Line6 DL4 No Power

[Stevo711]

Hello everyone, I picked up a non-working DL4 for super cheap. There is no power on the unit. I've tried all the initial troubleshooting; reseat the EPROM, factory reset, tried with battery power and adapter. I moved onto the diodes. The 3.6V zener was bad. From what I've read, it's only in the circuit when using the adapter, not battery power. I'm not sure that is correct.
I'm moving onto U14, the MAX887 chip. How should I test this? I don't think I have 3.3V coming off pin 7, but I'm unsure how to test it properly. I'm assuming one would use pin 5 as ground with negative test lead and the positive test lead on pin 7. Is this correct? If so, then U14 is bad, as there is no voltage on pin 7.

[GibsonGM]

Hi Stevo, is there any kind of schematic for the thing available that you could post?
Welcome to the forum

[Stevo711]

Hopefully that link to the schematic works.

[GibsonGM]

Pin 5 is ground, yes, and you should get something at pin 7, and 3.3V at the point indicated after the inductor. 

If D15 was shorted that would cause no voltage on pin 7.   Check C58, tantalums can go bad...
Make sure D11/D12 are good.  Should take about 1 minute.

SO to test, step 1:

Tell what you are powering this with (batteries, adaptor...), what the supply voltage is (pin 8?  small numbers), what you get at pin 7 and if you have voltage, is that present where it's marked "3.3V" after the inductor...

[Stevo711]

I'm powering it with 6VDC from my bench power supply, across the DL4's battery connections. I believe I've found at least one faulty component. D15 reads just .1vdc forward voltage and around 250 ohms. That seems low to me. I also replaced the electrolytic, 220uf cap C58. It was leaky on a bench test. I'll try to find a replacement for D15 and see where that gets me.

[GibsonGM]

Not sure if your chips will still be living, but that is a start!   Me, I'd lift that inductor so there's no path beyond the 887, and see if I get anything from the 887 - but be warned, I'm not sure if that zener diode is required to protect the chip, so make sure there's no path thru the inductor.

The 887 appears to be a step-down DC-DC converter, and it powers the rest of the circuit. If you can manage to find a 3.3V 'lab' power supply, you could disconnect it and see if the rest powers up without it (bypass the entire 887 network), which would reveal the 887 (and/or diode) to be bad...the PS would have to be able to source enough current to run the rest of the thing, though.    Alternative, replace the diode and cross fingers, then be ready to replace 887...

[Stevo711]

We're on the same page. I went ahead and placed a Digikey order for D15, the Max887, Max660 and the LM3480. They're inexpensive and I'm just going to assume that they're toast after the shorted D15. I really want to bring this thing back to life! Thanks for your help!

[GibsonGM]

You're welcome!   If there's any way to do it, I suggest isolating the 887 first, described above, and making sure it functions OK before it feeds the rest of the thing.  I found some references to this issue online - seems often it's that 887 that dies.   Want to make sure it's doing ok before taking on the load of the rest of the ckt...but it sounds like you're headed in the right direction!

[Stevo711]

I'll remove the 887 and power it on the bench and see if there is a voltage present. It's a big variable supply rated for 12VDC, 10A so it should handle the job. 

[GibsonGM]

Well, dunno if I'd do it out of circuit!  Those are SMD, tiny ICs, right?   Just break the connection from its output to the next chip.  Apply the voltage at the IC voltage input.    Go look up the data sheet for it. You should be able to do this in-circuit, even if you have to cut a trace (that you will later jumper)

I suggest measuring the suppy to say, 4 or 5V, and THEN connecting it, LOL  The 887 doesn't need the whole 6V to function, safer to keep it lower than needed.

[Stevo711]

I removed it, lol! Easy as pie, I have a ton of soldering experience. It's good, applied 5 volts on pin 8 and have around .3 out of pin 7. I think that schottky is the culprit, based on what I saw with the meter. Parts should be in next week.

[GibsonGM]

You should have 3, not POINT 3...seems the chip is dead, if you performed that test correctly. go check the data sheet...probably needs the other parts connected to it to function...I'd do it in-circuit after removing diode and lifting the inductor or cutting trace to isolate the chip....just sayin'...

Good thing is that many say after changing out the chip (and hey, the diode too might as well), the rest of the ckt runs fine. Maybe yours is having the same problem!

[GibsonGM]

It 'needs the other parts', if you will. Check this out, and get data sheet if you want.   Or just change it and diode & cross fingers.
There is an 'on' voltage at pin 1 that's needed, and feedback....
Cut trace to pin 3 of next chip if you want, to test a new IC and diode safely...then just jumper the cut trace (easy).

https://www.analog.com/en/products/max887.html

[Stevo711]

In continuing to troubleshoot this, I found D1 to be dead short as well. It is in series with D2 so I'm assuming it's part of a protection circuit? You can find D1 and D2 after the TL072 on the Mono Left input. I think this would definitely not cause it to power up as well, since the L Mono input must be connected to power it on in the first place. Thoughts??

[Stevo711]

I suppose I could remove one of the 3.9V zeners from the Right input circuit and try it to see if it powers up

[GibsonGM]

I'd wait on getting the part, and not cause some strange issue by removing and replacing the probably-good diode.  The fact you get nothing out of the 887 suggests highly to me that it's dead UNLESS D15 is shorted (is it?).   Cutting that trace would isolate that section and allow you to know one way or the other.  It's not hard to repair a trace you cut after the fact, once you have a new diode/887 installed and tested.  Standard procedure.

D15 seems to be for protection in case of over-voltage....it's regulating that 3.3V output.  With it removed, and 887 block isolated, you should get something close to that I would presume!   Based on the data sheet.     This test will confirm or refute that the 887 is bad, provided you can establish that the caps etc there are good.    Make sure D15 isn't shorted, it COULD be tho not as common as an open.   Failing short may kill the 887, BTW.

Diode test inconclusive?  Cut trace, test power at 887 output.  Remove diode...test power at 887 output. Post results.  *shrug*

[GibsonGM]

https://www.test-and-measurement-world.com/Terminology/Zener-diode-failure-modes-open-vs-short.html

[Stevo711]

Good call. Parts are ordered and they'll be here in a few days. I ordered the replacement for D1 as well.
As far as D15 goes, I read 0.1VDC forward voltage with my meter and 200 ohms. The data sheet states forward voltage to be .26V so .1 is a long way from that. I'm assuming that it's bad.

[GibsonGM]

It probably is toast; removed, you should/would have voltage coming from the 887 if it was good...which is why I think it's not...the death of the diode probably killed the 887...

[antonis]

https://www.test-and-measurement-world.com/Terminology/Zener-diode-failure-modes-open-vs-short.html

With all the respect Sir, I've never managed to measure the breakdown voltage of a healthy Zener with only a DMM..
(probably due to not enough I_Zk DMM current..)

BTW, an excellent reference about Zeners : https://www.onsemi.com/pub/Collateral/HBD854-D.PDF