Hammond M3 Vibrato/Chorus Reduced Volume

[jabney]

I just bought a Hammond M3 and the volume level is greatly reduced when either the vibrato or chorus is engaged.

 

Clues so far: The vibrato sound is not 'choppy' (to my ears) which Google seems to flag as the symptom of scanner dendrite growth (metallic 'whiskers') so I'm hoping a rebuild is not needed.

 

I'm not sure if the percussion is working all that well, but it is there. Doesn't seem as percussive as the Roland VK8, though.

 

The gentleman I bought the M3 from also told me there was no pedal output. I learned why, when I adjusted the pedal pot on the back of the amp, turning it so the pedals would sound. The pedals now sound loud, but the amp now sounds crackly. Even when warmed up for quite a few minutes.

 

What I'm thinking of trying:

 

V10, a remote-cutoff pentode (6BA6) is suspect so I will try to find a replacement.

 

I've read that Hammond used some sub-par carbon resistors at one point (they are described as having a rough surface) so I'll try shotgunning a few carbon replacements if the values have changed. The resistors around V4A (I think, very blurry schematic) seem like a good place to start checking. That's because feeds from vibrato, pedals, and percussion all converge on that tube's grid.

 

Does it sound like I'm on the right track?

 

And what does a 'choppy' vibrato sound like :-)

[mate stubb]

Sounds likely that something needs fixin in the vibrato channel of the preamp.

 

Not sure if the M-3 preamp works like the AO-28 on the consoles, but if there are two inputs to the preamp (straight channel and vib channel), try reversing the wiring and see if the volume drop now appears on the straight signal. If so, it's a bad cap, resistor, or tube in the preamp.

[jabney]

hi Moe,

 

Thanks for the reply.

 

I need to locate a better schematic, since the pre-amp section you mention may be there, but darned if I can tell :-) Perhaps V4B is the culprit (if that's a pre-amp in the non-printing 'gutter' space of the schematic), so checking around V4 seems like it may still be worth a try. I like your idea, but I will need a clearer schematic to do it. And, I have no idea what the solid-and-dashed-at-the-same-time lines represent. Grounds maybe?

 

It is kind of fun trying to track this stuff down, but I don't want to do any harm to the M3.

 

Thanks again!

[paully]

Clear schmatic: M3: downloads as 2 seperate files/pictures: left & right.

 

http://www.captain-foldback.com/Hammond_sub/schematics/M3_early.zip

 

Paul

[Ricochet]

I had a similar problem plus a scratchy sound and random popping and crackling with the vibrato channel on an M3 I recently fixed. Turned out to be the 6AU6 on the right side, which had a flickering short in it. I'd bet on a bad tube in the vibrato section, anyway.

[jabney]

hi Paul,

 

Thanks for the link. That's much better than the schematic I had.

 

hi Riccochet,

 

Thanks for the tip. Now that I can see the vibrato section on the schematic, what you say makes a lot of sense. Funny thing about tube stuff, it's almost always a tube that's the culprit :-)

[Ricochet]

Originally posted by jabney:

Funny thing about tube stuff, it's almost always a tube that's the culprit :-)

Not always, but there's a reason they made 'em to plug in and unplug so easily.

[paully]

Senior Member

Member # 41213

 

posted 03-23-2005 6:29 AM

 

quote:

----------------------------------

Originally posted by jabney:

Funny thing about tube stuff, it's almost always a tube that's the culprit :-)

----------------------------------

 

Not always, but there's a reason they made 'em to plug in and unplug so easily.

 

----------------------------------

 

Ric,

Absolutely right. Anyone who has done repairs knows that you "fix the loose wire, THEN charge 'em for a tube".

[Ricochet]

I've got radios from the '30s that have still-working original tubes.

 

On this particular M3, first thing I found was that its 5U4GB rectifier (made in April 1958, surely the one originally installed in the organ) was just about dead. Replacing it boosted up the volume considerably. The bad noises were from a bad 6AU6, like I said. The thing was still too quiet. Somebody had replaced (maybe intentionally) the intermediate amplifier's 12AX7 with a 12AU7A and dropped the volume to a tiny fraction of what it should be. Good tube, but I swapped back a 12AX7 and it'll rock & roll now. Changed out a 6C4 while I was at it that read marginally low on an emission tester. Shoulda left it alone. Tube testers were mainly invented to sell more new tubes, and they still work for that. (I had all the tubes on hand anyway.)

[WesG]

Try exchanging the 6AU6 tubes, and see if the quietness moves to the non vibrato channel. If so, bad tube.

 

The newest M3s are over 50 years old. Chances are high there are capacitors and resistors on there that have drifted in value enough to really matter. Owning a vintage Hammond always means restoration of some kind...but it's worth it.

[Joe Muscara]

http://www.motorcycleforums.net/forum/attachments/suzuki/25297d1309921005-intruder-recall-zombie-thread.jpg

[Threadslayer]

 

The newest M3s are over 50 years old.

Ouch.