[Johnson] 250-39 TR switch.

[Dennis]

Another point of view on these TR switches....

I have three of them in use and they work great.  As long as you keep the connection between your transmitter and the TR switch short the suckout mentioned will be slight; its because of this issue that Johnson put an RF amp in the TR switch so signals won't be degraded.  You will notice some changing of signal levels when you change the loading on your transmitter; but thats normal.  In my case I have my CE 100V & 600L coupled with a NC-303 using a Johnson TR switch and I have two Johnson Desk KW's using the Johnson TR switch and they do a great job.  Into a 50 ohm load they will provide you over 4kw of switching protection to your receiver.....  Not too bad...

With regard to the TVI, not an issue any more with cable and satilite but in the good old days Johnson recommenced using a Low Pass filter with the TR switch;  In those days most shacks already had a LP filter in the antenna line, I know I did, is it wasn't ever an issue....

I remember I wrote an article for ER (Electric Radio)  magizine discussing the many different TR switchs and their pros and cons.  Maybe you can get a copy....

Ciao, Dennis, k0eoo


---- Roy Morgan <k1lky at earthlink.net> wrote: 
> 
> On Oct 22, 2008, at 9:54 PM, <kb2wig at twcny.rr.com> wrote:
> 
> > I'm considering  the acquisition of the Johnson switch.
> >
> > ? what do they go fer now?
> I don't know.
> >
> > ? who has one fer sale?
> Not me.
> >
> > ? do I really need one?
> 
> Probably not.  Here's why:
> 
> -The main disadvantage of these things is suckout.  The transmitter  
> being attached to the receiver/antenna during receive sucks signal  
> from the system.  This may not bother you if signals are strong enough  
> and/or your receiver has enough gain.
> 
> - A second disadvantage is that they are reported to generate TVI.   
> These reports came at a time of 21 mc TV IF frequencies, however.  You  
> may have no trouble at all, but good citizenship would call for very  
> careful testing and corrective measures if needed.
> 
> If you DO get one of the Johnson units:
> 
> - They run blistering hot.  Three reasons are higher line voltages  
> than in the past, the tube used (6BL7?) dissipates a lot of heat in  
> both filament and plate, and just plain inadequate ventilation.  A  
> small computer fan located nearby would solve the problem.
> 
> - There is no fuse in them as built, and only a two prong line cord.   
> The solution to these somewhat dangerous conditions is pretty obvious.
> 
> - The instructions say to locate it as close as possible to the  
> transmitter.. Apparently this really does matter.
> 
> - The instruction sheet is on BAMA as I remember.
> 
> Roy
> 
> 
> Roy Morgan
> k1lky at earthlink.net
> 529 Cobb St.
> Groton NY, 13073
> 
> 
> 
> 
> 
> Johnson mailing list
> Home: http://mailman.qth.net/mailman/listinfo/johnson
> Help: http://mailman.qth.net/mmfaq.html
> Post: mailto:Johnson at mailman.qth.net