M-100 loud hum, popping etc.

[tubmyk2]

I just got finished hooking up a Leslie to my M-100 the other night and all was working fine, until instantly it started popping and humming very loud. I disconnected the Leslie from the 26-1just to make sure it wasn't that. It is definitely internal to the organ, but it transfers to the Leslie when connected. The popping more or less stopped after I pulled and reseated the tubes. But I feel like the loud hum has some vibrato to it, so I started playing with the vibrato switches and i can hear it change tone slightly. I pulled one of the vibrato tubes out of the amp and fired it up and the hum went away completely but there was a significant drop in volume. I also had at one point noticed stepping on the expression pedal made the hum go away for like half a second so I then thought maybe it was hitting something...

The amp is original and as far as I know it's never been modified other than when I put a new power cord on it 9 years ago (which I didn't ground so I have to do it again, but this hum issue just cropped up).

Suggestions? Could it be a faulty or dying tube? Should I just pull the amp and start doing a re-cap since it's way overdue anyway? I just hate replacing stuff that isn't needed when I already have projects piled up. The hum is very loud and now is consistent. I don't think the Leslie is what affected it, it may have been just moving the organ around brought it on...

[muckelroy]

You shouldn't be surprised that an amp of this age has problems if it has its original electrolytic caps. Those are worth replacing, even if not the root cause of this problem.

Look at the shielded cables that run to and from the vibrato scanner, as well as the other places in the organ. Make sure none of those braided shielded cables are making contact with the top of the vibrato line box, as that can cause an interruption of vibrato signal, and all kinds of mayhem.

What do you mean by "the vibrato tube"? V1 (6AU6) or 1/2 of V3 (12AU7)? You've got 2 6AU6 tubes, so if it is a bad 6AU6, put V2 in V1's place and listen. 12AU7s are super inexpensive and common.

Open the swell pedal housing and clean out debris with compressed air, as well as wipe down the cover, as dendrite growth is common.

Those are the only easy fixes. Beyond that, you will need to tilt the chassis up for work.

[tubmyk2]

I lucked out and it turned out to be V3. I had a 12AU7 on hand, and thank goodness for that. Crisis averted (for now)!