No chorus and distorted SS voicing after noise mod

[Moulashaker]

Hi all,

I've owned a version 1 mp1 for a couple of years now and decided to do the noise mod (3 caps and 2 op amps; U4 and U9).

I bought some OPA2604 op-amps after reading a few discussions on here but it appears they are counterfeit as the instrument input produced dreadful humming until I put the original op amp back in. (The line input on the back seems to have quite a bit less gain than the instrument input. I run it through a Helix Rack so have made sure it outputs the correct line/instrument level to the mp1 depending on which input I was using).

Since then I bought a few TL072s (for U4) and some LF412s (for U9) and put them in this afternoon and they sounded fine until I went to my chorused SS clean sound. It now appears that the chorus effect no longer functions at all (I run in stereo through a Helix Rack but also tried the headphone socket with no change) and the glorious cleans have now got a bit of gain on top (think of an edge of breakup tone).

Im now stumped as to what to look for when trouble-shooting the issue. I've swapped the op amps around and put the old ones back in just to see if I get a difference but no joy so far.

Hopefully someone can give me some pointers to try and solve the problem.

Thanks in advance,

Andy

[MarshallJMP]

Did you turn any trimpots?

[Moulashaker]

Thanks for the reply.

As far as I am aware I have only adjusted the two furthest left trimpots which I believe set the overdrive but perhaps I have accidently turned the one to the right of the trimpot above U4? They aren't too far away from each other so it could easily have happened.

[MarshallJMP]

Can you post a pic of all the trimpots settings?

[Moulashaker]

Here's how it looks at present.

[Kazinator]

Let's focus on the chorus thing for a second.

What happens if you enable or disable chorus in tube programs?

It seems odd that your mods would have an effect on the chorus switch because the components related to that are far away.

How are you getting output out of the preamp? The chorus mix is only present in the A and B outputs and the headphone jack, not the effects send, which is dry.

[Moulashaker]

Thanks for the reply.

I'm running out of the A and B outputs into the stereo return of my line 6 Helix Rack. I've made an incredibly simple stereo patch in the Helix to make sure nothing is summing to mono but it's the same with headphones straight from the back of the mp-1.

I'll test whether it works on the tube side of things when I get in from work shortly and let you guys know. 👍🏼

[Moulashaker]

No luck with any Chorus in any of the voicings, unfortunately.

I've now just got the ADA through a stereo line input on a mixer to take everything else out of the equation.

I'm more than happy to use a multimeter to do tests but haven't really got a clue where to start! I've had a look at the schematics which involve the chorus circuit but can't really figure out where this actually is inside the preamp.

[Moulashaker]

I also notice that one of the new caps, C131 (the horizontal one by itself), is shorted (only when in circuit as I lifted a leg to check).

[MarshallJMP]

The bias pot for the chorus looks correctly set. How did you determine that C131 is shorted? If it was the mp-1 wouldn't start up at all.

Now I agree with Kazinator that the parts you changed have nothing to do with the chorus.

Take a multimeter and see if the voltage is going up and down between around 2 and 10-11V on pin 9 of U24 (CD4046)
Set chorus depth to 100 and rate at 0.1.

[Moulashaker]

C131 has continuity with ground on both legs. I'll check all of this after work tonight.

The transformer has that electrical, almost burning smell as well and seems very warm. This all may be normal as the preamp was up on top of my half stack which is raised on a small table so I might well not have noticed this before.

Many thanks,

Andy

[MarshallJMP]

What kind of resistance do you measure? The negative side should be around 0 ohms and the other side is?

What's the voltage over C131?

[Moulashaker]

8.06V across C131 (3300uf).

O ohms on negative side and off the scale of my multimeter on the positive side. This is measuring from ground.

I've attached a photo to where I believe is pin 9 on U24. It is a stable 11.69V (doesn't change at all) with Depth 100 and Rate 0.1.

No change if I make any changes to either of the chorus controls in Edit mode.

[Moulashaker]

Strangely, I just had a few seconds of crackly buzzy noise through my monitors so I switched the mp1 off.

Turned it back on and the voltage is now fluctuating between 2V and 11V and I can hear a tiny bit of the chorus effect but it is barely noticeable.

The worst kind of troubleshooting!!

[Moulashaker]

Pushing the rate to 10 and pin 9 hovers around 5.47V +/- 0.01V