Boss jb2 malfunctioning LED when switching between modes

[Isaac.t]

Hi All,

Ive knowingly purchased a boss jb2 pedal that has a malfunctioning LED. The mode LED colour is JHS "red", "Boss BLUE" & Bypass as usual clear. What its doing now is JHS red, Boss clear and Bypass mode a faint blue.

I'm new to fault finding boss pedals and have noticed alot of micro electronics that I have never experienced before. I'm thinking it has something to do with a faulty faulty diode resistor or possibly the diode itself?

[ElectricDruid]

Hi Isaac,

It's a recent pedal, so I doubt we'll find any schematics about, unfortunately. And the boards are likely to be all surface-mount parts, so it's not the easiest to work on.

That said, something simple is still most likely. Have you taken it apart and checked it over? Can you post some photos of the guts and the boards?

[Isaac.t]

[Isaac.t]

Cheers for your input its much appreciated.

I'm not sure that the photos have come through clear.

Ive taken photos of both the LED & Mode switching rotary pot from the front and back including the main connection to the pcb for both pieces.

[MikeA]

According the manual, the LED scheme should be Boss = blue, JHS = red, both = purple.  That's achievable with a two-color LED in red & blue, with both on = purple.  I also see 3 terminals on the LED which makes this idea more likely. 

Questions:  Does the foot switch work correctly?  If not, start there. If yes:

Do all the positions of the Mode switch work correctly (sound right)?  If not, start there.  If yes:

Do you ever see blue or purple LED?  If not, that would point toward a fault in the blue side of the LED or something closely connected to it (power, ground, current limiting resistor or switching.)

Let us know!

[Isaac.t]

Ill start on the mode switch pot, it may have an internal issue causing the blue led to "faintly" operate on bypass mode and be clear on boss mode. Switching in the opposite direction

Footswitch is operating correctly.

Thanks alot!

[Rob Strand]

Not sure if this helps or hinders,  but it's an example of a modern Boss pedal (OC-3),




Adding a bit more,

It's highly likely the JB-2 uses the Red/Green circuit with CN6-1, CN6-2, CN6-3 which then plugs into CN1-1, CN1-2, CN1-3.   The parts R66, R68, Q14 would be loaded like  Q13 etc.

Does the pedal actually work?    It should be RED=JHS, Blue = BD-2.

Perhaps a faulty mode switch, or mode switch wiring, is messing with things instead of the LEDs.

[Isaac.t]

Pedal works & sounds like my good jb2 that I don't want to open for warranty purposes.

Thanks for the information much appreciated

[Rob Strand]

Pedal works & sounds like my good jb2 that I don't want to open for warranty purposes.

Thanks for the information much appreciated

Yeah, no need to open the good one.

If it works correctly in all modes and selects the correct mode then what's very puzzling is the Blue LED lighting in clean mode.

In clean mode both micro outputs should be high.    So for the blue LED to light in clean mode that means the micro going high and is driving current through the transistor and then to the LED.   Since the micro output doesn't swing to full LED supply voltage it explains  the dimness of the Blue LED (also the current flows through the high value base resistor).    So perhaps the transistor for the Blue LED is blown, or perhaps it has been incorrectly replaced with an NPN type instead of a PNP type.    That would also explain the Blue LED not lighting in BD-2 mode.

The fried blue-LED transistor theory seems to fit a lot of the observed symptoms!

[Isaac.t]

Rob.S I think you were spot on, I found Q25 and Q26 readings were inconsistent from one another when switching between boss and jhs mode.

My only issue is when I removed Q25 it kind of crumbled.... maybe too much heat?
Also would anyone know what kind of chip Q26 is? They're both the same which appear to say S.G 725

They seem very delicate and hard to replace

[Isaac.t]

[Isaac.t]

[Rob Strand]

Rob.S I think you were spot on, I found Q25 and Q26 readings were inconsistent from one another when switching between boss and jhs mode.

My only issue is when I removed Q25 it kind of crumbled.... maybe too much heat?
Also would anyone know what kind of chip Q26 is? They're both the same which appear to say S.G 725

They seem very delicate and hard to replace

Good news then.

The schem I posted indicates 2SA1586-GR.  If you look at the datasheet for that part the GR version is marked SG.   So it looks like that's it (sometimes Boss change the parts over time but its not the case here).    Any PNP with a similar gain (package and pinout) will work.

https://pdf1.alldatasheet.com/datasheet-pdf/view/419394/SECOS/2SA1586.html

I wouldn't worry about the part crumbling.  Sometimes they fry and crack othertimes they already have physical damage.

They do require some care in soldering but it is possible to do with normal tools.

[Isaac.t]

It looks like the BJT with 400hfe isn't readily available in Australia.

Pretty annoying, postage costs more than the part itself 🥲

[Rob Strand]

Off hand, it might be easier to get,

BC857BW; BC858BW, BC859BW

B = gain range
W = package (SOT232)

Please check details of gains, package and pinouts.

[Isaac.t]

Thank you Rob really appreciate it.

One thing I also noticed in the OC3 schematic there is a 2SC4116GR transistor inline with the 2SA1586GR, one is NPN and the other PNP.

[Rob Strand]

One thing I also noticed in the OC3 schematic there is a 2SC4116GR transistor inline with the 2SA1586GR, one is NPN and the other PNP.

The emitter of the NPN is the output of fixed low voltage power supply.  The voltage on the emitter (when any LED is on) will be about 2.6V to 2.7V.

If the PNP fault was mechanical then the NPN is unlikely to have suffered at all.  If the PNP fault was electrical then the NPN might have suffered a bit.   The resistors around the NPN should help protect it.  Given the unit works on one LED the NPN is probably OK and there's a low risk leaving it alone.