M-100 vibrato ground loop/thump

[Mastasheo]

So a while back I replaced three capacitor in my ao-29 amp. After I got everything back together I noticed when I hit vibrato small I sometime get a heart beat sounding thump that goes away if I turn vibrato off. I have noticed that the ground braid wire that goes from the scanner to the amp is picking up sound when I hit it with a chop stick. Also if I touch and wiggle the gray, brown and black wires coming out of scaner lugs 0,1,2 I will get a dramatic increase in the volume and rate of the thump. I just removed the ao29 and replaced it with a older m3 ao29 I had lying around and got the same problems so I don’t think it could be a problem with the amp or my cap replacement. I have also dismantled and cleaned the scanner of zinc hairs and replaced all the capacitor in the delay line with no solution. It seems to me like it’s some kind of ground problem or loose wire but I’m a novice at this stuff and could really use some help from the experts here before I go and try some more extrem fixes. I have been going mad over the fact I can’t use the vibrato when I play!

[Sweet Pete]

Does the thump happen when you engage the vib switch and then go away,or is it a constant thumping?

I reflowed solder on C14 and got rid of a repetitive thump. Refer to the M service manual.
I cured the on/off switching thump with new caps.The short is still in your scanner somehere.Take a continuity meter to check your ground again.
Reflow your solder work.Somethings not tight there.I thought I had done a great job soldering in my new caps.......not.

[Mastasheo]

It’s a constant thumping. Just tested all the lugs on my scanner and none had any shorts to ground. Also cleaned up some of the solder joint with no success.

[Mastasheo]

Ok so I previously replaced c64, c30, c64 and c64a (black can cap). I cleaned up the connection yesterday but I don’t think it could be in the amp work because I tried the m3 amp that is stock with wax caps to the same effect. Today I poked around looking for all the places that was “picking up” noise and found it was both of the braided ground wires connecting to B terminal of the AO-29 (one from the scanner and the other from the volume tab switch) it then goes into the 12ax7 tube and out to the green wire inside the expression pedal housing. I cleaned up all the weak looking ground connection around the organ also. Mostly around the tab bar. So what does it mean when I parts of the organ becomes michroponic like that? I also swapped the 12ax7 on the off chance it was a tube problem.

[Roger Memphis]

Is the cover of the matching transformer microphonic?
Are you sure it's the braided wire that's microphonic, and not a tube?
Tap gently on the tubes with a wood pencil.
Roger Memphis

[Sweet Pete]

Reflow the solder joints on your 6AU6's and associated caps after you tap your tubes? And like Roger suggests,check 'em all again!
A shorted dirty/loose transformer cover can cause issues. One dirty tube pin can cause issues.
I once cleaned removed the MT's and cleaned behind on the mounting area of the frame and cured a scratchy sound.
Failure to secure the braid at the cable end on the AO29 with the little screw can cause issues.Having wires A/B K/P uncovered can cause issues.
Another time I cleaned the rca plug that occupies the doghouse connection to keep it from corroding,ahahahahah.

[Jaim]

You checked the scanner for shorts. An open connection can also sound like motor boating. With the organ turned off, connect one lead of your meter to the C terminal of the preamp and check for continuity at each scanner terminal with the vib switches set to normal and small. You will be reading through the delay line inductors so the value you read will change. The important thing is you should read something.

Jim

[Mastasheo]

Ok so I manged to get some free time today to try some of the suggestions. First I read if the scanner was open like Jim recommended and found I was missing a couple readings. Maybe from some dead inductors or bad connections. I decided to just remove the scanner, delay line and switchs and roughly installed the vibrato set I pulled from the same M3 (forgive me. I dismantled the M3 to get the parts to add full fold back to this m100. ps the mod went great) Now the motor boating/ thumping is gone with the new vibrato set up! I’ll just keep it set up this way until I have the time to work with the vibrato more. The microphonics is still in the amp however. The matching transformer isn’t microhonic but the bass pedal drawbar houseing is. Tubes V4 through V9 seem to all be slightly microhonic with V7 being the worst. I’ll try a tube swap next when I can get ahold of all them and I’ve been meaning to get a tube tester that would probably be useful right now. Thanks for all the help! I’m slowly making progress.

[Roger Memphis]

I'm glad you got the thumping stopped... a lot of work, but worth it - I'm sure. There's probably only one device that's actually microphonic and it's being transmitted throughout the entire chassis. I had a horrible case in my C-3... it was the matching transformer cover... I de-magnetized it, and that stopped the problem dead!
Good luck on your continuing investigation.
Roger Memphis

[enor]

Ok so I manged to get some free time today to try some of the suggestions. First I read if the scanner was open like Jim recommended and found I was missing a couple readings. Maybe from some dead inductors or bad connections. I decided to just remove the scanner, delay line and switchs and roughly installed the vibrato set I pulled from the same M3 (forgive me. I dismantled the M3 to get the parts to add full fold back to this m100. ps the mod went great) Now the motor boating/ thumping is gone with the new vibrato set up! I’ll just keep it set up this way until I have the time to work with the vibrato more. The microphonics is still in the amp however. The matching transformer isn’t microhonic but the bass pedal drawbar houseing is. Tubes V4 through V9 seem to all be slightly microhonic with V7 being the worst. I’ll try a tube swap next when I can get ahold of all them and I’ve been meaning to get a tube tester that would probably be useful right now. Thanks for all the help! I’m slowly making progress.

You scrapped an M-3 to do foldback on an M-100???

To me, that's the very definition of "backwards". ;)

[Mastasheo]

I have a A100 also and I like having the differences between that and the M100 (diving board, double percussion) vs the M3. But the M3 was in much worse condition.