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Topic: Seeking advice on Pro-I changer anomaly |
Mark McCornack
From:
California, USA
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Posted 18 Jul 2018 12:29 pm |
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Hi all,
I'm looking for some advice on a mechanical issue with an early 70's Sho-Bud Pro-I. The changer is a universal, double row type (top row for raises, bottom for lowers) and I'm seeing some non-linear effects on lowering my string 4 from E to Eb. What is happening is when I begin lowering the 4th string with the knee-lever, the bell crank rotation and pitch drop are all fine and tracking together, until I get to about 0.130†of pull at the changer. Then, what I am seeing is the 4th string RAISE bracket in the changer starts to engage too! From this point, the slope of the pitch drop versus bell crank rotation starts to soften and at 0.180†or so, the string pitch actually starts to raise rather than lower! Bad deal!
Here is a graphic representation of what I measured on the steel. I am showing both E strings here ; string 4 and string 8 dropping in pitch. For purposes of illustration, I have multiplied the string 8 frequency readings by 2 so as to normalize the comparative frequencies (made unison, not an octave a part). Note that string 8 remains pretty linear as I rotate the crank, a full step down to a D note. String 4 goes non-linear near Eb (unfortunately, very near the target pitch) and actually starts to raise (see the tail end).
So, looking at the other strings on the changer, this behavior of the RAISE mechanism engaging engaging when lowering a string seems to happen on ALL strings. The thing is, for the other strings I have looked at, it seems like this does not occur until the pitch of the string has dropped by around a minor third or so. String 4 however is going whacky at near ½ step drop, and this is encroaching on the target for that lever.
Anyway, does anyone have any thoughts on this? Is it something I can fix, and if so, how? Perhaps someone has written a treatise on this mechanism on the SGF site already. Anybody know?
Thanks and regards,
Mark |
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Ian Worley
From:
Sacramento, CA
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Posted 18 Jul 2018 3:19 pm
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Is there a raise helper spring on string 4? If so, get rid of it. This can/will cause the problem you describe (raise helpers are the ones against the underside of the top deck, connected directly to the actual finger, not the lower return springs on the bottom of the scissor).
With no helper spring, the tension in string 4 is more than ample to keep the raise arm seated tight against the stop bezel during a lower. If that's not the issue, then it's likely that something in the string 4 scissor mechanism is binding. Could just be a lubrication issue, or a stray string ball end or something lodged somewhere out of view.
If it's nothing obvious, you can pull the finger and it likely will be. It's pretty simple, just relax ALL the strings, remove the lower return spring and pull rods from that finger. Push the 5/16" axle over just past the finger in question with something similar in size like a chopstick or a pen, then pull your push stick back just enough to release the one finger from the bottom. |
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Mark McCornack
From:
California, USA
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Posted 18 Jul 2018 3:59 pm
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Hi Ian,
I REALLY appriciate you taking the time to reply to my question. I'll study your suggestion and dive in as soon as I can muster the courage. I think my main problem is that I don't understand fully how the changer is supposed to work. All these terms "top bezel, raise helper, return spring" etc are foreign to me. I feel as though I am resonably mechanically inclined, and if I understood the mechanism, I'd better understand the problem.
Do you know if anyone has publushed a Theory of Operation anywhere (such as the Forum) for this changer? I have tried to find this, but no success as of yet. The guitar is about 45 years old and has been played a fair bit, so I wouldn't be surprised if it is just seriously out of whack.
Thanks again,
Mark |
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Ian Worley
From:
Sacramento, CA
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Posted 18 Jul 2018 5:31 pm
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Here is a pic and an explanation as relates to your issue. The one on the far left is what you have.
It's a fairly simple compound lever that pushes the bottom of the aluminum finger either forward or back. The slots you see in the two changer arms rest over projecting stops adjacent to the end plate window (that's the "bezel", the stamped metal thingy just inside the window). The rivets in the middle of each arm (raise and lower) provide a fulcrum for the other arm when a pedal or lever is engaged to raise or lower a string. The system is kept in equilibrium by the tension of the string in one direction, and the tension of the lower return spring in the other.
If you look at the pic you can connect the dots and see how it all works. The small hole on the bottom of the lower scissor arm (far left edge of pic) is for the lower return spring, the other end connects to the bottom edge of the bezel at the end plate. The optional raise helper spring, if you have them, would attach to the small hole in the bottom of the aluminum finger and to an L bracket on the underside of the top deck.
With no pedals/levers, the string tension through the compound lever is pushing the raise arm against the upper stop on the bezel, while the lower return spring is pulling the lowering arm against the stop on the bezel directly. If you remove either source of tension, the corresponding arm will pull away from the stop bezel.
When you activate a lower, the rod pulls the lowering arm away from the stop bezel. At this point, the rivet in the raise arm becomes the central fulcrum, and the vertical center arm of the scissor pushes the bottom of the finger back toward the end plate, thus reducing pitch. If in this situation the raise arm is lifting off the stop as you describe, something (such as a raise helper spring) is overpowering the string tension's ability to hold the arm firm against the stop. String tension should be ample to keep the arm firmly seated under normal conditions.
If it's not caused by a raise helper spring, then it's likely that one or more of the three rivets is binding, or something in the lowering linkage is hanging up on the raise arm. I suspect if you flip it over and examine it closely you'll be able to see where the problem is originating. It the scissor is binding it's easy to remove just the one finger ass'y, tweak/lube/reassemble.
Good luck
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Kevin Fix
From:
Michigan, USA
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Posted 18 Jul 2018 7:24 pm
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| I know on the Super Pro changer that the mouse ear will wear a notch in the changer and cause issues. JB Weld will fix the groove in the finger. |
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Mark McCornack
From:
California, USA
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Posted 18 Jul 2018 9:24 pm
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Thanks again Ian for this very helpful and lucid explanation! The picture of the whole finger assembly out of the changer makes it all clear.
I tried removing the helper spring as you suggested, and unfortunately the “easy†fix didn’t address the problem. The next step is to remove the whole 4th string finger and clean it up, and fix it as required. It will also give me a chance to get some of the 40 year old cat hair out of there as well  I suppose since I have never cleaned this since I’ve owned it, ‘bout time!
Mark |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 22 Jul 2018 10:02 am
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| Quote: |
| ...What is happening is when I begin lowering the 4th string with the knee-lever, the bell crank rotation and pitch drop are all fine and tracking together, until I get to about 0.130†of pull at the changer. Then, what I am seeing is the 4th string RAISE bracket in the changer starts to engage too! |
Many times, that's simply caused by the lowering spring being too tight. You can substitute a lighter spring, and that will usually take care of the problem. Note also that installing a lighter spring sometimes works better than just reducing the tension with the spring tension screw, if your guitar is so equipped. You can also try installing a heavier string - sometimes only a thou or two heavier gauge will allow the changer to function normally. |
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Mark McCornack
From:
California, USA
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Posted 23 Jul 2018 7:32 pm
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Problem resolved, and many thanks to all for your input!
It turns out this was a self inflicted problem caused by a custom bell crank I made to time the glisendo on my two E’s lowering to Eb (tracking through glis).The result of my longinsh custom bell crank on the 4th string was that the rod to the changer was entering it at a slightly oblique angle, and that was enough out of whack to cause binding in the finger assembly.
Long story short, I changed my cranks around to minimize the weird angle and all is now good. As a bonus, I now have a much better understanding of the changer assembly on this old Bud.
Thank you all for your input
Mark  |
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Mark McCornack
From:
California, USA
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Posted 23 Jul 2018 7:37 pm
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Problem resolved, and many thanks to all for your input!
It turns out this was a self inflicted problem caused by a custom bell crank I made to time the glisendo on my two E’s lowering to Eb (tracking through glis).The result of my longinsh custom bell crank on the 4th string was that the rod to the changer was entering it at a slightly oblique angle, and that was enough out of whack to cause binding in the finger assembly.
Long story short, I changed my cranks around to minimize the weird angle and all is now good. As a bonus, I now have a much better understanding of the changer assembly on this old Bud.
Thank you all for your input
Mark |
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