Some time ago prompted by some previous posts on jammed width control cores I started to experiment on a scrap analog board I just happened to have, and very quickly cracked and jammed my core quite badly into many fragments while using the correct size alignment tool. After some appropriate words I set it aside. Today I started examining the situation a bit more and here are some observations.
Removing a jammed core is easy if the core is mostly outside the part of the coil form covered by the litz wire coil winding. Carefully longitudinally slit the plastic coil form using a razor blade box cutter using precisely repeated shallow strokes until the blade cuts thru. Make the cut avoid the four inner ridges that provide the inner thread of the coil form, so there is a slight air space that the cutter encounters as it breaks thru, rather than core material. Now insert a jeweler's screwdriver blade into the slit near the coil and twist it a bit to widen the cut and relieve the grip the plastic coil form has on the core, or core fragments. It should be possible to back out the core, or at least the outer fragments of the core once the inner diameter of the coil form has been thus increased. Any fragments still remaining inside the coil proper might need some additional fragmentation to motivate them along.
After some partial reassembly my core fragments say they were once 1/4-28 x 1 inch long with teflon thread coating originally intended to provide some mechanical stability of position after initial factory adjustment. If you look under a microscope and play with this white material you will find it similar in consistency to teflon thread seal tape. Problem is that over the years, it balls up in clumps, incorporating any core wear particles until it jams the threads against the inside of the coil form. So once you get the core out, clean it completely free of teflon, and clean the inside of the coil form using a 1/4-28 threaded rod or a pulley tap.
My core fragments are black, more likely powdered iron rather than the gray of manganese-zinc ferrites associated with high frequency adjustable coils. So I went searching in my parts bins for adjustable coils and found a black core .76 inches long and another black one .38 inches long. There are many different grades/permeabilities of powdered iron but hey, this all I found. Then getting out the inductance bridge and the inductance meter as a cross check, I made the following measurements:
No core: 7 uH
.76 long core centered in winding: 33 uH
thread in additional .38 long core until it kisses against end of .76 core: 40 uH The inductance jumps up right at the contact point so cracks and gaps make for a significant decrease in inductance.
No claim on what's right for the Mac, and I'm not inclined to swap this in for a trial. I might try adhesive reassembly and measurement of the original core fragments, or open up a good unit but not today.
Some thoughts on long tight cores: if you use a plastic hex alignment tool to turn a tight core, the plastic twists until all the turning force is concentrated at the mouth of the core which tends to split it. A precisely ground metal hex tool would be better. But a poorly surfaced or plated metal hex tool might be worse because it could put all the turning force at a single high point anywhere inside the core and split it from there...
If a core does not need adjustment I would leave it. But if I find I need to do an adjustment on a teflon coated core, I would attempt to back it out completely and remove all teflon and wear particles from the threads inside and out while I still could. Repeated back and forth rotations are inviting thread jams while the core is deep inside the copper winding area.
Removing a jammed core is easy if the core is mostly outside the part of the coil form covered by the litz wire coil winding. Carefully longitudinally slit the plastic coil form using a razor blade box cutter using precisely repeated shallow strokes until the blade cuts thru. Make the cut avoid the four inner ridges that provide the inner thread of the coil form, so there is a slight air space that the cutter encounters as it breaks thru, rather than core material. Now insert a jeweler's screwdriver blade into the slit near the coil and twist it a bit to widen the cut and relieve the grip the plastic coil form has on the core, or core fragments. It should be possible to back out the core, or at least the outer fragments of the core once the inner diameter of the coil form has been thus increased. Any fragments still remaining inside the coil proper might need some additional fragmentation to motivate them along.
After some partial reassembly my core fragments say they were once 1/4-28 x 1 inch long with teflon thread coating originally intended to provide some mechanical stability of position after initial factory adjustment. If you look under a microscope and play with this white material you will find it similar in consistency to teflon thread seal tape. Problem is that over the years, it balls up in clumps, incorporating any core wear particles until it jams the threads against the inside of the coil form. So once you get the core out, clean it completely free of teflon, and clean the inside of the coil form using a 1/4-28 threaded rod or a pulley tap.
My core fragments are black, more likely powdered iron rather than the gray of manganese-zinc ferrites associated with high frequency adjustable coils. So I went searching in my parts bins for adjustable coils and found a black core .76 inches long and another black one .38 inches long. There are many different grades/permeabilities of powdered iron but hey, this all I found. Then getting out the inductance bridge and the inductance meter as a cross check, I made the following measurements:
No core: 7 uH
.76 long core centered in winding: 33 uH
thread in additional .38 long core until it kisses against end of .76 core: 40 uH The inductance jumps up right at the contact point so cracks and gaps make for a significant decrease in inductance.
No claim on what's right for the Mac, and I'm not inclined to swap this in for a trial. I might try adhesive reassembly and measurement of the original core fragments, or open up a good unit but not today.
Some thoughts on long tight cores: if you use a plastic hex alignment tool to turn a tight core, the plastic twists until all the turning force is concentrated at the mouth of the core which tends to split it. A precisely ground metal hex tool would be better. But a poorly surfaced or plated metal hex tool might be worse because it could put all the turning force at a single high point anywhere inside the core and split it from there...
If a core does not need adjustment I would leave it. But if I find I need to do an adjustment on a teflon coated core, I would attempt to back it out completely and remove all teflon and wear particles from the threads inside and out while I still could. Repeated back and forth rotations are inviting thread jams while the core is deep inside the copper winding area.