- #1
bwinter
- 27
- 1
Hi there!
So here's the deal.
I'm restoring a friend's old Philco radio for his 1968 Mustang. This includes changing out the internals to tune FM instead of AM. The old AM internals include a set of three inductor coils with magnetic cores that move in and out of the coils as you turn the tuning dial, which change the inductance of course. I wanted to continue using this mechanism, so to this end, I've designed and sent out for some custom PCBs for a whole new FM circuit which uses an external LC circuit to tune the desired frequency. This way, I can reconstitute the inductor coils and retain the clever mechanical preset buttons as well. Now, onto the problem.
I've calculated that the inductor(s) must vary 42% in value to tune from 88MHz to 108Mhz, the FM spectrum. This is regardless of the capacitance. The problem is, the coils as they are now are machined to cover the AM spectrum, and thus vary quite a lot more (more than 200% percent from lowest to highest needle position). The magnetic cores, if you are wondering, are lowered into and out of the coils as a fixed set and go from being completely outside the coils, to being inserted completely inside them as you turn the dial.
What I have to do is come up with a relation that will tell me just how many turns I must rewire these inductors with so that:
[tex]0.42 = \frac{L_{max}-L_{min}}{L_{mid}}[/tex]
All I have are the min (air-core) and max (magnetic-core inserted fully) values of the current inductors, and the geometry of the inductor tubes (think slightly smaller than a cigarette).
Any ideas other than trial and error? Luckily I do have a fairly accurate LC meter.
So here's the deal.
I'm restoring a friend's old Philco radio for his 1968 Mustang. This includes changing out the internals to tune FM instead of AM. The old AM internals include a set of three inductor coils with magnetic cores that move in and out of the coils as you turn the tuning dial, which change the inductance of course. I wanted to continue using this mechanism, so to this end, I've designed and sent out for some custom PCBs for a whole new FM circuit which uses an external LC circuit to tune the desired frequency. This way, I can reconstitute the inductor coils and retain the clever mechanical preset buttons as well. Now, onto the problem.
I've calculated that the inductor(s) must vary 42% in value to tune from 88MHz to 108Mhz, the FM spectrum. This is regardless of the capacitance. The problem is, the coils as they are now are machined to cover the AM spectrum, and thus vary quite a lot more (more than 200% percent from lowest to highest needle position). The magnetic cores, if you are wondering, are lowered into and out of the coils as a fixed set and go from being completely outside the coils, to being inserted completely inside them as you turn the dial.
What I have to do is come up with a relation that will tell me just how many turns I must rewire these inductors with so that:
[tex]0.42 = \frac{L_{max}-L_{min}}{L_{mid}}[/tex]
All I have are the min (air-core) and max (magnetic-core inserted fully) values of the current inductors, and the geometry of the inductor tubes (think slightly smaller than a cigarette).
Any ideas other than trial and error? Luckily I do have a fairly accurate LC meter.
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