I came across this cart on the 'bay and it is from an Admial 6F10
One interesting thing about it is the way the cantilever and stylus appear to be formed from a single piece of bent metal.
The other quirk is that it has three terminals with rather high resistances.
The end to end resistance is 150kΩ and the center pin is 100kΩ and 50kΩ to the outers. "Tweaking" the stylus makes the resistance change by 1-10kΩ+ so it really seems to be a strain gauge and with a date of "Pre-1952" that would make it an early one. The idea that the three terminals all seem to be connected to gauges has me puzzled an I haven't fully digested the schematic so have at it.
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Looks to me like there's a DC voltage on A, C is connected to ground through R18, and B is connected to the volume control through C15. Looks like the pickup behaves like a stylus modulated potentiometer...
ReplyDeletethe whole way a strain gauge generates a voltage is by putting a constant current through a varying resistance. If one of the series resistors was a fixed high value i see it working but since both resistances internal to the strain gauge vary it would need R18 be the external resistor that is a large fixed value. It states that R18 is 33K and only used in models with a "UL" suffix. R13 is also in series with the string and is 22K which also seems too small to approximate a current source. If the SG resistances were more like a panasonic @ 1K then I see it working so maybe my cartridge sample is 10-100X our of spec. The toher thing that still baffles me is the three terminals. A tapped potentiometer where both values vary dynamically will not create its own signal unless they two resistances vary at different rates.
ReplyDelete" A tapped potentiometer where both values vary dynamically will not create its own signal unless they two resistances vary at different rates."
ReplyDeleteOr different directions....
Indeed.... it does seem that the resistances vary in different directions and this thing is literally a 100K pot with the wiper riding the groove. Add 100V DC, cap couple the wiper and you get music.
ReplyDelete