[AMPS] Technical Question #1

[Jon Ogden]

>>However, Rich, I have problems with the concept of VHF oscillations 
>>causing bandswitch damage.  
>
>See page 33 of the Oct., 1990 *QST*.  The most severe damage you see in 
>photographs A and B is to the 10m bandswitch contacts and the 160m Tune-C 
>padder contacts, which are the contacts nearest to the VHF-resonant anode 
>circuit.  

I've seen them.
>
>>The tank circuit rejects VHF signals.  
>
>Would the HF tank reject VHF signals that are near a resonance in the 
>C-Tune?

Well, you bring up a good point.......

>>So I fail to see how the VHF voltage could get high enough.
>>
>The typical L in a 10m tank coil is approx. 1uH.  Assuming that the open 
>bandswitch contacts have 2pF of C (f=112MHz) and the frequency of 
>oscillation is 110Mhz, could there perhaps be a problem if the Tune-C is 
>resonant in the vicinity? .  

Yeah, I guess this is quite possible.  However, I would like to analyze 
the circuit a little more before just agreeing with you.
>

>>Has anyone measured the impedance of the plate RFC at VHF?  The reason I 
>>ask is that by the time we get to 100 MHz, the RFC has already gone 
>>through several points of series resonance.  With each resonance each 
>>succeeding peak impedance is less and less.  So, my unproven hypothesis 
>>is that at 100 MHz the choke could actually look like a low impedance.  
>
>In the SB-220, there is no HV-RFC resonance near the anode-resonance.  To 
>know where the resonances are, one needs to employ a dipmeter.    

I am not talking about a resonance.  I am talking about the HV-RFC having 
a low impedance because it has already been through multiple resonances.  
That's what I am saying.  What is the impedance of an SB-220 choke at 100 
MHz?  Anyone know?
>
>
>> Blown bandswitches from VHF oscillation is something I have a real problem 
>with.
>>
>I have heard from a number of people who had real problems from blown 
>bandswitches. TL-922s are probably the worst.  .   Have you perused the 
>(TL-922) bandswitch photos in the 10/90 *QST* and those on my Website?

Yes, I have.  I think I have read every page of it!  It started me 
thinking.  I must admit, I don't know anything about the construction of 
a TL-922.  Some might criticize me then for commenting on bandswitches.  
However, I am looking at it from a purely technical RF theory/design 
point of view.  I am not trying to deny a problem.  I am just curious 
about what is going on.  Is it a VHF oscillation or an HF oscillation?  
Perhaps over this long weekend, I can work out some of the math and post 
it up here for all to see.

73,

Jon
KE9NA



--------------------------------------------------------------------------
Jon Ogden

jono@webspun.com
www.qsl.net/ke9na

"A life lived in fear is a life half lived."


--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm