Debugging a Line6 DL4. Help needed!

[ataro]

Hi!

I hope you can help me fix a dead Line6 DL4 that won't power up and it's giving me headaches.

First, the schematic:

https://www.docdroid.net/JYZyjup/tmp-5225-delay-dl4-stomp-box-modelers1118748780.pdf.html

I disassembled it and made a visual inspection first. Nothing wrong. I did check voltages, all correct till U14. Touched U14 (MAX887) and was very Hot and giving wrong voltages. Pin 7 was giving 0V instead of 3.3v.



A bunch of components are fed by the 3.3v line, with U2 (DSP56364) being one of them.



After a lot of tracking, I found out that if I cut the 3.3v trace near pin31 of DSP56364, the voltage of the whole line is restored and MAX887 operates correctly.



I thought that the DSP56364 was shorted, so I replaced it with a new one (super expensive, BTW) and didn't made an improvement. MAX887 is still very Hot and 3.3v drops to zero as soon as the pin31 of the DSP56364 is connected.

Here's the DSP56364 Datasheet :

http://www.nxp.com/files/dsp/doc/data_sheet/DSP56364.pdf

Any ideas on this? What could cause the short?

Many thanks guys

[eagleaudio]

I see that you posted the same question on several forums. And you got answers there - please check. The first supsected components are 100 nF blocking capacitors on 3V3 power supply rail. Also check that several other ICs are supplied from the 3V3 rail, for example memories. They may be shorted but I would check the capacitors first.

[ataro]

I see that you posted the same question on several forums. And you got answers there - please check. The first supsected components are 100 nF blocking capacitors on 3V3 power supply rail. Also check that several other ICs are supplied from the 3V3 rail, for example memories. They may be shorted but I would check the capacitors first.

Thanks for the reply. Caps seemed fine, and I isolated the 3v3 rail to U2 only, so no other IC's caused the short. Someone suggested U14 is probably Bad and couldn't deal  with U2 load, so I ordered one. Will post results!
Many thanks!

[Digital Larry]

I'd measure the current to check whether it was within normal limits.

[ataro]

Solved: replaced MAX887, the pedal came back to life. Many thanks to all for the help!

[Digital Larry]

That's interesting.  I wonder what the failure mode was?  i.e. is this a common failure for those products?

[hgamal]

My bet is the internal MOSFET responsible for switching.