weird boss problem please help

[cmat]

I have a Boss GE-7 that someone has modified.  This takes the ACA adapter my problem is that the pedal works fine off of a battery but wont on an adapter.  The power from the battery runs through the adapter and delievers 9v to the circuit .  But when I plug in the adapter I only get 4v to the circuit.  I can desolder the power wire from the board and measure it and I will have 9v but once I resolder it it goes down to 4v.  This doesnt make any sense to me I could understand it doing it with a battery also but not just the adapter.  Please help

[hairyandy]

I'm taking a quick stab in the dark here but the power adapter jack may have been wired wrong when it was previously modified.  I haven't looked in a boss pedal lately and I can't remember what they used, but the power jacks that most of us use kill the power to the battery when the adapter is plugged in thus saving your battery.  This is acheived by having two positive connections on the power jack.  One shorts when the adapter is plugged in (this is the one that goes to the battery) so they must be wired correctly for the circuit to work with the adapter.  I figured this one out the hard way myself so now I always remember.  You can figure out which is which with your DMM by plugging in an adapter and measuring.

Hope that helps...

[cmat]

Thats the odd thing.  It measure correctly when the power wire is disconected from the circuit but measure correctly when just the batery is used.  Something in the circuit is bringing the voltage of the adapter down but doesnt bring the batery down.  Both run through the adapter that is what is so odd.  I even tried another jack even though I didnt think it would fix it. 

[redeffect]

i have the same problem with a CE2. i thought i was the only one to be so afflicted. mine measures 5.6V. i also tried all the things you mentioned,to no avail. i will continue exploring this issue, and keep you posted. i think it might be something with the onboard circuitry too.the funny thing is the schematic and circuitry are "in sych" with each other. i am obviously missing something..
red

[cmat]

That is exactly what mine is doing!  BTW mine is 5.6 also I tried it with a psa to see if it made any diffrence since a psa is 9v but it brought it down to 4v with the aca I get 5.6v also.  When you have it in your hands it just doesnt make sense.  I had to put it down today before my head poped off but I am going to try it again tommorow.  If you find the problem PLEASE let me know and if I get it I will let you know also.

[johngreene]

Sounds like the pedal is drawing more current than the ACA adapter is capable of supplying. You say that someone modified the pedal, is there any chance that the modification would have increased the current needed?

--john

[cmat]

I dont think so.  Why would the batery be able to power the circuit?  This one has got me because both power points go to the board at the same point.

[johngreene]

A battery is capable of supplying a lot more current than your typical wall wart. For a while at least. If you can measure the voltage than you can measure the current draw. That's where I would start anyway.

--john

[MartyMart]

Perhaps someone stuck a 5v6 zener in line with the PSU connection on the board ?
Instead of say a 1N4001 protection diode ( reversed of course )
Could have happened when it was "blown out" with a too large PSU being used and
the "fixer" confused zener with standard diode .... just a guess !

Marty.

[Paul Marossy]

Perhaps someone stuck a 5v6 zener in line with the PSU connection on the board ?
Instead of say a 1N4001 protection diode ( reversed of course )
Could have happened when it was "blown out" with a too large PSU being used and
the "fixer" confused zener with standard diode .... just a guess !

Marty.

Sounds plausible.

[johngreene]

Except that it should also affect the battery in that case. Both the battery and the PSU connect to the polarity protection diode. Interestingly, the PSU negative returns through a 470 ohm resistor and another diode. If this schematic is correct: http://www.freeinfosociety.com/electronics/schematics/audio/bossge7.pdf

--john

[cmat]

That schematic looks correct by my pedal (thanks for that )  Is D1 just a 1n914?  Also D5 is a 1n4004 in my pedal.  What do you think would be bad?

[cmat]

Ok, I desoldered the adapter ground and soldered it to the batery's ground on the board bypassing the ACA voltage pull down parts and now it has the correct voltage with a PSA so the D1 diode must be bad correct?  If so what diode is it?  Thanks

[johngreene]

I can't understand the circuitry in the negative path of the power supply. The schematic says that it draws about 6mA. The 470 ohm resistor and the diode are going to drop 3.4 volts with that much current. 9-3.4 = 5.6v. Just what you were seeing. It's almost as if they were expecting the power supply to provide 12 volts instead of 9.

--john

[cmat]

This pedal takes the ASA adapter which is 12v but something is causing it to drop from 12v to 5.4 but when I bypass the diode and the 470 ohm resistor and use a standard 9v adapter everything is right.

[johngreene]

Well, the 470 ohm will drop voltage depending on current draw so if your pedal is drawing anything more than 6mA (you said it was modded, right?) then you are going to lose almost .5 volts for every mA of current.

--john

[cmat]

Yes the pedal is modified but just with chip changes and a few cap swaps.  When I recieved the pedal it wasnt working at all, pieces missing and parts were wrong.  The pedal sounds wonderful when a batery is used or if the ACA stuff is bypassed.  I should just convert it to PSA but I want to know what is causing this.  I think you have got it figured out but I would like to know if D1 is a 1N914.  Could it be bad causing the big voltage drop?  Or do you think that the 470ohm will have to be adjusted to a diffrent value?

[johngreene]

The 470 ohm would have to be adjusted to match the current current draw.

When you say chip changes, the chips were replaced with the same part number or different?

The schematic link I posted recommends a few changes that result in the current draw increasing from 6 mA to 20 mA. Once you know what the current draw of your pedal is now you can calulate the new value by taking 12V-.6V-(current*resistor) = 9V. Or rearranging ((12V-9V-.6V)/current ) = resistor.

--john

[cmat]

Ok Lets see if I got this.  I measured 19.8 as my current draw so I need to use a 2K2 resistor?  Does that seem correct?

[johngreene]

I get 120 ohms.