Mesa V-Twin (pedal format) with strange oscillation on headphone output and "clean" setting

[waspclothes]

Hey everyone,

So today I've been looking at a Mesa V-Twin preamp thing. I posted this in the amplifier section, because it's pretty much an amp pre-amp in a small pedal format box.

Schematic is here: https://schematicheaven.net/boogieamps/boogie_vtwin.pdf

There is some pretty hardcore noise coming out from the Headphone/Mixer jack, but only when the channel setting is set to "Clean". I'm confused because this noise isn't present on the Power Amp Output that the headphone output is derived from. I'm further confused at the fact that it's only on the Clean channel setting, even though the headphone amp section is after any of the channel switching stuff. It's like the fact that the clean channel is selected is what is causing the noise, not the channel itself.

The noise is a high frequency oscillation at like 400khz that is present even without the tubes. I'm not quite sure the schematic is an exact match though, I tried tracing the headphone output section and there is no TL071, etc.

[Helmholtz]

The noise is a high frequency oscillation at like 400khz that is present even without the tubes.

400kHz must be a typo.

Do you use the original 12VAC power supply?

[Mick Bailey]

My thought is to check the +12v supply to the LM386 headphone amp. A defective LED can produce noise and perhaps when the clean channel is selected there's noise on the +12v rail. The LEDs can be unplugged to eliminate this possibility. The LM386 is also reliant on the positive supply being bypassed with a capacitor to give acceptable PSU noise rejection - check that the cap C53 is OK by tacking another in parallel to see if this cuts the noise.

[waspclothes]

400kHz must be a typo.

Do you use the original 12VAC power supply?

No typo, the output of the LM431 is oscillating at very high frequency when on "Clean". Yes I'm using the original 12VAC supply. When I use my scope to probe in the area around the LM431 mixer output section (like on the ferrite bead, etc), it will start picking up radio that's audible in the output of the LM431 section. I've tried just shotgun resoldering this whole area to no avail but other than that I'm clueless.

[Helmholtz]

No typo, the output of the LM431 is oscillating at very high frequency when on "Clean". Yes I'm using the original 12VAC supply. When I use my scope to probe in the area around the LM431 mixer output section (like on the ferrite bead, etc), it will start picking up radio that's audible in the output of the LM431 section. I've tried just shotgun resoldering this whole area to no avail but other than that I'm clueless.

Well no one can hear 400kHz. So audible noise can't be 400kHz.

How did you detect/measure 400kHz?

I can't find an LM431 in your schematic. LM 431 is not an opamp but a zener shunt regulator.

[waspclothes]

Well no one can hear 400kHz. So audible noise can't be 400kHz.

How did you detect/measure 400kHz?

I can't find an LM431 in your schematic. LM 431 is not an opamp but a zener shunt regulator.

My mistake - I use LM431 in several other projects. I meant the LM386 amplifier IC!

The 400khz oscillation was measured on my scope. The output noise basically just sounds like white noise.

I kept noticing differences between the actual pedal and the schematic, which made it hard to further isolate this problem.. but I was also very annoyed by the construction of this pedal and how difficult it is to assemble/disassemble (mesa stuff seems to always annoy me in this regard). So I ended up just giving it back to my friend and told him to not use the LM386 output. My last course of action would have been to shotgun replace the LM386 and all associated components, but I hate doing that and the pedal is totally useable when the output is taken from the "Power Amp" Output section.

[John G]

Hi, Is there oscillation at the power amp jack?....if yes then that driving circuit may be at fault.
If no............. then the Headphone/Mixing desk area is faulty, including PSU.
Question, If a mono jack is inserted in the HP/MIX output jack and switch set to the mix position, is the op amp not shorted to ground............... Maybe an isolating resistor should be added ?? or socket wired for series tip/ring connection when in HP switch position.
John

[John G]

Hi, I ment 'Headphone switch position' looks to short the op amp output.................sorry
John

[Ivan Rimsky]

Hi there!
I've been having the same issue with my vtwin, however it also happens when the blues/lead channel is engaged. It''s odd as the noise sometimes goes away for like 5 minutes and comes back again. Couldn't it simply be a faulty jack?