Hi Robert. Intriguing problem, and I think I may be able to shed a little light. My supposition could be wrong, but if you look at the physics of the thing, it could point to not one issue (“…obviously the contact point between the leather nib…”), but more likely two issues. You have to raise your perception above where the symptom’s cause would appear to originate, and look at the whole picture.
The key to solving your mystery may be directly related to something you conveyed very precisely, that the issue occurs over an extremely narrow band of speeds. So, it appears that we have a primary cause (vibration source at the leather and flywheel), exacerbated or amplified by a secondary condition, which I will assume is marginal wear in the holes that the leather pad arm’s arbor passes through (the first of the two axles), in possible combination with the holes that the speed indicator needle’s arbor passes through, in the indicator frame. Given that the steel arm that the leather pad mounts to is supported on a smaller arbor, and given that this arbor is first in line and thus closer to the governor flywheel where the vibration originates, it’s likely that wear would be more present at those holes and possibly the associated arbor as well, than at the larger holes and arbor of the indicator needle. The same wear on smaller parts will occupy a larger percentage of area, thus having a more noticeable effect. The wear I describe would be very subtle. If for example, the holes that the indicator arbor passes through are normally three-thousandths of an inch larger in diameter than the arbor (just guessing on this — I have not measured it), and it’s known that the arbor touches only a specific portion of the wall of those holes at a specific speed, then you may find that the holes and/or the arbor itself have worn -slightly- out of round. It would be hard to see, but if the holes are normally for example .003” larger than the arbor, and now they’re subtly egg-shaped and .005” in only one direction (out of round), then any vibration whatsoever at the contact point (leather pad & flywheel) would cause the arbor to vibrate in a unidirectional orientation, within its holes. Wear would naturally occur on the associated parts only in the specific areas of the arbors and holes that would be in constant contact when the machine is set to run the correct speed, assuming that for most of the machine’s life it was set to run at the correct speed, thus causing the wear at those limited locations. Keep in mind that even though the holes may appear to be a very close fit to the arbors and thus touching them all around, the reality is that the contact point between arbors and holes can only be over a specific small part of the ID of the holes. For them to touch all around, the holes would have to be the same size as the arbors, which would be a zero clearance, zero tolerance hole — almost a press fit. The holes are larger than the arbors to provide freedom of movement. Freedom of movement when constrained within certain engineered limits is a good thing. Freedom of movement outside of those limits can either cause problems or allow problems originating elsewhere to come to light. Chances are that this machine has lived with a slight vibration at the governor most of its working life, and this has in effect caused more wear to occur in a very narrow and specific way. ANY vibration will lead to greater wear on associated parts than the same arrangement with no discernible vibration. There are multiple causes of vibration at the governor and pad, which could be explored as part of another investigation. To correct your immediate problem, and assuming nothing is done to correct the vibration, then you might approach where the visible effects of this vibration manifest, by sourcing a different indicator frame and arbor. While the indicator needle serves to show the turntable speed, it also makes a marvelous vibration indicator! All of the above supposition is based on imagining all of the related parts working in the mind, in the absence of physical inspection, and therefore remains in the realm of theory. To check to see if the arbor holes are worn slightly out of round, a clockmaker would insert a "smoothing broach" into the holes (tapered steel tool of round cross section), until the tool gently seats in the hole. Then they would hold the indicator frame and broach up to a light source and hunt for a slim, crescent moon shaped irregularity showing at the hole, around the broach. If present, this would prove a worn hole. The test would be repeated for each hole. Wear would likely be greater at the hole closest to the steel arm and hairspring that supports the pad, than at the opposite end of the same arbor. You should also inspect the arbors themselves, where they touch the holes. I would expect to see that a narrow portion of their OD would at minimum appear more polished, indicating a history of (normal) rubbing. They could still be in tolerance however unless they measure out of round with a caliper. All this conjecture about worn holes is based solely on your comments about the problem manifesting only across a narrow portion of the speed range. For a condition where vibration of the needle is present across the entire speed spectrum, I would focus on resolving the source of the vibration, which could be as simple as refreshing the mounting pads between the motor housing and motor board. Vibration transferred to the motor board from the motor, could result in sympathetic vibration from the motor board to the indicator frame mounted under it, even if there is no wear at the indicator frame holes and arbors. In this case, you can ignore nearly all of the above. Ultimately, if the inidcator assembly is worn and replacements weren’t readily available, they could be restored to their factory tolerances. Best and good luck. Keep me posted on what you learn, Andrew Baron Santa Fe, NM > On Sep 12, 2019, at 7:47 AM, Robert Wright via Phono-L <phono-l@oldcrank.org> > wrote: > > Hello to all! I hope someone can help me with this incredibly annoying > problem: the speed indicator mech of one of my Credenza X's chatters at 78rpm > It's fine at 80, it's fine at 90 (I have a Hughes adapter I use for Pathé > verticals), and it's fine below 77. But right at 78 to 79rpm, it chatters. > > It's pretty obviously the contact point between the leather nib and the > governor disc -- once it's up to speed, it bounces instead of riding smoothly > against the surface. I've cleaned the disc and even held some 2400 sandpaper > against it, I've replaced the leather nib with fresh leather shoestring of > two different gauges (one which was sold specifically as material for > phonograph brakes), I've soaked the nibs in oil, I've slathered them with > lithium grease… Nothing helps. I can get a few plays without chatter if I > apply more grease, or clean off the grease that's there (same result -- > "disturbing" that contact point in either way helps for a few plays). > > Anyone else ever had to deal with this? > > Thanks in advance, > Robert > > > _______________________________________________ > Phono-L mailing list > http://phono-l.org > Unsubscribe: phono-l-unsubscr...@oldcrank.org
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