BSIAB2 debugging. please help me out? (:

[darron]

I tried to make a BSIAB2. It fires up but once I play a loud chord through it, it goes into a mode where everything is really splatty and farty, and will be muted unless the signal going in is of a high enough volume. I can reset the unit into working mode (which will continue working indefinitely until i play a loud chord) by touching one of the drive lugs. While in 'working mode', the gain knob doesn't actually work... The gain sounds very high, maybe maxed? The bias knob stops the unit working completely at one end, and everywhere else doesn't sound very different. Am I supposed to adjust this exactly and it will fix the problem? Suspecting when I touched the gain knob I was moving dodgy wiring, I replaced the pot and wires and realised that the fault was funny.

The only part I substituted was the 250pf ceramic cap with a 270pf. I have confirmed all of the resistor values as being correct. Everything that I put in it i bought specially and separately in packets so I don't think there's a problem. Unless a transistor is fried?

Here's a link to the GGG resources that I used: http://www.generalguitargadgets.com/index.php?option=content&task=view&id=150&Itemid=178

Running off a battery had the same effect.

Here are my voltages:

Source: 9.29v

Past the zener: 8.21v

Q1
G: 3.63
S: 8.39
D: 8.53

Q2
G: 5.37
S: 8.53
D: 8.71

Q3
G: 0 (sometimes going up 0.1. something going on here near the gain section?)
S: 8.03
D: 8.71

Q4
G: 5.59
S: 5.7
D: 8.71

Q5
G: 7.42
S: 8.05
D: 8.05 (not shorting to above that i can see. soldering is clean.)



Here are some MASSIVE images (sorry, photoshop isn't working...):

Of course under the flash everything looks like it's shorting...







Thanks heaps guys. (:

Hope I didn't stuff up any readings... Let me know if there's anything that I should double check?

(:

[Auke Haarsma]

I'd check the section around Q3-gate. There's coming 8.5V from Drain Q1 and Source Q2. Voltage will drop some due to the filters, but I don't think you should read 0V at Gate Q3.

hth

(haven't build the BSIAB2 myself btw, just looked at the schematic and your readings)

[darron]

if you take a look at the build instructions here and scroll to the last page it has the suggested voltages off the pins:

http://www.generalguitargadgets.com/pdf/ggg_bsiab2_instruct.pdf

mine don't really compare logically to those at all.... erm.... ? :S

[Auke Haarsma]

ah thanks for the link. Seems the voltage divider isn't working properly. Drain of Q1 should read 4.5V. Could you check the voltage between R4 and R5?

[darron]

ah thanks for the link. Seems the voltage divider isn't working properly. Drain of Q1 should read 4.5V. Could you check the voltage between R4 and R5?

it's sitting at 3.1 volts. i was thinking along the lines of the divider being out, throwing everything out.... don't know why....

[darron]

i left the biasing pot in the middle for all of the readings by the way.

[Auke Haarsma]

couldn't see it too good on the pics, but could you check R4 and R5 (at the side to ground), the solder seems very close to each other. Maybe resolder there?

[darron]

couldn't see it too good on the pics, but could you check R4 and R5 (at the side to ground), the solder seems very close to each other. Maybe resolder there?

they were okay but i resoldered them. same deal. the effect sounds really really good until it goes into bad mode and i have to reset it by touching the drive control. if i play quietly there's no problem until i hit a loud chord... maybe i should just make another one? and see if it does the exact same thing or works... or doesn't at all

[Auke Haarsma]

Since it takes some time for the error appears, could it be a bad cap?

Otherwise, let's hope for a more experienced forumite to jump this train

[darron]

Since it takes some time for the error appears, could it be a bad cap?

Otherwise, let's hope for a more experienced forumite to jump this train

it's not a matter of time, it's only once i hit that loud chord... otherwise i can keep playing with a non-adjustable drive and with my guitar turned down (: still sounds pretty cool. that wouldn't be practical though. all the caps are new and im pretty fast and clean to solder. thanks for your help so far pong.

[petemoore]

Q1
G: 3.63
S: 8.39
D: 8.53*
Q2
G: 5.37
S: 8.53*
D: 8.71
  These two are high, been a while, correct me if needed, Q2 sets bias point for Q1, the difference between Q1 S/D voltage is small, and near V+, test for ground on Q1S resistor, that there is a very small resistance to ground, looks like the sources are 'floating' [not pulled toward ground through S resistors].
Q3
G: 0 (sometimes going up 0.1. something going on here near the gain section?)
S: 8.03*
D: 8.71
Q4
G: 5.59
S: 5.7*
D: 8.71
  I think somethings not grounded or the divider isn't working, what voltage do you read at the middle of the divider? [where the two 1megs meet].
  Q5
G: 7.42
S: 8.05
D: 8.05 
  All the sources are near V+, and should be nearer ground, [above gate though, it appears the gates are 'following' the sources here...but 'floating' up near V+] it's like a ground they all use isn't connected.
  The output of each Mu amp [where bottom fet D meets top Fet S] should be about 1/2v.
  I used two little divider resistors [10k?] single divider feeds both Mu's 1/2v through very large resistors [<1m]...noiseless biasing, IIRC I put 4m7's between 1/2v and each Mu for big impedance there.
  GEO shows how Mu Amps work, the first two stages are [2 x Jfet] mu amps.

[darron]

for Q1, here are the resistances to ground for each pin:

G: 993k
S: 500ohm
D: 603ohm

thanks petemoore

edit: could i have a fried transistor?