Markbass combo ii oscillation

[Bigdrums]

Repaired a friends bass amp that had some bad output transistors. Put it back together and seemed to work OK, little low on power and drawing more line current than I would expect for a SMPS unit, about .8 amps at idle. Hooked up the scope and saw oscillation of 80mV p-p. and peaks every 4uS or so. Not sure what frequency that works out to.. Don't know where the oscillation is coming from or how to suppress it. Any suggestions?

[x-pro]

The frequency of the power supply, F=1/4uS =250kHz
Well, yes, insufficient filtering at the source output. But you will not hear that.
If you want sufficient filtering, this is a different price category of the device.

[Bigdrums]

x-pro
So this is a through away unit ? Basically too cheep to fix?

[x-pro]

I didn't understand the question.

[The Dude]

......Any suggestions?

Yes. Please link or upload a schematic.

[Helmholtz]

I didn't understand the question.

I had the same problem, but think he means: "So is this throw-away unit? Basically to cheap to fix?"

[g1]

My thoughts were that x-pro was referring to the 250Khz on the output being filtered out. Which would be done in a more expensive unit, but is probably not the source of this fault.
In other words, the 'fix' should be found elsewhere than filtering out HF noise that is not audible (and only 80mV so not stealing power).

[Helmholtz]

My thoughts were that x-pro was referring to the 250Khz on the output being filtered out. Which would be done in a more expensive unit, but is probably not the source of this fault.
In other words, the 'fix' should be found elsewhere than filtering out HF noise that is not audible (and only 80mV so not stealing power).

Let's see.

When you need to translate, wrong spelling can be a severe barrier.

[Bigdrums]

x-pro:

"If you want sufficient filtering, this is a different price category of the device."

Not sure what you mean with the above comment

[Bigdrums]

parsek_mark_bass_little_markii_mark2_combo_head.pdf

Here's the schematic

[Bigdrums]

My thoughts were that x-pro was referring to the 250Khz on the output being filtered out. Which would be done in a more expensive unit, but is probably not the source of this fault.
In other words, the 'fix' should be found elsewhere than filtering out HF noise that is not audible (and only 80mV so not stealing power).

I'm sure it is stealing power, it's drawing almost an 1 amp at idle. Never saw a SMPS pull that amount of current with no signal input

[The Dude]

I THINK what x-pro means (speculating a bit) is that this unit doesn't have a well filtered supply or output section and that you might expect some slight power supply noise at the speaker out, and that it isn't a big deal because it's well out of the audio range anyway. Again, that is a bit of speculation from me.

That said, did you bias the amp after you replaced outputs? Your high idle current could be from an overbiased amp. I don't think it's the power supply causing your problem. The schematic doesn't give any amp current specs or bias current specs (that I could find), so we don't even know for sure if it's idling high, although that does seem a bit high to me. I found some bias info here:

https://www.diyaudio.com/community/t...repair.352890/

It's for a different amp, but from the description in post #2, it seems to be the same output section, so I'd check your bias current and see how it stacks up against those numbers.

[Bigdrums]

Dude: great observation and suggestion to remedy my problem. I was able to lower the bias current and watched the variac amp meter drop to .3 amps and the oscillation on the scope diminish to nearly nothing. Can't thank you enough!