Sound issues after noise mod

[Deregorn]

Hi,

I recently did the noise mod on my MP-1 and now I'm having some weird sound issues...

After replacing the caps and 2 op amps I turned the amp on, started playing and thought "yeah, nice distortion" but the I realized I was on my clean preset . Well it turned out the op-amps had too much bandwith (10 MHz I think), but after turning the gain settings down it sounded good again.

But after a while the sound became somewhat muddy and lost it's punch. At first I thought the reason might be the op-amps, so I searched on eBay and got some TL072IN (the ones recommended in the noise mod document). I just installed them (IC sockets are great!) and the problem's still there. It's hardly noticeable when playing clean, but on mid-gain settings it's starting to become a problem.

And on high gain it's really awful. I have a EMG PA-2 preamp booster on my guitar, which gives the guitar signal just a flat boost. I use this to kick in some more gain when needed or to overdrive my cleans a bit. That already worked really nice before the mod, and now it turns my mid/high-gain sound to some ugly, muddy crap. Almost sounds like some kind of clipping, but I'm not sure about this, it's really hard to describe.

Any thoughts on this issue? Did I accidentally damage some components? I really don't know what to do now... :-[
Thanks in advance

[MarshallJMP]

Bandwith can't be a problem.

What exactly did you replace,just the opamps or more stuff?What caps did you replace?

[RobbHell]

Did you install the Caps and op amp the correct way?

[Deregorn]

I replaced caps C1, C2, C8, C129, C130 and C131. I'm pretty sure they are installed correctly but I can check them again. The op-amps I replaced are U4 and U9 (installed IC sockets). I used NE5532P first, but they seemed to amplify too much, the gain went from slightly crunchy when OD1 was at 2.2 to really high gain when increasing to 2.4. So I replaced them with TL072IN. Again, I'm pretty sure they are installed correctly.

[Deregorn]

Ok, I just checked everything again. All the caps and op-amps are installed the right way.

By the way I forgot to mention that I changed the tubes aswell. The unit came with PL (Penta Laboratories) 12AX7A, I replaced both of them with Tung Sol 12AX7. However I did this before the noise mod and there were no issues, so I guess something went wrong when I replaced the caps and op-amps...

Anything else I can check? I could provide pics if that helps.

Edit: I just realized the OD1 Clip LED lights up all the time when I play. My OD1 gain is at 7.0 and I've seen that LED lighting up before, but only once in a while, not all the time...
Edit 2: Just turned OD1 gain down a bit. At 5.0 it's still lighting up like crazy. When going below 5.0 it's getting better and finally at around 4.0 it almost never flashes.

[MarshallJMP]

Do you still have the orignal LF412 (U9)?

[Deregorn]

Yeah I do, but most of the pins broke while desoldering it (had to apply some force).

Also checked the trimpots - the one between U4 and U9 is almost all the way down, the others are around half way up.

[Deregorn]

It's getting worse... I turned the OD1 gain down to 4.0, because as I said there was no clipping at that level. I played a bit and now it's clipping like crazy even at 4.0. So it looks like the longer I play the amp, the worse it gets. Any ideas?

[MarshallJMP]

I would take a ohmmeter and measure all the traces going to U4 and U9.

[Deregorn]

I have a digital multimeter. A cheap one. Guess it's not very accurate, but maybe it gets the job done. The problem is I have no idea where and how to measure. Would you mind giving me detailed instructions? I'm a newbie :-[

[Deregorn]

Since no one answered here for 2 weeks: should I give up on this unit and just buy a new one? It really makes me sad that I cannot use it atm... :(

[rnolan]

Hey Deregorn, I'm sure MJMP will get back to you, sorry I can't help more, this is much more his thing....

[MarshallJMP]

Sorry must have missed your reply,(next time shoot me a PM or email if i don't reply).You need to follow the traces comming from U4 untill you see another component.Then measure the resistance between the pin of U4 and the other component.This should be around 0 ohm.Hope this makes sense.
Another way is to use the schematic and the pcb layout to track the traces.

[Deregorn]

Makes perfect sense, I will do that tomorrow and let you know. Thanks!

[MarshallJMP]

Ok,again sorry i didn't reply sooner.