MN3005 Dirtbag: huge earpercing hiss and huge distortion

[djaaz]

Guys,

My dirtbag is still on my test rig. While biasing and playing with the gain trimmer, i have so much hiss coming out of it that i don't think boxing it is not a good idea.

Any idea on how to troubleshoot on this one? I haven't probed it yet. Will do that as a next step.
In the meanwhile, if any of you have a brilliant idea...

I spent a lot of time trying to bias as well as possible. In the end, i have an awful delay:

1. The pedal distort the dry part.
2. The repeats distorts and are very dirty after the second repeat.
3. The delay is short (i measured 480 ms but somehow it sounds shorter, don't ask me). It's definitely way shorter than my memory toy.

Any idea what i should look at first?

[djaaz]

Would a video or sound sample help?

[madbean]

At which point does the hiss get introduced? Have you tried checking it out with an audio probe?

[djaaz]

It's not about just the hiss anymore.

I built some kind of ugly fuzz / delay pedal. I will send some sound sample, that will be worth a thousand words.
And i will use the audio probe.

As the repeats during the bias did not sound as bad as the pedal output, i'm guessing the distortion comes after.

[djaaz]

I spent an hour toying with the audio probe.
I have hiss very early in the circuit going out of IC1 & increasing.
The mod pot varies the frequencies of the hiss from pot full counter clockwise: "grave", full clock "treble".

Now the distortion:

Clean almost all along the circuit and then out of the NE570 everything distorts.

Faulty chip or should i check & post the voltage?

[the3secondrule]

Check the components around the compander...

[djaaz]

So some more probing & some more finding. But no changes yet:
VC= 15,1 V regulated with a road rage.

Blend pot, full clock.

Level pot: i have distortion on the delay as soon as the level exceed the first third of its course.
IC8A: I have some distortion at Pin1 (not the same), replaced it with TL082 (less noise & no more distortion)
NE570: Signal enters clean at pins 5, 6, 14 & 15. Distortion on pin 10&11.

Here's the voltage for NE570:

1: 1              16: 1,7
2: 1,8           15: 2,45
3: 1,8           14: 2,43
4: 0              13: 15,1
5: 1,8           12: 1,43
6: 1,8           11: 6,6
7: 9,94         10: 8,5
8: 1,83         9:   1,8


Kind of lost here. Looks like the thing distorts from a lot of different places. With the level pot at a low level, i can pinpoint the main source of distortion on the ne570, but i feel like it's not the only issue i have.

Any help is welcome!   

[djaaz]

Two videos:
First one if in & out of the circuit & the output of the second mn3005.

Second is delay in & out of ne570

[djaaz]

Anyone with a working circuit willing to share its voltage?

[djaaz]

No one?
C27 is a bit more than 3,9n (around 4,1). i'll change that if i can find another one in my cap stash.

[gtr2]

The ear piercing hiss is  the sound you get when you audio probe BBD's.  The distorted dry signal is certainly a problem.

C27 at 3n9 is fine, don't change it.

What are pickups are you running in to the delay to test it?

Josh

[djaaz]

Thanks for your answer! I was feeling a bit lonely.

The hiss is present at output as well. Of course, not as loud but certainly enough to be a showstopper.

I use humbuckers. The signal when the blend pot is at zero does not distorts easily. Clean does not break until the level pot is way up the clock.

I changed C27 to a value closer to 3,9n without change as expected.

I played a bit with R42 (11k). R42 connects PIN 12 to the ground on the schematic.
I removed R42 & moved a bit R62. I thought there could be a bridge there cause the layout is tight.

I tried the circuit without R42 and i had just the delay without distortion. I socketed the side of 42 that is connected to the compander & hooked a 100k pot from there to ground.

I had very different behavior. I could not find a suitable value but i could get rid of some distortion. At least on the delay. Nothing pretty though, nothing usable.

Running out of idea..

[djaaz]

Anyone have any idea on where to go from there?

I'm thinking of trying another ne570 and from there socket all components around the compander and try different values, but i can't help thinking there must something more obvious to check first.

[madbean]

It's early and I've got to get the family off for the day but I will take a look at this a little later this morning and try to offer some help.

[djaaz]

Thanks Brian, I ordered a new ne570 just in case but i'm pretty sure there's more problem there than the compander.

I took a look at the mods for the V3205 versions. I was wondering if some of these changes could help.
But then again, why my build is acting so weird when there are so many working ones out there.

I'm pretty sure, there's something messed up with my build, i just can't find it by myself.

What about the chip voltage?