Mesa Boogie F-30 Hum Problem After Repair

[Tom Phillips]

Got it.
1. Have you substituted preamp tubes in the troubled area?
2. Have you measured the heater balance at the preamp tubes in the troubled area?

Apologies if this is repetition. Just let me know if so.

Measure the heater balance at the top of the socket pin sleeve with the tube pulled. That insurers that the solder joints are in the mix.

Grasping at straws here!

[Helmholtz]

The B+ ripple shows a significant 60Hz component, which is not normal.
Might be due to asymmetrical winding voltages wrt CT, a bad diode or simply a bad solder joint.

[bobloblaws]

Got it.
1. Have you substituted preamp tubes in the troubled area?
2. Have you measured the heater balance at the preamp tubes in the troubled area?

Apologies if this is repetition. Just let me know if so.

Measure the heater balance at the top of the socket pin sleeve with the tube pulled. That insurers that the solder joints are in the mix.

Grasping at straws here!

Yes, have tried other tubes.

To check heater balance, am I comparing pins 4 and 5 measured to ground? I measured AC voltage comparing pins 4 and 5 wrt to ground as well as pin 9. In both case they look pretty much identical, no more than a few millivolts difference.

[bobloblaws]

The B+ ripple shows a significant 60Hz component, which is not normal.
Might be due to asymmetrical winding voltages wrt CT, a bad diode or simply a bad solder joint.

I'll look into the diodes and solder joints ASAP.

[Tom Phillips]

...To check heater balance, am I comparing pins 4 and 5 measured to ground?

The F-30 heater supply is a little tricky because of the way they implemented the AC/DC combination.
The attached schematic snipit shows the nominal voltages that you will measure at each V1 thru V6 heater pin with respect to ground.

[bobloblaws]

The F-30 heater supply is a little tricky because of the way they implemented the AC/DC combination.
The attached schematic snipit shows the nominal voltages that you will measure at each V1 thru V6 heater pin with respect to ground.

Ok, so in the case of V4 I'm comparing pins 4/5 to pin 9 wrt to ground. I measure 3.27 VAC in both cases.

[bobloblaws]

The B+ ripple shows a significant 60Hz component, which is not normal.
Might be due to asymmetrical winding voltages wrt CT, a bad diode or simply a bad solder joint.

So I checked the diodes in circuit and reflowed the solder joints, still have the hum. Note that at some point since I've been working on this there was far less hum at the output so I'm guessing there is something I've done to cause it while I was chasing down all these other noise issues. At this point I don't mind changing out the rectifier diodes, I have lots in stock, if there is any possibility the in circuit check I did is not sufficient.

I did check the PT secondary, I measure 204 ohms on one side and 219 ohms on the other side.

[Tom Phillips]

Ok, so in the case of V4 I'm comparing pins 4/5 to pin 9 wrt to ground. I measure 3.27 VAC in both cases.

No issue there.

I commend you for sticking to this long troubleshooting process but I am out of suggestions for the moment.

A significant clue is that the hum showed up after you did some work on the F-30. It's already been suggested that something happened with the grounding scheme and the fact that the bad hum seems to be 60Hz seem to be the other clues that stand out.

[Helmholtz]

I did check the PT secondary, I measure 204 ohms on one side and 219 ohms on the other side.

What are the voltages?

[Tom Phillips]

...At this point I don't mind changing out the rectifier diodes, I have lots in stock, if there is any possibility the in circuit check I did is not sufficient..

You can use the diode check on your DVM while the diodes are in circuit. Easy test but, if the diode polarity is correct, I think it's unlikely that they would be at fault and causing the problem you are working on.

...I did check the PT secondary, I measure 204 ohms on one side and 219 ohms on the other side.

That seems normal.

[bobloblaws]

No issue there.

I commend you for sticking to this long troubleshooting process but I am out of suggestions for the moment.

A significant clue is that the hum showed up after you did some work on the F-30. It's already been suggested that something happened with the grounding scheme and the fact that the bad hum seems to be 60Hz seem to be the other clues that stand out.

Thanks a lot for your help.

I should also mention that when I first posted I had a smaller amount of hum at the output. Originally I was dealing with hum that turned out to be coming from the -15V supply which no longer seems to be an issue. So what ever problems I have now may be self inflicted or just due to happenstance.

[bobloblaws]

What are the voltages?

336VAC and 340VAC

[bobloblaws]

The B+ ripple shows a significant 60Hz component, which is not normal.
Might be due to asymmetrical winding voltages wrt CT, a bad diode or simply a bad solder joint.

Any other possibilities? If the diodes are working correctly and the PT secondary is OK, there should be no 60Hz component at that point, yes?

In any case, I would expect any ripple at the level shown in that trace to be pretty much inaudible anyway, certainly nothing like what I am actually hearing in the output.

[Chuck H]

Any other possibilities? If the diodes are working correctly and the PT secondary is OK, there should be no 60Hz component at that point, yes?

In any case, I would expect any ripple at the level shown in that trace to be pretty much inaudible anyway, certainly nothing like what I am actually hearing in the output.

I think what you're missing is that it IS being amplified. Which means it's being fed in as a signal rather than cancelled in common mode. So the 60Hz component must be before the power amp.

[bobloblaws]

I think what you're missing is that it IS being amplified. Which means it's being fed in as a signal rather than cancelled in common mode. So the 60Hz component must be before the power amp.

Yes, I'm still missing something. My reasoning was that with various amps I'll often measure 2-4 VAC ripple at the reservoir cap/B+, yet it will be fairly inconsequential in terms of signal to noise ratio at the speaker output. What I'm seeing in that 1st photo is ripple of less than 1VAC. I don't understand how this scenario is different.