[VHFcontesting] [Fwd: Use of negative voltage to reduce power with FT-847]

[Bruce Herrick]

I have never trusted that method.  In theory it works fine.  But...if the ALC fails, or the TIB or other source of negative voltage fails, or the radio puts out a high power spike as Eric mentions (and many do), it's back to the shop.

In my case, I dug into my IF rig, a TS-690S, and brought out the driver signal.  A relay switches it between the transverter and the rig's finals.  It's a simple mod, if you can find room on the rear panel for a couple of connectors.  I used SMAs, which don't take up much space.  I got the idea from an article written by Rus Healy, K2UA, in the January 2000 QST.  Although I implemented it a bit differently, the article is worth reading.

Good luck!

Bruce WW1M


>
>I think you just nailed the reason many people use the 817 to drive 
>their DEMI gear. Since the TX won't make more than 5W even in it's 
>highest power mode, you don't have to worry about cooking your shiny xverter.
>
>Because I don't want to incur time for repair on the units, the last 
>thing I want to do is cook them with 100w spikes on TX due to ALC, 
>even if only for a few seconds. Running the bands with several 
>multi-op stations at each new grid can really add up.
>
>I know the 857 has a low power jumper on the power input cable. If 
>you ground it, the rig is supposed to put out no more than 10W. Not 
>sure if this is via ALC or if it somehow bypasses a PA. In any case, 
>if you only had a 857 that route might be worth investigating. Not 
>sure if the 847 has this feature, check it out.
>
>At some point I'm going to switch over to the low power flex radio 
>SDR, which is also a nice IF rig power wise.
>
>I'm curious to see what the results of your o-scope work finds. Please post!
>
>Have fun on 900!
>
>kr0ver
>
>At 09:30 PM 2/2/2010, you wrote:
>>I used it in the January VHF SS with the
>>power control set to minimum on 144 MHz to drive my 903 DEM transverter
>>with no problems.
>
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