mast mount preamp needs power hmmmm? - Videokarma.org TV - Video

newhallone · #1 10-13-2008, 04:22 PM

I was given a Winegard 8700 preamp but not the power supply. Their website has one for 16 bucks but I figure I have enough junkbox stuff to make one. The specs are 15V DC.
I have an archer supply that puts out 14V ac. I have lots of trannys around I could use. I could make a full wave rectifier. I'm just not sure what to use for filtering and a blocking cap on the tv side. Any ideas?

wa2ise · #2 10-13-2008, 04:43 PM

A blocking cap like 1000pF should do it. As for what to use to feed the power supply to the preamp, likely it would be a choke coil that passes DC but blocks RF. Something around 1 microHenry. Not at all critical. The 75 ohm impedances in this system should swamp out any resonances from the cap and inductor anyway.

newhallone · #3 10-13-2008, 06:25 PM

Here is a pic. of one coil already in the Archer supply and another pic of a junk box item. Values unknown....

wa2ise · #4 10-13-2008, 09:16 PM

Quote:

Originally Posted by newhallone

Here is a pic. of one coil already in the Archer supply and another pic of a junk box item.

Several turns of fine wire wrapped thru a ferrite bead ought to do it. A ferrite bead that looks somewhat like a lifesaver candy would work. Usually found inside computer monitors and such electronics. That ferrite bead in the picture would probably work. Any impact of it being not enough inductance would be most noticeable on Channel 2. Channel 7 and up should be okay.

newhallone · #5 10-13-2008, 11:40 PM

I didn't have time to finish it up before hero's came on so I temporarily kludged it . I used the ferrite bead in the picture and a 1000pf cap. For the power I didn't have time to whip up something so I just used a wall wart. It worked, except I had local channel 3 bleeding in. I suppose it didn't help having the guts hanging out and the wall wart. I may try something different for the choke.