dosmun,
Let me know what you think of it once you get to crank it up at volume.
Thanks,
Ed

[R.G.]

I am kinda surprised I missed it early on because I went over the circuit multiple times with my magnifying glass to check for solder bridges bad solder joints etc.

Mother Nature can hide her little surprises... 8-)

[jsleep]

I just posted this on the project page.  Sorry it took so long, maybe some future builders can find it useful.

http://www.generalguitargadgets.com/diagrams/bsiab2_voltage_diagram.gif

Also,   sorry Jack, I had forgotten that you were the first person that I saw mention the idea of cascading miniboosters that led up to this and other great sounds!  and thanks to Ed for reminding me :-)  I corrected some of the background descriptions text.

JD

[lethargytartare]

I don't suppose anyone has a similar voltage diagram for the BSIAB version 1??  I built that RIGHT before the ver2 came out...