Mesa Boogie F-30 Hum Problem After Repair

[bobloblaws]

No need for "tuning" as your scope has autoset and automatically displays period time and frequency.
(I would change the settings to 50mVdiv vertical and 5ms/div for a larger display.)
The hum pattern shows single sided (negative going) peaks, typical for power supply ripple.

Now I would scope the +/- 15V supplies with the scope input in AC coupling.

Here's a couple of different resolutions. Also a link to a video in case that is of any value. I'll see what I can do with the +/- 15V.
https://drive.google.com/file/d/1uHz...ew?usp=sharing

[bobloblaws]

Now I would scope the +/- 15V supplies with the scope input in AC coupling.

Like I said, I'm pretty green on the scope. This is what I got when I tried to scope the -15V supply.

https://drive.google.com/file/d/1wWl...usp=drive_link

[Helmholtz]

Like I said, I'm pretty green on the scope. This is what I got when I tried to scope the -15V supply.

https://drive.google.com/file/d/1wWl...usp=drive_link

You should get a steady trace showing the 60Hz ripple.
Do not feed a signal while scoping the power supply and don't forget to connect the probe's ground clip to amp ground.

[bobloblaws]

You should get a steady trace showing the 60Hz ripple.
Do not feed a signal while scoping the power supply and don't forget to connect the probe's ground clip to amp ground.

I can give it another go tomorrow.

Anyway, it sounds like you are convinced my issue is ripple in the +-15V supply. If that is the case, what is left to do other than pull the board out and remove the 1000uF caps and test and possibly replace them?

[Tom Phillips]

...what is left to do other than pull the board out and remove the 1000uF caps and test and possibly replace them?

You can first just bridge another cap across the existing 1000uF caps in the 15V supplies using whatever access points are available from the top of the board.

[bobloblaws]

You should get a steady trace showing the 60Hz ripple.
Do not feed a signal while scoping the power supply and don't forget to connect the probe's ground clip to amp ground.

I could not get the scope to lock onto a waveform (other than something around 90Mhz) when trying somewhere other than the output, even then I still had to prime it first with a 64Hz test signal. Like I said, I'm pretty green.
Anyway if I audio probe the positive rail of the +- 15V supply I'm hearing 120hz. Bridging another 1000uF cap did not reduce the hum at the output.

[Helmholtz]

I could not get the scope to lock onto a waveform (other than something around 90Mhz) when trying somewhere other than the output, even then I still had to prime it first with a 64Hz test signal.

I don't understand this, especially as your scope has "autoset", making it rather foolproof.
Do you use a real scope probe?
Did you connect the ground clip?
Is the scope in good shape?
Do you have the scope's manual?

To me working with a scope is pure fun (admittedly I've used scopes daily for many years).

[bobloblaws]

I don't understand this, especially as your scope has "autoset", making it rather foolproof.

Do you use a real scope probe?

Yes, the one that came with it. Tried it on 1X and 10X.

Did you connect the ground clip?

Yes

Is the scope in good shape?

It's still like new

Do you have the scope's manual?

3 files attached


ds1102e.pdf User's Guide for Oscilloscope.pdf Users+Guide+DS1102E.pdf

[Tom Phillips]

I could not get the scope to lock onto a waveform (other than something around 90Mhz) when trying somewhere other than the output, even then I still had to prime it first with a 64Hz test signal. Like I said, I'm pretty green...

When you are trying to see the hum component, even when it is riding on a noisy signal, it works well to use the Line Trigger function. This works with both 60 Hz and 120 Hz hum as well an other hum harmonics related to the line signal. This is an old school method that will probably work better than the modern Auto function in this particular circumstance.

Reference post #57 in this thread and the attached table from your scope manual.

[x-pro]

Probably the problem is not in the power supply, and not even in the transistor circuit. It is a banal induced background from improper installation of conductors and shielding.

[bobloblaws]

When you are trying to see the hum component, even when it is riding on a noisy signal, it works well to use the Line Trigger function. This works with both 60 Hz and 120 Hz hum as well an other hum harmonics related to the line signal. This is an old school method that will probably work better than the modern Auto function in this particular circumstance.

Reference post #57 in this thread and the attached table from your scope manual.

Thanks, Tom. I think that helped. I also figured out that I need to spin the time and volt knobs to find the wave. I'd been expecting something to just appear automatically, like when pushing the "Auto" button.

This is from the +15V rail: I believe it is what I heard yesterday when I audio probed the same spot ("H" on the drawing). I don't detect anything like that on the -15V rail (I had to turn the audio probe waaay up to hear any hum), which seems counterintuitive since it seemed like the section associated with the negative rail was where the problem manifested itself (bypassing the contour section eliminated the hum, right?).

I took a video as well, also scoping "H" to show that it is not locked on a single frequency, it is jumping around quite a bit.

https://drive.google.com/file/d/1TRc...ew?usp=sharing

[Chuck H]

The small dips on the positive side are spaced at 60Hz.
Not sure what it is but the output hum might be composed of 2 different hum signals.
I would ignore the small dips for now.

Got it. I didn't account for the actual time spacing of the peaks and simply saw that there were doubled peaks within the 60Hz image. Brain fart. Makes sense now as you've explained it. Thank you.

[bobloblaws]

Probably the problem is not in the power supply, and not even in the transistor circuit. It is a banal induced background from improper installation of conductors and shielding.

Do you mean from the factory? Or the repair I did?

[x-pro]

Both of them.
I'm sorry, I don't know your skill level.

[bobloblaws]

Both of them.
I'm sorry, I don't know your skill level.

No offence taken.

Anyway, the only thing I can think to do at this point is pull the board out again and reflow solder joints, possibly the whole board if revisiting the +-15V supply doesn't solve it.

I'm still perplexed why bypassing the -15V contour section cleared it up but the +15V rail is where I'm detecting noise when probing/scoping.