>>As I recall, he mentioned that, however, we were discussing bandswitch
>>arcing -- like that shown in photographs A and B on page 33 of the Oct.
>>1990 *QST* article, "Parasitics Revisited", which is why he called me.
>>According to Mr. Paul Pagel, Ameritron customers whose bandswitches
>>seemingly had failed quite similarly were telephoning Mr. Rauch and
>>asking questions -- which I presumed were somewhat "unfriendly".
>
>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.
>The tank circuit rejects VHF signals.
Would the HF tank reject VHF signals that are near a resonance in the
C-Tune?
>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? .
>Rather I would guess that the damaged bandswitches are either under-rated
>for high power use on HF
The bandswitches used in the TL-922, AL-82, SB-220, AL-80, and SB-1000,
have a withstanding ability of approx. 6000vDC.
> (and more likely) that the amplifier in
>question is unstable at HF and is potentially oscillating at HF.
>
>I haven't proved this, and I put if forth on the reflector for comment:
>
>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.
>Certainly much lower than the impedance of the Tune C. I would expect
>any VHF energy then to be shunted to ground via the choke. So blown
>chokes I would expect.
In Henry 2K-4s and 3K-As, toasted, but not blown chokes, are somewhat
common. Typically, a few turns near the hot end have burned insulation.
The anode resonance is near 95MHz.
> 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?
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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