DV Mark Little 40 L34 - now you've ruined everything

[xtian]

I bought another "project" amp on a whim. This is some modern, whiz bang engineering. 40 watts, only 17.5 pounds. Seller's warning was, "This amp makes intermittent popping and cracking noises." I played it for an hour with no problems exhibited. This is a little concerning, because I won't feel confident reselling it until I find an issue. This amp is discontinued, but DV Mark won't give me a schematic. I've reached out to my friends at Guitar Center, because they're a distributor.

The power scaling is a cut above the London Power stuff--EQ (harmonic content) seems to stay very steady as wattage is reduced. Maybe they even compensate for Fletcher Munson--it sounds very, very good scaled to 0.5 watts.

But...boy howdy this is a LOT of potential points of failure. Looks like a SMPS right in the middle of the main board, timed delay for the B+ on startup, µ-processor for the biasing and power scaling stuff, etc... And, two different Torx bits required to remove the cage = F.U. DV Mark!

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[TUBEDUDE]

The worst! Everything soldered to the board, pots, jacks, and crazy hot tubes.

[xtian]

The bug showed up. Bacon frying with occasional pops. Noise is post MV, and strangely, turning the Presence all the way UP causes the noise to become DARKER, like rolling the treble off. Weird. Not an FX loop issue. Same bad noises with different set of tubes. Noise is not sensitive to knocks or chopsticking.

Can anyone get a schematic from DV Mark?

[ViperDoc]

Is the Presence pot wired backwards or has reverse taper?...

My voltage scaler control on my Carr Telstar is scratchy every time I turn it. Could there be an issue with the power scaler?

[xtian]

Is the Presence pot wired backwards or has reverse taper?...

No, the Presence control effects the guitar tone exactly as you'd expect...but it affects the noise in the opposite!

My voltage scaler control on my Carr Telstar is scratchy every time I turn it. Could there be an issue with the power scaler?

Def possible. Moving the Power Control pot does not sound "scratchy" like DC on the pot, but it does perturb the noises, in a very random way.

New notes: Pulling the PI tube silences the noise. Grounding the PI input does NOT silence the noise. I was able to replace the PI's plate resistors with 1W, but no change in noise.

[GAStan]

Would a cap with the dielectric breaking down (internal arcing) cause the sounds you describe?

[maxkracht]

Is r36 supposed to be green?

[xtian]

Is r36 supposed to be green?

Yes. I replaced it last night anyway (one of the PI's plate resistors).

[maxkracht]

maxkracht wrote: ↑Wed Feb 21, 2024 9:16 am
Is r36 supposed to be green?
Yes. I replaced it last night anyway (one of the PI's plate resistors).

The lighting made it look like it could have originally been tan.

Coupling caps seem like a reasonable explanation. If it weren't a complete pain, I would suggest reflowing the socket. I've had a handful of strange crackly things caused by board mounted tube sockets and rohs solder. Isn't always visible or chopstickable like you would think. I wish more amps had a service panel to reach under the board...

[xtian]

I removed the main PCB, disconnected the microcontroller daughterboard, resoldered many points, painted all connections with DeOxIt, reassembled. No change in noise!

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[xtian]

I replaced the pair of post-PI 0.047u coupling caps. Still getting the same bacon frying and pops.

[xtian]

Photos of main PCB, just FYI.

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[Reeltarded]

Have you tried turing it off?

The first full images.. the last image.. looks like Mark Fowler collecting things from around the shop to make his before breakfast amp.