Bypass switching in a DD-3?

[perfectsoundforever]

Hello everyone. My DD-3 (last MIT version) just died on me and I'm having some trouble finding what's wrong with it. The LED turns on and off but there's no effect signal (I have a dry signal always in both outputs). I messed around with the power supply and I get a delayed pop sound, so the delay is working.

Schematic is here: http://www.synfo.nl/servicemanuals/Boss/DD-3T_SERVICE_NOTES_3rd.pdf

I tried to check if it was a switching problem but it doesn't look like the Boss pedals I've seen... Q2 is the closest it gets, and it passes the signal wether the pedal it's on or off (no idea if that means anything). Should I look for Q10? I get a very weak signal but it's not passing it. I've checked pin 14 of both IC3 and IC4 and the power seems fine (9v and 5v respectively). Pin 3 of IC1 reads only 0.9v. What should I look for?? Replacing C46 didn't change anything.

Sorry for not posting a detailed measurement of all IC's voltages but I don't know how relevant it's in this case. If anyone thinks I should, I will do it right away of course.

Thanks!

[R.G.]

It's likely not the bypass switching in the DD-3 that's dead. It may well be more ominous. The switching is all done with buffers and solid state switches, there isn't a real bypass switch that diverts signal.  It is entirely possible that IC9 is dead, letting +9V onto the logic chips specified for 5.5V absolute max.

Check the voltages on IC9 first. They should be +9V, ground, and +5V. If the "+5" is higher than that, the logic chips are dead.

What makes me suspicious is that there is no shorted-input protection diode on IC9. We find this a lot in pedals coming to us for repair. If there is not a diode from the lower voltage (+5 here) to the higher input voltage, and the input voltage is shorted suddenly, the bypass caps on the +5V regulated side discharge back through the regulator chip and fry it. It becomes, in the parlance of my old power supply lab, a Darkness Emitting Diode, or DED. Generally they fail shorted. Connection to an AC 9V supply can do much the same thing, but this generally burns out the protection diode as well, and the normal "bypass" LED wouldn't work.

I really can't understand why so many commercial - well, and boutique as well - pedals don't put a $0.03 diode there and save some pedals. It's really cheap protection.

I hope I guessed wrong on this, but this is where I'd start.

[perfectsoundforever]

Thanks for taking the time R.G.! Didn't see this earlier.

With fear in my heart I measured IC9 and, I-don't-know-yet-if-luckily, I get 9.7v in and ~4.98v out (and it's properly grounded too). What seemed strange, and it may have been a mistake when measuring, is that I got an oscillating voltage in C29... between 0 and 0.11. I resoldered and now it seems correctly grounded, but nothing changed.

And, yes, I don't think it's a power supply issue, at least not an AC related mistake. I'm using your Spyder supply which seems super reliable (those measurements were made with another supply, though) and it just stopped working when I left the Spyder on overnight... I don't remember right, but could an input short still be the cause of the problem? I could have plugged an EHX pedal with one of those 3.5 ugly connectors and just failed to notice the DD-3 problem until later.

Maybe Boss just wants people who are afraid of SMC to just buy a new pedal, whatever the problem might be. I sure panic everytime I have to fix one of this tiny things. I sure am clumsy when soldering, but how am I supposed to avoid burning the whole board? 

Here are some more measurements I blindly made. Sorry for the random guesses, but I'm completely lost with this one. Where should I continue looking?

IC4
1- 4.92v
2- 4.92v
3- 0v
4- 0v
5- 4.92v
6- 4.97v
7- 0v
8- 4.97v
9- 0v
10- 0v
11- 0v
12- 4.86v
13- 4.92v
14- 4.97v

IC7
30- 1,40v
76- 4.92v


IC11
1- 4.98v
6- 4.98v
20- 4.98v
21- 0v
35- 0v
40- 0v

Thanks again for lending a hand here.

[IsaacMoth]

I have repaired a lot of those Boss DD-3s and 2s and many times it is the input buffer IC has gone bad. IC1b on many of the schematics.

[perfectsoundforever]

I have repaired a lot of those Boss DD-3s and 2s and many times it is the input buffer IC has gone bad. IC1b on many of the schematics.

Wouldn't that mean no dry output as well? I'm getting a dry signal at the output when regardless of the on/bypass state.

[perfectsoundforever]

Anyone any ideas about this??

[slacker]

I would try and trace the signal through the circuit and try and find where it stops, that will give a clue as to where the the problem is. You said both outputs have dry signal so you know it is getting as far as IC1A pin 1, the signal that is going to be delayed then goes though Q5, Q7 and Q9 so check pin2 of Q9 and see if the signal makes it that far. If it does then check for it on pin 7 of IC2 and pin 9 of IC3. If it gets that far then for now we'll assume it's made it into the delay.
It should then come out of the delay and goes though Q8, Q4 and Q3 so check pin 2 of Q3, then pin 10 of IC2 and lastly pins 1 and 2 of Q10, I know you already mentioned those.

Completely off topic, but I've looked at the DD3 schematic a few times and never noticed that discrete ADC before (the stuff round IC5) pretty cool.

[perfectsoundforever]

Well, thanks and sorry for not being able to see the signal path myself. I followed the path as you said and it got as far as Q3 / pin 15 of IC2: I lose the signal at pin 10. I measured and I get 4.41v there, is that alright? The complete measurements for IC2 are these:

1 0.82v
2 1.76v
3 1.76v
4 0v
5 1.76v
6 1.76v
7 8.75v
8 1.76v
9 0v
10 4.41v
11 1.76v
12 1.76v
13 9.69v
14 1.76v
15 1.76v
16 ~0.69v and discharging


Pin 10 and pin 4 seem fine, and that's as far as my knowledge can guess. Thanks again for the help.

[perfectsoundforever]

OK, bumping this one more time to avoid having a Boss branded paperweight.

[perfectsoundforever]

(Just so you don't need to read the whole thread: DD-3 not passing delayed signal but makes delayed noise when plugged/unplugged. Tracing the signal leaves me at pin 15 of IC2, a SA571 compandor. Trying to guess if the compandor is dead or the problem is elsewhere.)
Still stuck with this. I'm guessing the problem is definitely on pin 9 of IC2...
here's the schematic for the compandor:
http://www.onsemi.com/pub_link/Collateral/SA571-D.PDF
Isn't the SA571 just divided in two? The more drastic difference I see is the difference between pin 8 and 9. Pin 1 and pin 16 look suspicious too at least for me (pin 16 just discharging?? what's that all about). Plus, both of those things happen in the IC2 half where my signal is missing. I've replaced C11, C14, C34, C45 and checked C7 for shorts with no luck. Does this mean the compandor is definitely dead? I'm really hoping that it's not, because it's almost impossible to get my hands on one.

Thanks again guys!

[slacker]

If it helps here are the Voltages from the SA571 on mine

1 1.06
2 1.80
3 1.80
4 0.00
5 1.81
6 1.81
7 3.79
8 1.80
9 1.80
10 4.84
11 1.80
12 1.80
13 9.00
14 1.80
15 1.80
16 1.00

Unfortunately I'm not familiar with the chip so I don't know if this shows anything obviously wrong with yours.