[Mar]

Hello, I bought cp4500 kit over a month ago. The build went all good. Decided to do more, wanted to buy mp512 kit but there was one build/used one eBay. Got it, and I am happy about your gear. Lately, on the mp512 I noticed when using DI, the front LED turns green and as it fades it makes audible distortion/clipping to the sound. Does not happen when using mic input. Any help would be appreciated. I am just an amateur in regards to electronics. Thank you.

[JPK]

What is the revision number of the PCB?

[Mar]

Rev1.4

[JPK]

You could try replacing C21, C22 and C23, in case they get old, but the easiest would be to disable the green LED by cutting R37.

[Mar]

Update. Decided to swap the caps first. Got a new 1uF 100Vc21. After replacement smoke started coming out . I did make sure it was put the right way onto the PCB +/- . DI is not working at all now. Mic input is working.
Looks like this capacitor was faulty. It had a small dent on side I did not notice..This is what you get when buying from eBay. I saved on capacitor but now going to have to spend a lot more on fixing. Learn on my mistakes guys. 

[Mar]

Another update. 2N7000 on DI board burned. Replaced : C21,C22,C23 and 2N7000 . No change, preamp producing quiet distortion on sound as it goes down in volume at the end. Next I cut down
R37. No change. Mic does not have that problem. Any more suggestions? Thanks

[JPK]

Next I cut down R37. No change.
Did it shut down the green LED? If true the issue has nothing to do with the LED.
Check that the supply voltage gets to the DI board, on both sides of R11, against 0V.
Check the lectrolytic caps on the DI board, should not be bumped.
Maybe replacing C3, C4, Q2?

[Mar]

After replacing C1,C2,C3,C4 and Q2 no change. It’s the same as on day 1. Both sides of R11 show voltage. Against 0V = 15.42V on CN2 side and 13.72V on the other side. Again, if signal is line via XLR at the back no problem. Green LED on and off reflecting signal ok.  What’s next ? All resistors and diodes?

Tried plugging different guitars, bass and cables. No change.

[JPK]

Does the problem persist after turning off the LED, by cutting R37?

[Mar]

Yes. Cutting R37 has no effect on the problem. I got all the components mentioned in the post replaced and the problem persist regardless of R37 cut or not. I can make a video with sound files if needed.

[JPK]

A sound clip could help me understanding.

[Mar]

Here it is. Dry guitar followed by a bass https://limewire.com/d/zfSxT#MU82BZAeTL

[JPK]

Can you try to reproduce the effect with pulses of sine wave coming from a generator pluggin?
This is because I don't understand how it could be caused by the preamp.

[Mar]

It has to be something on the DI side of things. When I am going DI on my other pre and coming out into the mic input of the 512 at the back problem goes away. I use DI a lot, this unit didn’t bring me a lot of joy. Not wasting any more time on it.