Troubleshooting a BSIAB2 - Power Circuit Issue?

[Juan1]

Hello everyone.  I've put together several pedals before, usually without any problems but this is the first time I've been unable to identify an issue.

So the story is I put together a BSIAB2 with contour knob, and all was good in the world.  The pedal sounded great out of the box, and even better when I removed C13 and found a resistor to lower the max gain to early EVH levels when the gain was full up (R10 @ 1.5k).  Great.  Jammed with the pedal that way for a few more days.  Finally, I tried changing R10 to something with lower gain, just to see if it was worth putting R10 on a switch so AC/DC tones would be easier to dial in.  Swapped R10 to 3.9k.  Now the pedal effect won't work and the LED won't go on. Changed R10 back, and the issue persists.  True bypass is fine.

I'm only measuring mV's at the Q's, but I'm showing 9.3V up to the input side of the diode, but then mV's on the other side of the diode.  Does that point to the diode being the problem? I think it has to be an issue in the power circuit. My fear is that with all the wire jiggling, testing with the pedal case open, and not securing the motherboard, that I've touched two wires and blown something out.

[antonis]

Hi & Welcome..

Could you plz post a schematic of particular BSIAB2 you refer on..??
('cause there are various diagramms of different items numbering..)

P.S.

I'm only measuring mV's at the Q's, but I'm showing 9.3V up to the input side of the diode.  Does that point to the diode being the problem?

In case of a blown out diode, measurements should depend on its actuall placement (i.e. series or shunt)
For shunt diode, your curcuit should work fine..
For series diode, you should get no voltage reading at all on its cathode..
(but many DMMs show some mVs for an "open circuit"..)
You can momentarily short (via a jumper) that diode to see if you get power on upper JFETs Drains / upper bias resistors / smoothing capacitor..

But better, make sure for that reverse polarity diode placement before shorting it..

[Juan1]

I don't know the setup names for diodes, but my guess is that series applies.  Here is the schematic.  Are diodes that sensitive?

[Govmnt_Lacky]

...but I'm showing 9.3V up to the input side of the diode, but then mV's on the other side of the diode.  Does that point to the diode being the problem?

Think you have a bad diode.

[Govmnt_Lacky]

For series diode, you should get no voltage reading at all on its cathode..

Huh??

[ElectricDruid]

Yes, that's a series diode.

If there's voltage on the power input side of the diode, but not on the circuit side, then it does sound like a fried diode. Given that it's a ¢5 part, I'd replace it and then see what happens.

As to whether they're "that sensitive", the OnSemi datasheet for the 1N914 says 300mA maximum forward current, so that's not that sensitive and implies that if you fried it, you probably shorted the power rails at some point. A short elsewhere would be a lot less likely to cause enough current to flow.

[Juan1]

Changed the diode and except for the LED, the pedal works great again.  A new LED and the pedal will be good as it was a few days ago.  Thanks everyone!

[Govmnt_Lacky]

A new LED and the pedal will be good as it was a few days ago. 

Make sure to orient the LED properly or you might be back in the same situation 

[antonis]

For series diode, you should get no voltage reading at all on its cathode..

Huh??

You didn't quote "blown out" ..

[antonis]

Make sure to orient the LED properly or you might be back in the same situation 

I wouldn't say so..
(a reverse oriented LED could make no harm..)

[amptramp]

Most LED's don't have a very high reverse voltage.  A 9 volt battery can fry them.

[antonis]

Most LED's don't have a very high reverse voltage.  A 9 volt battery can fry them.

Of course but a burned out LED shouldn't create any issue other than no ON-OFF indication..

BTW, the only way for fried both D1 & LED is R13 accidentally been shorted..

[PRR]

...OnSemi datasheet for the 1N914 says 300mA maximum forward current...

ISTR that it used to be 100mA.

Certainly if I suspected >100mA I would look for a 1N400x.

But I don't see any 100mA in that plan unless it got shorted.

[amptramp]

If you plugged in a very stiff power supply that was on, you would get uncontrolled current going through the diode to charge the 100 µF filter cap.  I agree that a 1N400X series diode would give you some more margin.  It would also extend the life of the diode to plug in the power supply when it is off then turn it on so the ramp up in power supply voltage will deliver charge over a longer period of time with less inrush current.  Some designers use a small resistor in series with the diode to limit the inrush current.  Since you are not drawing a large amount of current, you would not get much drop across a series limiting resistor.

[ElectricDruid]

Some designers use a small resistor in series with the diode to limit the inrush current.

...and it makes the power filtering cap much more effective if it's an actual RC filter, instead of depending on the resistance in the wires!
There's *lots* of good reasons to have a small resistor in series with the power supply!