Boss DD-2 not working at full 9V

[maximee]

Hey everybody

During this christmas season I finally, after a couple of years, found time to do some soldering again.
So this is my first post since a long time. Glad to be back!  

I'm experiencing a weird problem in my vintage MIJ Boss DD-2.

It runs fine on batteries, but as soon as I connect it to a 9V power supply (Ibanez, the original Boss PSA or my regulated voltcraft PSU) it starts humming. This humming occurs also on the bypassed mode and fully cancels any other signal.
I opened it up and at first expected to find some broken wires or alike. But it all seems fine.
As batteries seem to work i connected the PSU to the battery lugs of the DD-2. And I got the humming again. Weird.

Also, whenever it craps out on me I can smell burnt electronic parts. Not much but it is there.
After checking some voltages with my multimeter I found out that the pedal works only if the DC input voltage of the PSU does not exceed 6.9V.

The voltage measured at DC jack when operated with battery is 5,6V (Voltage of the battery itself when disconnected is 8,2V)

Maybe this sounds familiar to the more experienced users in here?

I'm thinking that there might be some bad component that is shorting when there is too much power present.
That would also explain the burnt smell. But that's just a guess.

As always: thanks for your help, guys!!!  


EDIT: When the DD-2 is humming, it is not enough to lower the voltage. The DC jack has to be removed and the pedal has to be "rebooted". Otherwise it just keeps on humming endlessly.

[VPIF]

Old BOSS pedals are supposed to run on 12V ACA adapters, I think...

[maximee]

This one still has the saying "use BOSS PSA adaptor only" on it. And the PSA I have doesn't work...

[VPIF]

Had to check my vintage BOSS CS-1.

It has a table printed underneath. It says to use BOSS AC adapter.

Linkaments: http://www.stinkfoot.se/andreas/diy/articles/bossadapt.htm

[maximee]

Thanks for the link, I was already looking at that site. As stated, mine says to use the PSA adapter, both at the label besides the jack as well as on the plate table printed underneath the pedal. Also I tried hooking it up to another pedal (a MIJ TS9) using a daisy chain DC cord (so they have a common ground). Unfortunately it didn't work. So I'd say it's not the PSU... or am I missing something?

[VPIF]

I'm one of those so-called newbies, so I guess someone else have to suggest what the issue with this pedal might be.

[Processaurus]

Check that IC10, a 78L05 Regulator (looks like a transistor) is working right.  There should be 9v on the input, 0v on the ground, and 5v on the output.

DD2 schematic:

78L05 datasheet

[maximee]

Processaurus, you are right, that seems to be the problem area. As PSU voltage exceeds 6,9V IC10 gets really hot.
Below 6.9V it is working as supposed with 5V output.

Now I'll have to find out why that happens! Thanks for pointing me in the right direction.

(Notice to self: Next time I smell hot electronic parts actually TOUCH the components and see where it's getting hot. That is such a no brainer but didn't come to my mind.)

[maximee]

There seems to be something wrong with D6, a 11V RD11FB3 Zener diode. It is getting so hot that it desolders itself from the PCB... 
I'll replace the Zener and see where that goes...

[maximee]

Woohoo!

D6 was faulty and shorting out.
I substituted it with a 1N914. Success!!! I'm happy! 
Thanks Processaurus, I owe you one. Gotta love the DIY community!
Now: delay away-ay-ay-ay-ay-ay.

[Processaurus]

Weird, the zener was there to both protect against reverse polarity power and to limit the correct polarity voltage to 11 volts.  You probably don't need the latter since you have a real regulated pedal power supply, but to work well as shunt type (violently sucking all the reverse current to ground )reverse polarity protection, you need a beefier diode, a 1n4001 or the like.  Or you can put your 1n914 in series with the power wire coming in off the DC supply board, that will achieve the same goal.

[jalmonsalmon]

 Thanks for sharing!
I Have a Boss DD-2 I bought brand new in the 80's, and about 16 years ago, I used the wrong adapter and it died. I thought it was fried and tossed the pedal in a box to sit and rot.
I figured it was a lost cause to fix, but kept the pedal just in case...   Few days ago I stumbled on this post and decided to try and replace D6 since after some research, that is what goes out when you use the wrong adapter.
BINGO!   DD-2 is back in action!!!    I had a 1N4148 laying around and used that, so far so good...  Hopefully that will keep the pedal running!

[Mike Burgundy]

It will - you could take it out entirely and it will work.
It's there for polarity protection, albeit not a very good one. If you accidentally apply reversed polarity the diode shorts out the power supply and then it's a waiting game to see which one fails first, diode or supply. If the diode fails open, the circuit is still hit with the reverse voltage. If it fails short, you're lucky. A 1N4148 will fail *fast*, so using something that handles more current is a good idea. Using a series diode is, as said, a much better idea (and that only needs to handle the regular current the pedal uses, no short circuit deathmatch). Use a low-drop diode like a Schottky for best results.