Mesa Boogie F-30 Hum Problem After Repair

[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.

Ahhh, it took me a couple of days, but I think my little brain just figured out what you were driving at. The 120Hz component is the typical AC power supply ripple and the 60 Hz component is hum that has been fed into the OT via the signal path rather than coming from the PT. They just happen to both be detected at the OT center tap.

[bobloblaws]

This is from the anode of the diode feeding the bias supplies. I had to set the probe on 10X for the scope to show this. does this look normal/expected?

[Tom Phillips]

I'd say that the waveform looks correct for a half wave rectifier without a filter cap connected at the node where that waveform was monitored.

[bobloblaws]

I'd say that the waveform looks correct for a half wave rectifier without a filter cap connected at the node where that waveform was monitored.

Thank you, Tom. In that case I have a couple of ideas as to why the bias supply seems to be the source of hum at V4. Helmholtz suggested that maybe the new cap I put in is defective. I figure an easy thing to try is to clip in another cap in parallel and see if there is any reduction in hum. The other idea theory I had is that the hum is coming from the PCB itself somewhere, these amps have that reputation and there is evidence of high heat/carbon as shown in photos I posted in #13. So I'm thinking I may need to scratch out one of the trace connections and run a jumper. The question is, where is the most logical place to try this (assuming I end of going that route)?

[Chuck H]

I'd say that the waveform looks correct for a half wave rectifier without a filter cap connected at the node where that waveform was monitored.

This ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If you're looking for hum you need to test where the 1000 uf cap meets the 120k resistor. There would have to be a lot because I remember Helmholtz mentioned that minor hum from that supply should cancel with common mode operation in the PI and power tubes. I remember because I wasn't sure the same common mode cancellation would happen at the PI so I learned something with his post.

[Chuck H]

Thank you, Tom. In that case I have a couple of ideas as to why the bias supply seems to be the source of hum at V4. Helmholtz suggested that maybe the new cap I put in is defective. I figure an easy thing to try is to clip in another cap in parallel and see if there is any reduction in hum. The other idea theory I had is that the hum is coming from the PCB itself somewhere, these amps have that reputation and there is evidence of high heat/carbon as shown in photos I posted in #13. So I'm thinking I may need to scratch out one of the trace connections and run a jumper. The question is, where is the most logical place to try this (assuming I end of going that route)?

I don't think you need to start scraping the board and adding jumpers. Remember there was a time in this process when the hum WAS reduced. That wouldn't have happened if it's cause were a constant in this process. As the carred board has been.

[Tom Phillips]

...In that case I have a couple of ideas as to why the bias supply seems to be the source of hum at V4...

In order to see the true health of the bias supply you should look at the signal at the bias filter cap on the other side of the 120k resistor and at point F. Reference page 4 of the schematic that you attached to post #1.

[Chuck H]

Please test for hum again at V3 pin 7. Keep in mind it will likely be a very low signal. This is the effects return so even very small signals here are amplified to feed the PI grid.

[bobloblaws]

I don't think you need to start scraping the board and adding jumpers. Remember there was a time in this process when the hum WAS reduced. That wouldn't have happened if it's cause were a constant in this process. As the carred board has been.

Point taken. I guess I'm grasping at straws.

[Tom Phillips]

I see that Chuck was posted while I was writing.
I'll add the point that, if the bias supply was the source of the hum, then the hum would not have gone away when you shorted the grid of the preamp tube mentioned in an earlier post. This is because bias supply caused hum is injected at the power tubes.

[Chuck H]

Point taken. I guess I'm grasping at straws.

I would be by now. Pacing around with a serious looking grimmace on my face. At which point my wife would say "Having any fun?" What can I say, she knows me.

[Chuck H]

I see that Chuck was posted while I was writing.
I'll add the point that, if the bias supply was the source of the hum, then the hum would not have gone away when you shorted the grid of the preamp tube mentioned in an earlier post. This is because bias supply caused hum is injected at the power tubes.

In this amp the same supply is used for the PI cathode. Which is the current suspect tube.

[Tom Phillips]

In this amp the same supply is used for the PI cathode. Which is the current suspect tube.

I totally missed that when I looked for point "E" marked on the schematic. That's a significant clue. I sure miss working with a schematic that shows everything on one large page.

[bobloblaws]

Please test for hum again at V3 pin 7. Keep in mind it will likely be a very low signal. This is the effects return so even very small signals here are amplified to feed the PI grid.

Here is V3 pin 7. I hear hum here with the audio probe but I had to turn the monitor up relatively high compared to V4 pin 7, low level as you suggested.

Note that the hum is still at V4 if I remove V3 or lift either of the caps between V3B and V4B.

[Helmholtz]

Here is V3 pin 7.

It is interesting that this signal is a perfect sine and doesn't look like 60Hz supply ripple.
I wouldn't exclude a conductive board issue.