Boss Pedal Ground / Common

[mr clack]

I have a Boss BF2 that I am trying to repair and I seem to be coming across problems like this again and again with boss pedals which means I just don't understand how they wire ground properly!

If I put my multimeter on the 0v connection of the input power connector and measure common on the board I get 3v and on the +v rail 9V

I understand that the 0v connection goes through a 56ohm resistor and a diode (for reverse power protection) across the input jack so it switches  only when the jack plugged in.

then obviously between the ground of the input jack and 0v of the power input I get -3v and 6v +V

that must mean the circuit only sees 6v??

I have read that when a boss pedal is opened up it wont work , but thats only because the output jack is not grounded as it goes through the case correct?

what am I missing? is this a fault or normal?

[thedefog]

The circuit should see the full 9v, unless there is a diode drop in there. In all of the Boss pedals I've worked with and repaired/modded, I've never experienced what you're seeing, never any +v on the G. Check your meter maybe? Make sure your battery/power supply is good, and make sure you have a 1/4" jack plugged into the input.

Boss pedals are no different than any other pedal when opened. If there is no ground running to the output jack and it uses the enclosure as a ground source (which is generally the case), just use an alligator clip from the ground lug of the output jack to the input jack ground. The real question is why would you remove the output jack from the case anyway?

[R.G.]

what am I missing? is this a fault or normal?

It's an oddity about some Boss Pedals that only becomes apparent when they're used with other pedals on a daisy chain power supply, plus the pedal you're working on probably has a problem if your measurements are right.

Look at the schematic, over on the left side by the input jack and battery symbol. Notice that the battery connects to the ring connection of the input jack and the switched battery connection of the AC adapter jack. The negative terminal of the AC adapter jack goes to board wire terminal 5, and then through a resistor (R61?) and a diode (D10) before connecting to the ring connection wire at wire terminal 4. What this means is that the DC from the adapter cannot be at the same voltage as ground on the pedal. It goes through a diode and a resistor, and so the negative terminal of the adapter must be lower than signal ground by at least one diode drop plus the resistance times the total current used by the pedal.

Pedals with this style of power connection were probably meant for use with one power adapter per pedal, not a daisy chain as used today. The oddity can be corrected by moving that diode to connect anode to signal ground on the board and cathode to +9, and shorting between board wire terminals 4 and 5. This converts the board to modern style input polarity protection and so it will work with other pedals sharing the same power and signal grounds.

However - if you're seeing 3V across the resistor and diode (that's how I translate what you told me) then there is way too much voltage across the resistor and diode for everything to be working correctly. Either the diode or resistor is causing way too much voltage drop, or the rest of the circuit is pulling way too much current and causing the voltage across the resistor to be too high.

[mr clack]

The circuit should see the full 9v, unless there is a diode drop in there. In all of the Boss pedals I've worked with and repaired/modded, I've never experienced what you're seeing, never any +v on the G. Check your meter maybe? Make sure your battery/power supply is good, and make sure you have a 1/4" jack plugged into the input.

Boss pedals are no different than any other pedal when opened. If there is no ground running to the output jack and it uses the enclosure as a ground source (which is generally the case), just use an alligator clip from the ground lug of the output jack to the input jack ground. The real question is why would you remove the output jack from the case anyway?

Yep all of those things are fine, I tested the multimeter across the power supply. I am testing it with a scope, so the output jack isn't the matter yet as I am tracing the signal through the circuit and found it stopped at the BBD chip (I can only see the clock come out of it then after the filtering nothing is there) and then noticed the power problems! one of them being that that 5v regulator was putting out 7.5v!! I replaced that but I just wanted to clear up that the power problems were real (and not some standard boss anomaly) before I got into the circuit.

... Look at the schematic, over on the left side by the input jack and battery symbol. Notice that the battery connects to the ring connection of the input jack and the switched battery connection of the AC adapter jack. The negative terminal of the AC adapter jack goes to board wire terminal 5, and then through a resistor (R61?) and a diode (D10) before connecting to the ring connection wire at wire terminal 4. What this means is that the DC from the adapter cannot be at the same voltage as ground on the pedal. It goes through a diode and a resistor, and so the negative terminal of the adapter must be lower than signal ground by at least one diode drop plus the resistance times the total current used by the pedal.

Pedals with this style of power connection were probably meant for use with one power adapter per pedal, not a daisy chain as used today. The oddity can be corrected by moving that diode to connect anode to signal ground on the board and cathode to +9, and shorting between board wire terminals 4 and 5. This converts the board to modern style input polarity protection and so it will work with other pedals sharing the same power and signal grounds.

So what was the purpose of doing that? is it just because they didn't want the battery to be affected by the diode/resistor drop? and the slight voltage drop was not an issue on the PSU as normally its a bit higher than a battery anyway?

However - if you're seeing 3V across the resistor and diode (that's how I translate what you told me) then there is way too much voltage across the resistor and diode for everything to be working correctly. Either the diode or resistor is causing way too much voltage drop, or the rest of the circuit is pulling way too much current and causing the voltage across the resistor to be too high.

Yes 3v between the power 0v and the jack GND, I tested the diode and resistor , there must be something in the circuit, I am using a bench PSU and it says its drawing 20mA (it was 10mA before I changed the 5v regulator. my psu only displays every 10mA so thats rough)

A cheeky question what should I be looking for? I always check out electrolytic's as increasing leakage = larger load?  

[Paul Marossy]

However - if you're seeing 3V across the resistor and diode (that's how I translate what you told me) then there is way too much voltage across the resistor and diode for everything to be working correctly. Either the diode or resistor is causing way too much voltage drop, or the rest of the circuit is pulling way too much current and causing the voltage across the resistor to be too high.

Yes 3v between the power 0v and the jack GND, I tested the diode and resistor , there must be something in the circuit, I am using a bench PSU and it says its drawing 20mA (it was 10mA before I changed the 5v regulator. my psu only displays every 10mA so thats rough)

That sounds right to me, it should be drawing a little more current with a lower voltage. 20mA seems like a lot for a guitar pedal, but OTOH there is a lot of active components in that circuit...

[mr clack]

Tip I found out for everyone, dont always blindly trust datasheets and new components!!

The pinout for my ST manufactured 78l05 I bought shows the pins reversed to what they actually are even on the data-sheet supplied with the part on the website!! burnt a couple out before I worked it out!

also found another issue which was a broken protection zenner

[thedefog]

Tip I found out for everyone, dont always blindly trust datasheets and new components!!

The pinout for my ST manufactured 78l05 I bought shows the pins reversed to what they actually are even on the data-sheet supplied with the part on the website!! burnt a couple out before I worked it out!

also found another issue which was a broken protection zenner

I've learned that lession with transistors. I always test them out on the meter first to make sure I have the orientation right. Although it generally doesn't, It can vary between manufacturers.

[Paul Marossy]

Tip I found out for everyone, dont always blindly trust datasheets and new components!!

The pinout for my ST manufactured 78l05 I bought shows the pins reversed to what they actually are even on the data-sheet supplied with the part on the website!! burnt a couple out before I worked it out!

also found another issue which was a broken protection zenner

Whoa, that's one you wouldn't think of looking for. I mean the data sheet is supposed to be correct.