DOD Grunge Distortion. Won't Power Up.

[beans_amps]

Hey all,

I have not been around in a long while.  I just picked up a broken Digitech Grunge distortion.  It does not work at all.  No sound, hiss, clicks, pops or anything.

The thing is completely surface mount with four 4580 (I think in the SSOP8 package) op amps and one 14007U.  It has at least 11 SOT23 transistors on the board as well.

I have looked about the net and found a link to a DOD FX69B which is purported to be same thing, but is not.

The pedal does not draw any power when connected, (Fluke 79 says 0 mA.)  So I assume a blown reverse polarity diode or something, but I cannot find anything wrong. 

Anyone have any ideas, or a link to a proper schem.

p.s.  This same board is apparently also used for the Death Metal Distortion.  A few components (C13, C38, C39, and Q12)

TIA

Sean Weatherford

[Processaurus]

Because it isn't drawing any current, seems like something is open early on from where the power comes in.  How far does the +9v get when you check with your meter? does it get to the board? opamps supply pins (pin ?  Could be something weird with the input if it uses the stereo jack trick to turn the battery on?

[beans_amps]

I am in the process of going through all the power points to see where power might be missing.  I just now realized that I have been attaching the -lead to the battery clip when measuring voltage.  If I have an open in the return path I would not see it.

I was just hoping someone had a schem or some known problems they could pass on to me.

I am not a big fan of SMT repair work. 

I may call digitech and see if they will help.

Sean Weatherford

[mongo]

I'd bet it is one of the polarity diodes...


Andy

[beans_amps]

I talked to Digitech.  They will not send a schem, even if I offer them money. 

Talking to the tech guy, he keeps referring to the 'ferrite bead' at the AC adaptor input.  I finally said I do not see a ferrite bead.  I do have a surface mount schottkey diode (SK14.)  He says, "We call it a ferrite bead."     I think I may be on my own.

I will check the input jack to see if something is wrong there.  I will also try to trace out the schem, but that is going to take a while.  Board is pretty heavily populated.  Anyone know where I can find a good list of surface mount transistor numbers?  I have found some but they do not list the trannys I have.

Sean

[John Lyons]

So you do know that it's not the same as the DOD FX69B?
I have one of those schematics.
The chips in mine are 4560D and one MC14007UBCP Nothing is surface mounted on mine though...
So you have checked battery to diode and the diode is ok? (reading +9v on both sides)
Usually this is the problem as people will stick a phone unit's 12 volt adaptor in or use the wrong polarity plug etc.

John

[beans_amps]

Apparently when I looked the first time, I clicked on the wrong link or something.  The first one I looked at only had one opamp and two or three transistors.  The FX69B does appear to be reasonably close, the online schem does not show the power supply setup though.  A little longer time spent troubleshooting last night.  The input jack does switch the battery in and out of the circuit.  Schottkey Diode OK.  Circuit draws 3-4 mA with cord in input jack.  Should draw 20mA or so according to Digitech.  Almost all the battery voltage is being dropped across two series resistors in the power supply.  So I assume I have a shorted opamp, transistor or biasing diode.  I don't think it would be a capacitor, but It could be. 

The power supply voltage on the 4580's and the 14007U is about 1.6 volts.  If the ic's were socketed, I could pull them one at a time and see if the battery voltage came back up.  Im not sure how to go about finding a shorted SMT component.  Perhaps I could have a look with an infrared camera.

I will post back as I find out wha tis going on in case someone else has the same problem.

Sean

[beans_amps]

The problem was a pair of 100 ohm chip resistors paralleled for 50 ohms in series with the power supply feed.  This was after the schottkey diode and before everything else.  They were reading very high.  9 volts on one side, 1.5 volts on the other.  Tack soldered a 47 ohm resistor across the back of the board.   Pedal works now.  Now I have to order some 603 size 100 ohm resistors.

Sean Weatherford

[John Lyons]

Gotta love SMT parts!
At least you found the problem. Cool!

Also There was a thread here about taking out C19 (15n) for a good bit more bass responce. Worth a shot as this one is pretty bass shy.
Adding series diodes to the clippers would clean it up a bit as well. Not to mention the op amps could be upgraded.......................

John