network

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network

Postby Fonotec » 03 Apr 2013, 03:47

The Ortofon Dummy Load box presents ~ 8.5 Ohms until, invoked via pushbutton, its higher Ohms appear, which trips the bi-stable switch in the Safety Circuit. This circuit tries to save the cutting head coils from profound heat by suddenly opening the audio path, since that's what's energizing them.

{The mercury-wetted reed relay offers fast action, and without loss of electrical continuity during the reed's "bounce-back on 'make'" [i.e., the repercussive spring-back from striking it.] The Mercury which pools in the lower part of the relay housing, is, of course, electrically conductive, and, due to capillary action, it climbs up part of the reed and is able to maintain the permittivity of the switched position, even when the reed briefly stops touching the contact. ...a pullable bead of liquid metal will span that small divide (of bounce-back), preventing the hot coil from being (briefly) re-energized by the audio program.}

High heat is dangerous to the coils, since the glue that holds the coils together could melt. Increased coil resistance is usually only due to a build up of heat caused by audio program high frequency levels being excessive. The indicator needle on the temperature meter starts at the 25 degrees Centigrade mark when the load is ~ 8.5 Ohms, which is the expected DC resistance when the rocking bridge is at rest and the coils are merely at "room temperature." The indicator needle points to 200 degrees Centigrade at the actual test load (i.e., 17 Ohms). Circuit reaction time and meter ballistics have to be compensated for. This is simply achieved by tuning the sensitivity of the heat meter to swing its needle a little bit further than the 200 degrees indication when the load becomes 17 Ohms. The Dummy Load box has large coil inductors and capacitors, in addition to power resistors, so it closely mimics the reactive impedance of the cutting head. Furthermore, the TEST pushbutton allows the output drivers to be bypassed minimizing the available power, so this is invoked for these tests.

(3rd order 2 Hz Low Pass filter, inside, for DC resistance measurements...)

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Last edited by Fonotec on 03 Apr 2013, 03:50, edited 1 time in total.
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Re: network

Postby Fonotec » 03 Apr 2013, 03:50

The box resistor is a God send for its ability to allow the user to sweep through a broad range of values while fine-tuning a circuit. The Ortofon Safety Circuit, and many other cards, have tuning resistors that have to have the values determined for the amp the card is in. The extra resistors are in parallel with ones that are factory-specified (and packed). Nice posts stand off from the board which allow alligator clips from a box resistor to be used. I got this one on eBay, and it was calibrated. I verified with a new Fluke and it is spot on. What's more, the name Shallcross is the same family responsible for Shallco switches. These are used in things ranging from Sontec Mastering equalizers to so-called "nuclear" power stations...
These switches have a satisfying "snick" as they find their respective steps.


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Re: network

Postby Fonotec » 03 Apr 2013, 03:52

...By now, I ran across an Ortofon 31-pin edge connector which makes for a much more convenient Impedance Board bypass connector. Someone had already jumpered two of the connections and had almost done that correctly (gotta peep every solder when bringing back these relics). I corrected the jumper pin which was wrong and added the extra (jacketed) jumpers, following the instructions in the field bulletin that came with my Ortofon manual. The bulletin reminds the clerk who is doing the work that resistors take "some time" to stabilize after soldering. This was definitely evidenced... I'd carefully cherry pick a resistor (or fangle the target resistance with a two-part network), pack it, and then power on and see a different behavior (at first) than I did with the calibrated box resistor. After waiting a few minutes and repowering, the resistors I had just packed suddenly started to behave correctly - just like the box resistor did...

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Re: network

Postby Fonotec » 03 Apr 2013, 03:57

Here's 1 kHz modulation at 3.54 cm/sec/channel/RMS, driven by the backup set of GO 741 Ortofon amps. This was cut with a base depth of ~ 2 mils (width) "across the top."


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