Hi @neonknight:
Before answering your question, many of the cartridges that are returned to us for service are ailing due to factors that are within the customer's power to control.
For example, inadequate cleanliness in the cartridge operating environment will lead to excessive accumulation of dirt / metal particles in the front yoke / magnet and coil area, which hampers the cantilever's ability to track properly. Normally this can be avoided by keeping the stylus and LPs clean, but if the LP played while still wet, or the customer is sloppy in applying stylus cleaning fluids, the excess fluids can solidify with the accumulated dirt to form a cement-like substance that is impossible to remove without damaging the front yoke / magnet and cantilever. We've had to rebuild / replace cartridges because of this, which is regrettable as it was entirely preventable.
Many returned cartridges show signs of side-swiping or side impacts, which can cause the vertical axis of the signal coils to become twisted, degrading channel separation / crosstalk.
For the past few years we have been building up a database, which as the final step of the production process, records each cartridge's serial number and performance measurements such as channel separation. When a cartridge is returned to us for servicing, if it is still playable we will measure it and compare the data against the numbers that were recorded when the cartridge was new. A marked degradation of channel separation is usually a sign of a side impact, and sometime we see physical evidence that backs this up (such as a front yoke that has been twisted, due to its receiving a whack from the side).
We've even had cartridges returned due to degraded performance, where the front yoke was clearly out of position. Upon querying the customer, some have admitted to taking a screwdriver to the screw that secures the front yoke in place and tightening it up, which is likely to twist the yoke and degrade tracking, channel separation, and sound quality. It seems that seeing exposed screw-heads makes some people want to get their tool chest out (lol).
However, sometimes the customer is not at fault. On one occasion a cartridge that had excellent channel separation prior to shipping, was returned to us with much poorer figures. After readjusting, the channel separation was in the mid-30dBs. But shortly after the customer received the readjusted cartridge, again the channel separation had plummeted to under 25dB. We took back the cartridge, but after readjusting the cartridge, we waited for about a week, to see what would happen to the channel separation. Lo and behold it worsened, and the problem was traced back to slippage in the plug used to secure the suspension wire, which in turn was caused by an inadequately mixed batch of glue. Since the fault was deep inside the cantilever assembly, hidden underneath the coil armature and joint pipe, it wasn't visible externally, and since the suspension slippage didn't occur immediately, we initially thought that the cartridge was OK, when it wasn't. In the end, Ogura owned up to their assembly error, the mystery was solved, and the customer was happy. (Note that replacing only the stylus, or only the cantilever and stylus, wouldn't have cured the problem.)
Other cartridges have been returned because they had become low-riders (insufficient or marginal clearance between the bottom of the cartridge and the LP surface). In many cases the cause will be an inadequately adjusted tonearm elevator mechanism that allows the cartridge to be dropped onto an LP from a good height, which is likely to stretch the suspension wire and possibly force a depression into the dampers. In other cases excessive tracking forces are the cause, sometimes due to the customer's habits, sometimes due to the tonearm counterweight accidentally slipping forward.
Regarding stylus wear and the contributing factors, excessive vertical tracking force will decrease stylus longevity, likewise for incorrect anti-skating.
It is not uncommon for new cartridges to be played with a tracking force that is at the top of the recommended range, or even slightly beyond, but if the intent is to eke out the longest stylus lifespan, using higher tracking forces than strictly necessary is counterproductive.
Next, cleanliness is next to saintliness. The dirt that accumulates in the LP groove and gets transferred to the stylus tends to have a high silicon oxide content (sand), and sand's abrasive nature will significantly hasten stylus wear. Keeping the LP scrupulously clean with an ultrasonic or fluid-and-vacuum cleaner, and cleaning the stylus after each LP (after each side is even better) with a liquid stylus cleaner will help greatly to extend stylus life. But as mentioned above, leaving undried fluid on the LP, or slathering stylus cleaning fluid onto the cantilever or front yoke will lead to other issues.
Yes, the cost of a quality stylus cleaning fluid is not insignificant, but compared to the cost of a cartridge retip or rebuild that is brought on by accelerated stylus wear, it is a bargain.
As an aside, Magic Eraser as a non-fluid stylus cleaner should be used with an awareness of its limitations, and not be relied on as the only means of cleaning the stylus.
Using a Magic Eraser scrapes off the dirt that has accumulated on the stylus, but pushes it upwards rather than removing it entirely. This helps in the short term, but since the cantilever forms a ceiling over the stylus, the pushed-up dirt becomes trapped between the cantilever ceiling and the stylus block, and over time will be packed tightly and extend downwards (like a stalactite). Eventually the tightly packed layer of dirt will extend downwards far enough to interfere with the LP contact edges of the stylus, and tracking will suffer.
We have received cartridges for service that had been regularly cleaned with a Magic Eraser, yet had lost their ability to track the LP groove. Meticulously cleaning the stylus with a liquid cleaner (and sometimes a judiciously-wielded scalpel) rectified the issue.
Careful use of the Magic Eraser can be OK, but it should be augmented with periodic cleaning with a good liquid stylus cleaner (but don't get the liquid onto the cantilever - apply it to the stylus only).
All right, finally time to answer your question directly.
Over time...
In addition to stylus wear, alloy corrosion can occur in the joint pipe that ties the forward section of the cantilever to the coil former and suspension section. Excessive corrosion of the joint pipe means that the mounting rigidity of the boron / ruby / diamond cantilever rod will be lost. If the cantilever is metal, corrosion can likewise happen to the cantilever itself, depending on the alloy and whether a protective coating or surface treatment has been applied. Any corrosion process will be hastened with exposure to excessive moisture, or airborne salt and other reactive elements.
The adhesives used to bond parts together will gradually lose flexibility, become more brittle, and may shrink and pull away from the components that they were intended to bond or encapsulate. If the process continues, the adhesive layer thicknesses can become thinner, and alter tolerances and clearances.
Greases (used for dampening as well as lubrication) tend to harden over time, and in some cases the solids and liquid components may start to separate and clump. And depending on how much freedom the greases have to move, the pull of gravity can cause shifting over time (Satin cartridges are a known example of this).
Most rubber dampers will exhibit decreased elasticity, and gradually deform due to constant compression by the armature and coils along with general hardening of the rubber compound. Other compounds will turn to goo over time, or liquify. A number of the temperature-stable damper systems used by the likes of Technics and Yamaha are prone to this issue.
The suspension wire can take on a kink or "set", which means that the cantilever moves less readily in some directions than others. This will worsen tracking performance.
Should the insulation layer of the coil wire have pinholes (even if they are so small as to be initially imperceptible), the long-term result can be breakage or shorts.
The leadout wires of the coil will gradually work-harden due to repeated flexing every time the cartridge is cued on or off the LP, which can likewise lead to eventual breakage or short circuits.
We and a few other cartridge manufacturers mitigate this by threading the coil leadouts through the center hole of the dampers, but this can lead to unpleasant surprises if a non-manufacturer retipper attempts to rotate the dampers without taking into account that doing so will twist the coil leadouts and possibly break them.
The grub screws that secure the cantilever assembly in place are very small, and can only take a very limited number of torque cycles before they break. Once the heads break, their small size makes it nearly impossible to remove the screws to free the worn-out cantilever assembly.
The magnets can lose strength.
With Alnico magnets, typical causes are impacts (dropped cartridges), or if the magnetic circuit was ever opened. Luckily, it isn't particularly difficult to recharge Alnico magnets.
Neodymium magnets degrade quickly without a surface layer to protect against moisture and oxygen. However the protective layer sometimes gets scratched or flaked; then it is only a question of time before the magnet loses much of its strength, and should be replaced.
Samarium-cobalt magnets are stable in my experience, and platinum-iron magnets are also stable (but the older formulations were not as strong as the newer grades).
It is also quite common to get back front magnet carriers (aka yokes or polepieces) that are damaged (cracked or fractured). In Lyra's cartridges the front magnet carrier is counterbored to form a mounting cradle around the front magnet, and having cracks or fractures in this critical area means that the front magnet can vibrate or move.
I have seen photographs of retipped Lyra cartridges where parts of the front magnet cradle had cracked off and were missing.
Signal output pins are typically plated with gold, silver and other highly conductive metals which are soft, and will pick up scratches and nicks with usage (due to abrasion against the headshell lead clips). This means worsened quality of the contact surfaces, and degraded sound.
And depending on how customer attaches the headshell lead clips to the output pins, the pins can be forcibly rotated, which strains the coil leadout wires inside the cartridge. In extreme cases, the output pins will loosen within the output pin block and spin freely or snap.
On some cartridge models I have used flanged inserts to take the 2.6mm mounting screws, rather than cutting the screw thread into the cartridge body. Certain customers have been prone to tighten the screws with so much torque that the flanges flexed and work-hardened, which eventually resulted in breakage. Each screw insert was covered by its flange, making it nearly impossible to spot the damage building up until the flange popped loose.
Scratches, nicks and damage accumulate with use (we have received cartridges that were badly gouged because whoever installed the cartridge, activated an electric screw driver without checking if the mounting screws were aligned with the screw holes or not).
A not-so-large minority of customers will accept a cartridge that retains scratches or dings on the body, but others have kicked up a stink when blemishes that they put into it were still present on the returned cartridge, leading to demands for before-and-after photographic evidence, blame vs blame, and general unhappiness.
On the other hand, restoring a blemished cartridge to as-new or even nearly-new appearance requires first checking the threads of the grub-screw-holes that secure the cantilever in place for damage, and the threads of the 2.6mm mounting screw-holes. If there is damage, the cartridge body should be discarded.
If the screw threads are in good shape and the cartridge body is free of gouges, severe scratches and dings, the cartridge needs to be taken apart completely. This includes heating the body in an oven to break all of the adhesive joints bonding the internal components together, and punching out the interference-fitted mounting screw inserts.
This will be followed by deep cleaning / degreasing the body, stripping off the anodizing or plating (which will trigger a minor shrinkage in body dimensions), shot-peening, re-buffing if the cartridge should have a mirror finish, and then applying a new anodizing / plating / coating.
Next replacements for for all wearable components must be readied (stylus, cantilever rod, joint pipe, armature, coils, suspension wire, dampers, grub screws, magnets, front magnet carrier, output block, output pins, mounting screw inserts).
Once the combination of new and refurbished componentry has been prepared, the cartridge "rebuild" can start. Finally!
Given the high amount of labor and time required to completely strip down and refinish the cartridge body, it has proven easier and faster to simply start with a completely new set of components, build a new cartridge, provide that new cartridge as a rebuild, and take a haircut on the difference in pricing between new and rebuild.
IOW, although we may refer to or provide a cartridge as a rebuild, in nearly all cases it will be a brand-new cartridge.
We have avoided "retips", as this implies that only a bare minimum of componentry will be replaced, which is at odds with taking pride in our craftsmanship and attention to detail (each Lyra cartridge is individually auditioned using electrostatic headphones, compared against a reference unit, and adjusted and voiced until it plays and sounds as close to the reference as possible).
I cannot comment on the policies of other MC cartridge manufacturers, but the situation should not be dissimilar for any brand who aims to provide the customer with a "rebuilt" cartridge that is free of all blemishes and performance that is the very best that it can be.
What determines the cost of a rebuild then becomes the manufacturer's production cost for a new cartridge (since in most cases a rebuild and brand-new cartridge are the same thing, only with different naming), how much of a haircut they are willing to tolerate on the difference between new and rebuild pricing, how much margin the importer / distributor needs, how much margin the dealer accepts, and the tariff / tax rate.
Non-manufacturer retips have a major pricing advantage due to the transaction being conducted directly between customer and service provider - no importer / distributor margin, and no dealer margin. But in return, much of the componentry that should be replaced, won't be.
This doesn't mean that all cartridges will not perform satisfactorily unless every potentially wearable component is periodically replaced. However, as a manufacturer we have to aim for top performance and build quality in 100% of our cartridges, and a belt-and-suspender approach (which may seem overly cautious and conservative to the budget-minded customer) is the only way to assure this.
FWIW, I discuss related issues that may interest you in two posts on the following Lenco Heaven forums page.
https://www.lencoheaven.net/forum/index.php?topic=44559.30
hth, jonathan
