>>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."
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