BSIAB II 2 troubleshooting.

[tjburton]

I want to love this pedal... because I love my friends BSIAB2, but mine is not right- help.

It "works" but has a ton of headroom. If both the gain and level are set to noon, it is incredibly hot. Also the tone is very scooped in the mids. Very different than the other BSIAB's I've played with.  I could deal with the levels, but the tonestack is not right.

Below are my Q values. There are a few strange things I see, but haven't been able to track down why. The following voltages were taken while using a regulated 9v power supply (reading (9.01v) with a jack in the input. I've replaced all of the trannies (I thought I might have fried one- so I put sockets in). The 0v on Q3 and Q5 Gate is throwing me for a loop. Any ideas on component areas to look at for these issues? Thanks guys!

Q1
G: 0
S: 0.366  about a third of the GGG values
D: 4.3

Q2
G: 3.8
S: 4.35
D: 8.3

Q3
G: 0   should be .3 per GGG
S: 0.037   about 1/2 of GGG value
D: 4.0

Q4
G: 3.8
S: 4.00
D: 8.29

Q5
G: 0  should be .5 per GGG
S: .663
D: 4.0

[oldrocker]

Did you build this on PCB, perf or vero?  Do you have an audio probe?

[liquids]

I'm no expert, but Check your pinouts, first...

http://www.muzique.com/lab/trans.gif

[tjburton]

I built it from the GGG kit (PCB). So things are laid out to be pretty much dummy proof... almost. I'll double check the pin outs, but I'm pretty sure I have them in there correctly. Not sure what the audio probe will provide since signal is passed and all controls work. I just know it's not functioning correctly due to playing through other BSIAB's (and the voltages are off).

[tjburton]

Just checked the pin outs... All is good there.

[oldrocker]

An audio probe can be used to find where these scooped frequencies you described originate.  Possible cap value or orientation issue and other misplaced - misread components.
I actually have a bsiab2 on my breadboard right now.  It worked without too much hassle.  When it's right it does sound great.  You can't fight the fact that there is an error somewhere.  Study the schematic and double check every node and component value.

[tjburton]

Thanks, I need to build an audio probe...
Yep, I've been studying the schematic and have about 3 ideas to try tonight.

[tjburton]

Okay... built the audio probe. I definately can see where I will use this in troubleshooting a completely broken part, but on this issue I'm not sure what I'm looking for... but in any event. I pulled out my boxes of parts and started replacing tranies again. All I can say is WOW are these things fickle!!!

the pedal is working better... through switching out Q1 Q3 Q4 and Q5 my voltages are much closer to what GGG has listed.... Except the Gate of Q5 has no voltage... there is sound there with the audio probe, but no voltage. Can this actually be correct?
https://www.generalguitargadgets.com/pdf/ggg_bsiab2_sc.pdf

[Talon5051]

there is sound there with the audio probe, but no voltage. Can this actually be correct?

I measured mine and it also reads 0.  I love the sound of mine so I am thinking that this is not so bad.

[Hupla]

Okay... built the audio probe. I definately can see where I will use this in troubleshooting a completely broken part, but on this issue I'm not sure what I'm looking for... but in any event. I pulled out my boxes of parts and started replacing tranies again. All I can say is WOW are these things fickle!!!

the pedal is working better... through switching out Q1 Q3 Q4 and Q5 my voltages are much closer to what GGG has listed.... Except the Gate of Q5 has no voltage... there is sound there with the audio probe, but no voltage. Can this actually be correct?
https://www.generalguitargadgets.com/pdf/ggg_bsiab2_sc.pdf

Yup the gate of my Q5 has no voltage either. Works great. What a pedal

[liquids]

I've not checked mine, but my 'understanding' of Jfets is that the gate of Q1, Q3 and Q5 should basically be at ground.  R1, R19, and R14 ensure that the gates are referenced to ground.   Like a tube amp, where the tube stage's 'inputs' (aka 'grid' for a tube, 'gate' for a JFET) need to be grounded via a resistor 1M or smaller.   This is as opposed to, say, a BJT or a mosfet, where the 'base' or 'gate' more or less must have voltage on it to function properly.  

The little bit of voltage mentioned on the GGG page is within 5% tolerances, but it's minuscule voltage, and it may as well be 0v.  

So on both accounts, don't worry if your getting 0v readings.  Your DMM, in the setting you had it or altogether may not just not be sensitive enough to note a few tenths of a volt, or there may have been some other oddities or variables going on with the GGG build not worth accounting for.  0v is probably right, actually, and either way It's close enough.    As for the source voltages being low or 'off' the GGG readings,  that is likely affected by, as you noted indirectly, the particular jfets you are using, and the resistor value from source to ground. Increase the values and you'll get higher voltage and 'less gain' (advisable, IMO), and it may even up the drain voltage readings a bit.  Lower the value toward no resistance and you'll get lower voltage, toward 0v.  You might want to check those resistors and connections - R3, R10, R15, for one.  Or, if you follow my mods, get rid of all j201s, and increase the first two source resistors  for a nicer, more usable gain range, for starters.  

The confusing part is that, if you don't understand the circuit, Q2 and Q4 are functioning in a completely different way than Q1, Q3, and Q5 are, and the voltages of each of these two 'groups' are hence completely different.  However, you see that Q2 and Q4 share similar voltages (gate and Source ~4.5v, D ~9v) , as do Q1, Q3 and Q5 (Gate ~0v, Drain ~ 4.5v, Source ~1v).    Especially given how much JFETs vary, that is probably more useful info than listing them all with voltages on each pins, as if they all have their own individual voltage requirements and are consistent--not something Jfets are known for.  I completely ignored those voltages even with my first build, which was a BSIAB, because I breedboarded it, modded it for days and days, and had audio probed a ton already, so I had grown to get familiar with it by the time I put it in solder.  The voltages on mine were significantly different by then.   If I had compared, I'd had been pretty confused and thinking something was wrong, due to lacking any understanding that my mods and other factors (power supply, etc) had a big impact on said voltages, so I'm glad I didn't!  

I think voltage readings are odd without some background.  With Op amp circuits are pretty straightforward, as the pins are often meant to read full voltage, ~1/2 voltage, ground, or 'continually variable' in some cases with lfo's, etc.   But outside of that, the concepts of what voltages you should be getting at various points in a circuit, in general, are not straightforward unless you understand the principles of biasing and/or are quick with math, let alone if you've modded things in anyway.  Wide variations are common and not necessarily an indication that things will sound much different, let alone not work.   Resistor and capacitor tolerances alone throw it off a bit.  Batteries can even be problematic.  So when your looking for exact matches it's unlikely, and even close consistency between your voltages and someone else's in a circuit like this without any of the core concepts can be disturbing or disheartening, as was the case, despite yours probably working just fine.  

It is a preliminary help, however, if you see something is way off, like something being at ground that is supposed to have voltage on it, or something at supply power that is supposed to be near ground etc.  Outside of those cases, oftentimes the variety of power supplies, battery power, regulators, diode drops or not, trimmers etc, means comparing to xyz's voltages can be as confusing and misleading as they are helpful unless you understand the principles, do the math, account for the variables, etc.    

Also, I say ignore what that sheet is says about the voltages on Q5.  It's just good practice to first read the voltage at the banded side of the diode (and/or at the drain of Q2 and Q4, should all be the same) as accurately as possible.  Whatever that voltage is (say, 8.6v), set the drain of Q5 to be as close to half that as you can (4.3v in this example)...and note that small turns on the trimmer makes a big difference.  Start there, tweak the controls, and then turn it either way to and see if you prefer the sound, if you are so inclined.   Turning it either way of that voltage will just yield more clipping and more 'asymmetrical' type clipping, up to a point where you reach true misbiasing.   I don't feel the stock BSIAB needs any more clipping or assymetry, but that's up to you.   But 1/2Vref is the best starting point for any comparisons, since the setting of the tone, gain, and volume pot at any given moment will have a big effect on where the trimmer 'sounds best,' as it is highly interactive with those settings...that is just my opinion.

But most importantly, have fun playing with it!    

[tackleberry]

The tone stack cap/resistor values in the GGG BSIAB2 will really suck the mids out. I played with the duncan calculator to reduce the huge mid scoop, which made alot of difference. The GGG layout I built never could get it to sound right or get the values they list. A book I have shows 3 trim pots 1 for Q1/Q2, 1 for Q3/Q4 and 1 for Q5. A little engineering got the other 2 spliced into the circuit without etching a new board. Very easy to get the GGG listed values and great sound. Many have had successful builds with the single trim pot. I guess swapping out trannies might get it to work right, but I prefer the dialability of having the extra trim pots.

[liquids]

careful with the BMP tone stack on the duncan calculator.  It assumes being driven by a low impedance (1k) load.  I don't know the typical mu-amp load as it drives the BMP tone stack on the BSIAB, but it's probably closer to ~30k like a fender, if I had to make a poor assumption...has a big impact on the range of sweep and frequency response.

[Ranger_Steve]

Also, I say ignore what that sheet is says about the voltages on Q5.  It's just good practice to first read the voltage at the banded side of the diode (and/or at the drain of Q2 and Q4, should all be the same) as accurately as possible.  Whatever that voltage is (say, 8.6v), set the drain of Q5 to be as close to half that as you can (4.3v in this example)...and note that small turns on the trimmer makes a big difference.  Start there, tweak the controls, and then turn it either way to and see if you prefer the sound, if you are so inclined.   Turning it either way of that voltage will just yield more clipping and more 'asymmetrical' type clipping, up to a point where you reach true misbiasing.   I don't feel the stock BSIAB needs any more clipping or assymetry, but that's up to you.   But 1/2Vref is the best starting point for any comparisons, since the setting of the tone, gain, and volume pot at any given moment will have a big effect on where the trimmer 'sounds best,' as it is highly interactive with those settings...that is just my opinion.

But most importantly, have fun playing with it!    

This was my first stompbox project and I was in pretty far over my head.   After I finally got it "working" it sounded terrible.  It was sputtering and fizzing out.   My voltages were similar to the OP's, and in particular Q5 was G: 0v, S: 0.69V, and D: 0.8.  (versus 0.5, 0.9, 3.4 per GGG).  Not really knowing what I was doing in the first place I didn't know where to start on the troubleshooting.  I checked everything I could think of, connections, traces, solder bridges and then I ran across the above post and it totally solved my problem.    After adjusting the trimmer to yield 4.3v on the drain side of Q5 (per the above) the rest of my voltages fell into tolerance with the specs.   More importantly my BSIABII sounds awesome!

I know I'm replying to an old post, but thanks a million Liquids!

Steve

[liquids]

Great!