Line6 DL4 No Power

[GibsonGM]

I was assuming the diode has failed (or was killed by the 887), so I wanted OP to find an open or a short...given the circumstances, I would simply replace the 887 chip and the diode also...and then test that circuit block independently by cutting the trace to the next IC.

I looked up how this Line6 likes to fail, and the 887 was often (always) the cause.    I agree - the DMM will not be able to overcome IzK!     My DMM has difficulty finding Vf of simple LEDs...

[Stevo711]

I'm not measuring breakdown voltage. I'm measuring the voltage drop of the diode out of circuit. Super easy test with a good DMM.

[Stevo711]

And what exactly is the function of D1 and D2? Seems like a sort of clipping circuit on the output of the TL072 from the guitar input. Maybe a limiting factor before the signal heads into U4?

[GibsonGM]

Your meter can test a Zener?  It provides enough current to cause breakdown and then measure the Zener voltage?     Kind of irrelevant. With that zener out of circuit, you should have about 3.3v coming from the 887, per its data sheet. If it died, it may well have taken the zener with it. It is low enough cost to just change them both, as this appears to be a common failure of the L6 unit you have.  This doesn't guarantee that the surrounding components are ok, however.

Can't say for sure what D1/D2 are doing, except that I suspect they are some form of crowbar circuit that would clip off excessively high signal voltages, perhaps...

[Stevo711]

All I know is that D1 measures zero VDC with my bench meter in the diode setting and with the meter probes both ways. D2 measures ~0.65VDC forward and open reversed. That is a dead shorted D1, no other way to look at it.

[GibsonGM]

Depends on what's going on, I can't see what you're doing, LOL.  There can be a few ways of looking at things, sometimes.  Are they both out of circuit?   

"Open" reversed - could that just be that the resistance is so high, the meter's not picking anything up on diode setting?  Ohmmeter setting can often tell much.

If in doubt, change them out!  Since they are (I assume) out, I'd change them just as a matter of principle.    My only actual point is that either the zener failed and shorted the 887, or the 887 failed and blew the zener...887 + diode should be replaced...WHY that happened, hard to say, but others report the same issue.

[Stevo711]

Well, I'm going to have to declare this DL4 dead, at least for now anyway. It seems the issue is downstream from the voltage regulator. It's probably safe to assume that one of the larger surface mount IC's are toast. Those just aren't worth the time and money, IMO. Time to cut my losses and save up for the new Boss RE-202! 

[GibsonGM]

Awww, that sux! At least we all got a look at how the power section is set up with these.  You're sure you got power applied downstream? 

[Stevo711]

Yes, there is power after U21. It has to be one of the large IC's somewhere after that. I gave it a best effort and I'm moving on.   

[GibsonGM]

Sometimes, we have to, lol...been there done that.  IMO, after some point it's not worth trying to save these SMD, tiny chipped things.  Rack it up to a learning experience!

[Stevo711]

Absolutely! Maybe it's time to finally build that pedalpcb clone of the BuggFX Daydream....