MP-1 possible issues after noise mod (OD1 LED clipping at low gain)

[Loktite]

Best way to learn something is to mess up and ask questions, right?

If that something is an ADA MP-1, I cannot fully agree. They don't make those any more! 

Careful with those traces. When you are pulling out parts, the top pads and traces can be torn off. Apply heat from the component side and tug gently. There are special soldering iron tips for heating all the pins of an IC at the same time, from the component side.

Usually removed components are discarded, so you don't have to worry about damaging them, only the board.

I am being a bit cavalier with that statement, I apologize if I've caused concern for the well-being of my preamp. I am a persistent person and follow repairs through to the end even if it takes a lot of troubleshooting.

[MarshallJMP]

Did you got my email?

[Loktite]

An update on this short journey (this is what I e-mailed Philippe most recently):

I have all the components soldered in again. I checked all the traces on U4 before, during and after. I've only had to jump the top trace from pin 8 of U4 and pin 3 of the vactrol. I have very little sound, OD1 has no change in sound in all voicings including SS. My suspicion is the vactrol.
R103 now measures at 13.4V at anything above OD1 = 0.0.

Here is a quote from Philippe in another post that I've followed:
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.

I've made those measurements. They are all in spec, except for pin 8 to starground. That measures at 0V. Being that this is connected to the vactrol, would I be safe to assume that we have found our culprit?

[Loktite]

Did you got my email?

What timing, I did and I just responded to you with the above post!

[Iperfungus]

Did you got my email?

Welcome back!   

[MarshallJMP]

Pin 8 of U4 is the +15V connection and is not connected to the vactrol at all? Also there is no pin 3 on the vactrol either?

So you measure 13.4V over R103??

[Loktite]

Pin 8 of U4 is the +15V connection and is not connected to the vactrol at all? Also there is no pin 3 on the vactrol either?

So you measure 13.4V over R103??

Maybe I was not clear with me response. The trace from pin 8 of U4 to pin 3 of the vactrol was lifted, so I had to jump it with some wire. There is now a solid connection between the two. However, measuring from pin 8 of U4 to the star ground gives me 0V.

And yes, I measure 13.4V across R103.

Let me know if there is anything I should double check, I may have made a mistake measuring. I ordered a vactrol from small bear, I'm hoping the issue is just the vactrol. OD2 works as normal. OD1 does nothing right now, and acts as if it is set to a very low setting.

[MarshallJMP]

Are you sure you have a correct understanding of the pin layout of the NE5532? I think you have the pins mixed up. Only  pin 2 of U4 goes to pin 5 of the vactrol, that's it.

[Loktite]

Are you sure you have a correct understanding of the pin layout of the NE5532? I think you have the pins mixed up. Only  pin 2 of U4 goes to pin 5 of the vactrol, that's it.

I can guarantee you based on the diagram you are showing to me that I have the pin layout WRONG. This is my current understanding based off the top trace picture you sent me. But it appears I'm still not understanding something correct. Pin 1 of U4 has a dot near it correct? Do I even have it located correctly? It's still not matching up with your description.

[Loktite]

This is how I have the components in currently.

[MarshallJMP]

No

pin 2 of U4 connects to pin 5 of the vactrol
pin 5 of U4 connects to pin 4 of the vactrol

Maybe this will help

[Loktite]

No

pin 2 of U4 connects to pin 5 of the vactrol
pin 5 of U4 connects to pin 4 of the vactrol

Maybe this will help

I can confirm that I have continuity between those points according the the diagram. Why are there 6 pins listed for the vactrol when there are only 5 pins physically there? Is it correct to say that pin 3 of U4 connects to pin 2 of the vactrol? This is also confusing to me because the data sheet for the vactrol I ordered shows this:

https://eadn-wc04-3257698.nxedge.io/audio/wp-content/uploads/sites/2/2021/11/Xvive-5C4-2-Data-Sheet.pdf

[Kazinator]

You shouldn't be needing to follow traces to do most part replacement mods.

(An exception might be jacks, since the schematics are super confusing, not to mention shared with another product (MQ-1), and replacement parts are not exact.)

There is a diagram which gives the locations of the parts: every resistor, diode, capacitor, IC, and so on.

Look at that first, then follow traces if you feel you need to double check something.

R103 having 13.4V across it at all times, except when OD1 is off looks as if the feedback diode D16 of U9B is an open circuit. Thus the amp acts as a comparator: on, off. I'm actually surprised a bit by the voltage because you'd think that the LED inside the Vactrol would drop a bit of voltage. And the NE5532 (which I think you are using) cannot go much farther than 13V on 15V power. What is the voltage at the output pin of that U9B and how does that vary with the OD1? I.e. between ground and that point, not across the resistor.

[Kazinator]

Am I misunderstanding something or is the LED inside the vactrol not drawn backwards in the schematic?

Based on my understanding of the log amp with the diode in the feedback, the output should be negative.  But that makes the schematic's version of the LED reverse-biased, which can't be.

[Loktite]

You shouldn't be needing to follow traces to do most part replacement mods.

(An exception might be jacks, since the schematics are super confusing, not to mention shared with another product (MQ-1), and replacement parts are not exact.)

There is a diagram which gives the locations of the parts: every resistor, diode, capacitor, IC, and so on.

Look at that first, then follow traces if you feel you need to double check something.

R103 having 13.4V across it at all times, except when OD1 is off looks as if the feedback diode D16 of U9B is an open circuit. Thus the amp acts as a comparator: on, off. I'm actually surprised a bit by the voltage because you'd think that the LED inside the Vactrol would drop a bit of voltage. And the NE5532 (which I think you are using) cannot go much farther than 13V on 15V power. What is the voltage at the output pin of that U9B and how does that vary with the OD1? I.e. between ground and that point, not across the resistor.

I'll try to check that tomorrow and get back to you on that information. I was only following the traces in case I messed one up (which did turn out to be the case). D16 would be the diode below U9 that I replaced with a 1N4004, correct? The leg on the striped side of the diode is the output, correct?