To: <amps@contesting.com>
> Date: Tue, 26 May 1998 22:59:49 -0500
> From: Jon Ogden <jono@webspun.com>
> Subject: Re: [AMPS] Amplifier Experiments!
> To: w8ji.tom@mcione.com, amps@contesting.com
Hi Jon,
> What do you call a proper fault protection resistor?
One designed to handle a fault without internal arcing or failure.
RCD makes a pulse rated wire wound resistor, and Stackpole makes
composition types.
>I did have two 10
> watt 10 Ohm wirewounds in series right after the HV DC bypass caps at the
> DC entrance point to the RF deck. They opened up during the arc or
> whatever. I just have to get new ones. Isn't that a proper fault
> protection resistor?
Besides dissipation and resistance, resistors also have a voltage
rating. If the resistor arcs internally, it won't offer the best
protection to the rest of the circuit or the resistor itself.
> >> this perhaps a little too close to 21 MHz? Perhaps putting the
> >That's pretty darned close, but workable in most cases. You
> >absolutely need to measure it with the cover on. I know you like
> >using the GDO, but there are MUCH better tests.
> >
> >One is to use a resistor, diode, and signal source to measure choke
> >impedance. The problem (I hear an echo) with a GDO is it tells you
> >nothing about choke impedance.
>
> Yes, yes, I know. The GDO doesn't measure impedance. I hear that loud
> and clear. All it measures is resonance. So how do you measure
> impedance with a resistor, diode and signal source? Are you talking
> about making a noise bridge or something? Enlighten me.
Disconnect only the TOP of the choke from the circuit. Add
a resistance in series with the top connection of the choke, with a
detector diode across that resistance, and measure the voltage drop
across that resistance. While somewhat crude, it's better than having
no idea at all of choke impedance.
A more crude method is just to use a small 12 volt wire lead lamp and
a T connector. Run one end of the T connector to your dummy load and
the other to the rig. The other empty port goes to the lamp through a
short flexible lead..
Ground the T to the chassis and sweep the choke with the rig. When
you get the approximate brightness for a given amount of power, you
can substitute a resistor and find the approximate impedance for a
given power and brightness.
This also lets you move things around and watch the choke current
change, without fussing with a GDO.
73, Tom W8JI
w8ji.tom@MCIONE.com
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