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Let's Get Technical => Troubleshooting Problems => Topic started by: Deregorn on May 06, 2014, 11:19:01 AM

Title: Sound issues after noise mod
Post by: Deregorn on May 06, 2014, 11:19:01 AM
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 :o. 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
Title: Re: Sound issues after noise mod
Post by: MarshallJMP on May 06, 2014, 01:59:09 PM
Bandwith can't be a problem.

What exactly did you replace,just the opamps or more stuff?What caps did you replace?
Title: Re: Sound issues after noise mod
Post by: RobbHell on May 06, 2014, 03:25:23 PM
Did you install the Caps and op amp the correct way?
Title: Re: Sound issues after noise mod
Post by: Deregorn on May 06, 2014, 10:23:05 PM
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.
Title: Re: Sound issues after noise mod
Post by: Deregorn on May 07, 2014, 09:07:47 AM
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.
Title: Re: Sound issues after noise mod
Post by: MarshallJMP on May 07, 2014, 01:15:17 PM
Do you still have the orignal LF412 (U9)?
Title: Re: Sound issues after noise mod
Post by: Deregorn on May 07, 2014, 02:10:38 PM
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.
Title: Re: Sound issues after noise mod
Post by: Deregorn on May 11, 2014, 03:18:05 AM
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?
Title: Re: Sound issues after noise mod
Post by: MarshallJMP on May 11, 2014, 01:52:49 PM
I would take a ohmmeter and measure all the traces going to U4 and U9.
Title: Re: Sound issues after noise mod
Post by: Deregorn on May 11, 2014, 02:32:27 PM
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 :-[
Title: Re: Sound issues after noise mod
Post by: Deregorn on May 25, 2014, 06:10:37 AM
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... :(
Title: Re: Sound issues after noise mod
Post by: rnolan on May 25, 2014, 08:51:22 AM
Hey Deregorn, I'm sure MJMP will get back to you, sorry I can't help more, this is much more his thing....
Title: Re: Sound issues after noise mod
Post by: MarshallJMP on May 25, 2014, 02:05:01 PM
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.
Title: Re: Sound issues after noise mod
Post by: Deregorn on May 25, 2014, 02:38:36 PM
Makes perfect sense, I will do that tomorrow and let you know. Thanks!
Title: Re: Sound issues after noise mod
Post by: MarshallJMP on May 25, 2014, 03:37:08 PM
Ok,again sorry i didn't reply sooner.
Title: Re: Sound issues after noise mod
Post by: Deregorn on May 26, 2014, 01:49:45 PM
No problem ;)
I just tried to measure everything as suggested. It seems that the traces to U9 are all fine (around 0 Ohm). However there are 2 pins on U4 that I cannot really follow, I can't see where they're going so I don't know where to measure. The other ones are fine too.

I think they're pins 5 and 8 (the first and last one on the left side when looking on it from above with the tube board to the left). I attached a picture to show you what I mean. It looks like the traces are going down and then disappear under whatever that big black thing is ???.

Title: Re: Sound issues after noise mod
Post by: rnolan on May 27, 2014, 06:09:35 AM
The black thingy is an op amp
(http://www.alldatasheet.com/view.jsp?Searchword=Tl072%20datasheet)
(https://www.google.com.au/search?q=tl072 datasheet&biw=1639&bih=906&tbm=isch&imgil=9jpAWKIarLu5wM%253A%253Bhttps%253A%252F%252Fencrypted-tbn0.gstatic.com%252Fimages%253Fq%253Dtbn%253AANd9GcS28YPvIIFihcN_L2N-1X6DVdL84xXrBPF1Erhl-YsC1DUq0jSCMQ%253B159%253B96%253BxxxFebmGjOW2BM%253Bhttp%25253A%25252F%25252Fwww.datasheetdir.com%25252FTL072%2525252BOperational-Amplifiers&source=iu&usg=__-F_wPW9dAapzelVvD0MSGuYBd34%3D&sa=X&ei=0IuEU62POs_XkQX-14DoDA&sqi=2&ved=0CEcQ9QEwBA#facrc=_&imgdii=_&imgrc=msUBp_yB0W-3jM%253A%3BUw6NgRm5pQBjNM%3Bhttp%253A%252F%252Fwww.st-andrews.ac.uk%252F~www_pa%252FScots_Guide%252Fdatasheets%252FOpamps%252F072dia.gif%3Bhttp%253A%252F%252Fwww.st-andrews.ac.uk%252F~www_pa%252FScots_Guide%252Fdatasheets%252FOpamps%252F071.html%3B470%3B272).
And indeed pins 5 and 8  :thumb-up:
Title: Re: Sound issues after noise mod
Post by: MarshallJMP on May 27, 2014, 02:45:33 PM
The "black thing" is an optocoupler.It's in this case a light depended pot which regulates the gain of U4.
Now pin 5 isn't important,pin 8 is.Pin 8 is +15V,pin 4 is the -15V,pin 3 is ground and you need to have 40kohm between pin 1 and pin 2.So go the power supply caps and locate the starground (this is where alot of traces come togheter near the big caps).This is the ground.Measure between ground and pin3,should be 0 ohm.Measure between pin 1 and 2,should be 40kohm.
Then turn the mp-1 on and measure between starground and pin4 ,you should read -15V,do the same with pin 8,this should be +15V.
Title: Re: Sound issues after noise mod
Post by: rnolan on May 28, 2014, 07:20:14 AM
I stand corrected My bad, the internet lied to me  :facepalm:
Title: Re: Sound issues after noise mod
Post by: Deregorn on May 28, 2014, 12:31:59 PM
Ok, I just did everything as suggested and this is what I measured:

Pin 3 - Ground: ~ 2 Ohm
Pin 1 - 2: 37.7 kOhm

turned on

Pin 4 - Ground: -15 V
Pin 8 - Ground: +14.7 V

By the way, if I just put the two contacts of my multimeter together it also shows around 2 Ohm, so I guess it's just inaccurate and actually is 0 Ohm.
Title: Re: Sound issues after noise mod
Post by: Deregorn on June 09, 2014, 07:05:33 AM
*bump*
What should I do now?
Title: Re: Sound issues after noise mod
Post by: MarshallJMP on June 09, 2014, 08:58:12 AM
Sorry missed your reply.That all looks good,the only thing that could be wrong is the connection of pin 2 to pin 5 of the optocoupler.(one side has 2 connections,the other one 3,it's the middle of the 3).So if you measure between these it should 0 (or 2 in your case) ohms .
Title: Re: Sound issues after noise mod
Post by: Sparker on June 09, 2014, 10:45:09 AM
Also, I would re-check connections/resistances around U9 with the meter. This is the PI control loop for the Vactrol, which is part of the OD1 gain cell. If you have your meter out, check the voltages at U9 pins 1, 2 and 3 with the MP-1 turned on. Pins 2 and 3 should be around 0V. Pin 1 should be above 0V, but constant, not rising over time.
Title: Re: Sound issues after noise mod
Post by: Deregorn on June 12, 2014, 03:05:44 PM
@Sparker: Checked U9 - Pins 2 and 3 show 0 V, Pin 1 shows around 1.6 V (constant)

@MJMP: I'm still a bit confused about that optocoupler. I attached 2 photos (top and bottom view), maybe you can help me to see things clearer... :-[
From the top I can only see 2 pins at the right side. I marked them yellow and green, because I have no idea which pin is which (are these 1 and 2?).
At the bottom view I also marked those same pins green and yellow. Are the other 3 pins you talked about the ones I circled in red (with the question mark next to them)? And what exactly do I measure? :???:

Sorry for my bad understanding of electronics. ;)
Title: Re: Sound issues after noise mod
Post by: MarshallJMP on June 13, 2014, 05:24:16 AM
These are the pins with the red circle.The middle one should go to pin 2 of the opamp.
Title: Re: Sound issues after noise mod
Post by: Deregorn on June 13, 2014, 10:05:01 AM
Ah, okay. Well in that case it's around 0 Ohm, so it should be fine.

I hope you're not running out of ideas ;D
Title: Re: Sound issues after noise mod
Post by: Sparker on June 13, 2014, 11:02:06 AM
Can you do the same check on U4, i.e. measure voltages on pin 2 & pin 3 = 0V and pin 1 = ? (keep the meter on DC volts), with the guitar lead from the MP-1 unplugged.
Title: Re: Sound issues after noise mod
Post by: Deregorn on June 19, 2014, 10:33:08 AM
I measured all the pins on U4

Pin 1: 0.35 V
Pin 2: 0 V (it actually changed between -0.00 and +0.00 so there might be some really tiny voltage)
Pin 3: 0 V
Pin 4: -15 V
Pin 5: +14.7 V
Pin 6: 0 V
Pin 7: 0 V
Pin 8: 0 V
Title: Re: Sound issues after noise mod
Post by: Sparker on June 19, 2014, 01:44:00 PM
I'm assuming that you might have read pins 5,6,7,8 back to front - i.e. pin5 at +14.75V is actually pin8, pin6 is pin 7, etc. In which case this could be ok, but could you double check this just to be sure. Then might be time to trace a test signal through the front jack to the output of U4 (at pin 1), or if you have a spare vactrol for U5, try changing that. MJMP might have some.
Title: Re: Sound issues after noise mod
Post by: MarshallJMP on June 19, 2014, 02:41:35 PM
Another thing to try is measure the voltage over resistor R103.The voltage should rise if OD1 rises.
R103 is the resistor in the bottom right corner of the opto_top pic.It has brown black red gold color rings.Just measure over the resistor.This way we know that there is a current flow trough the led of the opto.