Sound issues after noise mod

[Deregorn]

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 ???.

[rnolan]

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:

[MarshallJMP]

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.

[rnolan]

I stand corrected My bad, the internet lied to me  :facepalm:

[Deregorn]

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.

[Deregorn]

*bump*
What should I do now?

[MarshallJMP]

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 .

[Sparker]

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.

[Deregorn]

@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. ;)

[MarshallJMP]

These are the pins with the red circle.The middle one should go to pin 2 of the opamp.

[Deregorn]

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

[Sparker]

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.

[Deregorn]

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

[Sparker]

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.

[MarshallJMP]

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.