Extreme motorboating

[bdodds]

Hey guys. Just talking through a problem I have to tackle tonight, I've got a '59 C3 with bad motorboating, sounding like more than half the positions in the vibrato cycle aren't passing any audio. I thought maybe it was the scanner since a lot of folks look there first and I could see that all the terminals were thoroughly oil-soaked. I'd never touched a scanner before so I thought I'd try - took it all apart, degreased everything, oiled the bearing points, reassembled, no change. So I probably should have gone through and done some continuity testing to see if anything is shorted or wide open - the 6-position knob's switch matrix looked practically brand new last I looked at it a few weeks ago and I cleaned the contacts while I was in there, but I'll check with a meter for sanity. The line box is where I'm thinking might have issues since when I recapped the TG last year some of the cap values were way off scale, some several orders of magnitude. But basically just spitballing an order of what I should check - re-check the terminals on the scanner to make sure nothing is shorted, use a meter to verify the switch matrix, then finally look at the line box. Anything blatant I'm overlooking?

[bdodds]

WAIT here's another thing I just remembered now (which would have been handy before rebuilding the scanner, though that was kinda fun) I remember a few weeks ago before a gig firing it up and chorus working properly for just a second, then I turned it off and back on and it was back to the motorboating. I think I could get it to pop back to normal by vigorously flipping the tablet for the manual, but it only happened a few times and then never since. That's when I opened that up and checked inside and everything looked pretty clean, but now I'm thinking I may need to do a more thorough cleaning of those switches. I think I was afraid to poke too much at them since I had no experience with them so much and didn't want to ruin something before a gig. Hit it with de-oxit and that didn't seem to make a difference. Plus since this motorboating symptom affects both manuals equally I was thinking it wouldn't be a component that only affected one or the other so I figured it was a red herring. I could obviously be wrong. I'll be a bit more methodical in my troubleshooting tonight so I can rule out larger sections of the system. This doesn't sound like it could be amp related, but I'll swap tubes and I've got a good AO-28 I can swap in completely as a quick thing to try. I mean, as quick as swapping an AO-28 is (quicker than a scanner rebuild, slower than cleaning the rocker switches possibly).

[Jaim]

If you take the time to think about the vibrato signal flow, there is NO way an AO28 could cause motor boating. No vibrato - yes; motor boating - no.

The vibrato drive signal exits the preamp on terminal C. Brown wire to delay line. The red and blue wires from the delay line go directly to the scanner. The remaining delay line taps go through the C1-C3 switches. Take a set of headphones - one lead to ground and connect to the delay line taps. This will checkout the delay line.

One other common failure point is the wires running through the DB base from the switch. These can get pinched.

Jim

[muckelroy]

If you take the time to think about the vibrato signal flow, there is NO way an AO28 could cause motor boating. No vibrato - yes; motor boating - no.

It kind of depends on your definition of "motor boating". The OP should post a recording of this sound, in the absence of us having a teleportation device.

Be careful. Though rare, it is possible for there to be a positive DC voltage on terminal C if C13 (1 uf) is leaky. Disconnect the wire from the C terminal and measure C terminal with a DC voltmeter to verify no DC voltage is present.

[Sweet Pete]

Next time you have the vibrato switch open have a look at the resistor.There is only one.
In '59 when the A100 was introduced this resistor had been changed from 22K to 12K in '58 when the open frame line box was introduced.
Unlikely an earlier vibrato switch was used on a '59,however Hammond used up all existing stock before a modification.
Always good to make sure it's a 12K with an open frame line box! If it reads above 15K? the chorus shallows out......less resistance=more depth.
Also make sure you have the correct Bristol Spline wrench to do any work on the axle/rotor gear if needed.

Before the internet and the Organ Forum there were techs and the service manuals and service bulletins.
They are a really dry read.
Big on info,not very entertaining.

Cleaned up,that scanner should sound great!
If you take the time to read the service manual it will warn you not to bend the pickup pin/axle! Wouldn't want wobble in your vib/chorus.....

[bdodds]

If you take the time to read the service manual it will warn you not to bend the pickup pin/axle! Wouldn't want wobble in your vib/chorus.....

Oh yes, I was very paranoid and careful about this pin. Turns out the whole clutch and rotor spun nice and free and I didn't try to get into it since I didn't have to. I'll take a look at those values next chance I have. I figured out what it was, and, well, it takes a big man to admit when they're an idiot, and I'm not a big man, so I'll see if you can spot my problem:

[Sweet Pete]

I don't recognize the black caps on the line box? I'd start there troubleshooting.If the scanner is clean then the line box can be tested just like Jaim said!
Headphones!Test each section no worries!

'59 was a transition year.The A100 was introduced.Your '59 C3 looks correct!
Last year of cloth wire insulation,oiling funnels,wood cheek blocks on B/C/RT was '60. The A100 was new and already had a 12K chorus resistor and plastic insulation on the wiring.

[Jaim]

I figured out what it was, and, well, it takes a big man to admit when they're an idiot, and I'm not a big man, so I'll see if you can spot my problem:

The shield on the scanner output cable shorting several vib line box terminals?

Jim

[Sweet Pete]

The shield on the scanner output cable shorting several vib line box terminals?

Jim

Hard to tell for sure at that distance if the shield makes contact but if it does? Bingo!

[bdodds]

Hard to tell for sure at that distance if the shield makes contact but if it does? Bingo!

I couldn't take the picture so close that it would be that obvious, but yes, that's what it was. Sigh.

Well. At least my scanner is clean, and bonus when I reassembled it I put the oiling wick back on the run motor, it had been missing since I got it. Also inside the scanner the wick to the rotor had been cut, so I repaired that also. So not a waste after all!

The odd-man-out capacitor on the line box looks like they replaced the original with two of half-value in parallel (i assume, i've got the caps to rebuild it, just no time)

Thanks for the tips, everyone!

It's funny, I'm new here, but only because I haven't organed on the internet in a long time. Once upon a time I helped administer the Hammond/Leslie FAQ, at some point in the 90s.