[AMRadio] Power Resistor

Jim Wilhite w5jo at brightok.net
Sun Mar 20 21:21:05 EST 2005

This link explains the drive pot replacement for the DX 100 you are speaking 
about Jim.


If it is a drive pot replacement with low current, the transistor should not 
need heat sinking; and the article explains how to mount it on a solder 
terminal strip.

In my case, I need voltage at both ends of the power resistor and a slider 
in the middle at ground, then sliders between each end and ground to give me 
voltage.  I can redesign the circuit I am fooling with by using pots but do 
not have the metal tools to make brackets, unfortunately.

I am going to have to do something, for my radio is in too good electrical 
condition to part out.

73  Jim

> Byron,
>   I recall someone in your situation using a lower wattage pot, and a HV
> NPN Horizontal output transistor as an emitter follower. A resistor from
> emitter to ground is required, maybe 47K 2 watts. You might think about 
> that
> option. This transistor will need to be heat sinked, and the case isolated
> from ground with a mica insulator, and a little "owl sh_t" grease. If the
> transistor beta is 100 at say 20ma emitter current, then the base current 
> is
> .02/100 or 200 ua. Your pot will stay cool as the transistor does the 
> work.
> In case the output gets shorted, maybe add a 470 ohm series resistor that
> should fry in the case of a short. I'd put that on the collector side 
> going
> to the low B+ since it might quench out a parasitic before it gets 
> started.
> Regards,
> Jim

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