>Hi,
>
>a few days ago I posted a question regarding a KENWOOD TL922 amplifier
>where tuning was very strange on most bands, more precise, the LOAD
>control was most of the time on the fully left position (smallest C)(SWR
>on the antennas was ok!).
>
>I got four replies, Charley - W1TE was right: The problem was the small
>choke in the grid circuit. This choke burned away, making one tube
>intermittant before failure... This changed the input and output
>impeadences and required retuning...
>
>So have a look at this choke if you have a similar problem (soon?!).
>
>Regards, Carsten - DL6LAU
- A grid/ground RF choke that is rated to carry 1A suddenly fails. The
copper wire suddenly melts in two at the instant when a noise is heard .
// Does it seem likely the normal 3-500Z grid current could have sent
the 1A choke to Kaputsville? . . Hmmm.
{Twilight Zone theme}
. . What if:
1. The amplifier oscillated at it's anode resonance of 130MHz.
2. There was no path of escape for the resultant kilowatt or so of
130MHz energy -- due to the low-pass characteristic of the HF tank
circuit.
3. Since there was virtually no VHF load on the possibly-oscillating
3-500Z high-Mu triodes, the grid current temporarily became too much for
the choke to handle.
4. High grid current caused high EM force on the hot (1800 degree C)
tungsten filament -- which caused the filament to bend and short against
the grid. (see page 15 in the September 1990 issue of *QST*, "Parasitics
Revisited".)
Murphy said 'nothing is as easy as it looks'.
Rich---
R. L. Measures, 805-386-3734, AG6K
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