Pedal Fried with AC instead of DC

[64fx]

A friend of mine got a GGG ITS8 (with the "Very Expensive Boutique Mods") from me and loaned it to someone who plugged a 9 volt AC power supply into it.  It passes signal when bypassed, but when engaged, the LED will light up, but it passes no audio.

Here are the voltages I measured:

Battery = 10.04v

IC
Pin 1 = 8.96v
Pin 2 = 8.89v
Pin 3 = 6.08v
Pin 4 = .4mv
Pin 5 = 8.73v
Pin 6 = 9.07v
Pin 7 = 9.09v
Pin 8 = 9.7v

Q1
Collector = 9.63v
Emitter = 4.95v
Base = 4.6v

Q2
Collector = 9.57v
Emitter = 4.96v
Base = 4.58v


Here are the recommended voltages from GGG:

IC
Pin 1 = 4.5v
Pin 2 = 4.5v
Pin 3 = 4.5v
Pin 4 = 0v
Pin 5 = 4.5v
Pin 6 = 4.5v
Pin 7 = 4.5v
Pin 8 = 9v

Q1
Collector = 9v
Emitter = 3.0v
Base = 2.5v

Q2
Collector = 9v
Emitter = 3.0v
Base = 2.5v


I'm assuming it probably fried the transistors or the chip, but that's just my guess.  Any help would be appreciated.

Also, is there a way to put in a protection circuit to keep this from happening again?

Thanks!
Daniel

[jdub]

I did this once to my Phase 45- got lucky and only fried a TL072.  Bummer nonetheless, though- I feel yer pain.

A good thread is here: http://www.diystompboxes.com/smfforum/index.php?topic=55003.0

If your ITS8 is GGG, then it probably has reverse-polarity protection.  Unfortunately, this won't help with AC; in fact, as far as I know, about the only thing that will protect against AC damage is...not plugging AC into a DC pedal.  I protected my pedals against AC by clearly marking my AC adapter (for a foot controller) with red electrical tape.  It's worked so far.

I'd bet that the IC is probably toast.  I'd try replacing it and the trannies (4401s are cheap) and see if it fires up, as these components are most likely to fry first. 

[64fx]

Thanks for the link.  I did a little bit of searching, but apparently not enough...

My favorite line in his post is "The worst thing you can do to a pedal short of melting it down in a pot of liquid iron or pounding it to bits with a sledge hammer is to connect up a power adapter that provides AC instead of DC. "  Yikes!!!

I think I've got some extra transistors and IC's, so I'll give that a go first.  In R.G.'s post he said something about the protection diode being fried.  Aside from the germanium diodes for clipping I think there's only 1 other diode, so I assume that would be it?

Thanks!
Daniel

[jdub]

If you have the GGG PCB, it'll be the 1N914 at the lower-right corner of the board.  Actually, if I had read the thread I mentioned a little more closely  , R.G. also suggests that the electro caps could be damaged, too.  It certainly won't hurt to swap those out, too...little more work, though...

[petemoore]

  Ouch !
  Pin 4 should be grounded.
  Inspect for burnt anything with close-in mag and light.
  You can measure the resistors, last thing to go though I think.
  Rework the board, tease in the new polarized components, Electro's and Opamp [socket], or use solder-wick to get the old solder out, install with correct polarity !
  You can also measure the diodes, clipping diodes might survive that.
  To prevent this from happening, require all the worlds AC adapters to use a different type of connector, or replace the connector you wish to have supply yet protect a circuit with something unique enough that it can't be RP'd or AC'd.

[JKowalski]

Unfortunately, this won't help with AC; in fact, as far as I know, about the only thing that will protect against AC damage is...not plugging AC into a DC pedal.  

If you put a schottky (lower V drop) diode in series with the power supply line you should be fine, provided it (the circuit) can withstand the peak voltage of the AC.

And if your pedal has filter caps too, which it really should have, then you might possibly have it functioning using AC as well as DC since the diode + caps = half wave rectifier DC supply  

I wonder if there are any DC power supply protection chips out there? Over voltage, AC protection, short circuit protection, TO package? That would be nice. Of course, a discrete overvoltage protection circuit shouldn't be too complicated at all.

[Paul Marossy]

Just an idea... could you do the paralled diode arrangement like an opamp clipper to prevent AC from damaging the circuit? It seems like it would be able to pass DC but not AC. But I suppose one would pass DC but the other diode would block any DC, right? So it would be a stalemate. 

[jdub]

And if your pedal has filter caps too, which it really should have, then you might possibly have it functioning using AC as well as DC since the diode + caps = half wave rectifier DC supply

Wow, hadn't thought of that...

[R.G.]

If you put a schottky (lower V drop) diode in series with the power supply line you should be fine, provided it (the circuit) can withstand the peak voltage of the AC.

You know, if you used a MOSFET in a slightly odd arrangement where you used the MOSFET body diode for that series diode, and arranged to have the incoming voltage turn on the MOSFET, the MOSFET channel would "short out" the body diode and achieve even lower voltage drop than a series diode. That's kinda the basic idea of the MOSFET polarity protector at GEO. 

I wonder if there are any DC power supply protection chips out there? Over voltage, AC protection, short circuit protection, TO package? That would be nice.

None that I know of that could be applied to pedals. And I've looked.

Of course, a discrete overvoltage protection circuit shouldn't be too complicated at all.

It's not. I'm been down this path a couple of times. A MOSFET "rectifier" bridge is possible, and would make DC out of any polarity applied, even AC. However, pedals are extremely sensitive to ground shift and offsets for low noise. There are ways around that, too, but it involves mods to the pedal's signal path that most people would object to. A Low Drop Out regulator (LDO) will do the OV function nicely, and you can hack in some current limit. The LDO's internal current limit will be far too high for most single pedals. It's possible, but the complexity of the any-power-any-time circuit is comparable to the complexity of most pedals by the time you get done.

[JKowalski]

If you put a schottky (lower V drop) diode in series with the power supply line you should be fine, provided it (the circuit) can withstand the peak voltage of the AC.

You know, if you used a MOSFET in a slightly odd arrangement where you used the MOSFET body diode for that series diode, and arranged to have the incoming voltage turn on the MOSFET, the MOSFET channel would "short out" the body diode and achieve even lower voltage drop than a series diode. That's kinda the basic idea of the MOSFET polarity protector at GEO. 


Yeah, after I posted I remembered you had some stuff on your site about this. The MOSFET approach is an excellent one.

[JKowalski]

Here's my attempt at a circuit protection circuit - no negative voltages allowed, no voltages over the first zener limit allowed, regulated to second zener voltage.

Needs 12V to operate at 9V, though. Not very appealing in that respect, I suppose.  



Green is the intput voltage, red is the corresponding output voltage.