>Rich Measures wrote:
>> ...snip...
>> IMPROVING AMPLIFIER STABILITY
>> ...snip...
>> If you discover that some of the output-bandswitch wafer contacts are
>> burned in your amplifier, you can telephone Kenwood, but their standard
>> answer is that "bandswitch contacts can only be burned by the (stupid)
>> operator (that's us) rapidly switching the bandswitch while transmitting."
>> If you would like to see a photograph of a TL-922 bandswitch which was
>> crispy-crittered by intermittent VHF parasitics, see the magazine
>> article: "Parasitics Revisited" in the September and October 1990 issues
>> of QST. To their credit, QST's staff had no qualms about publishing this
>> photograph since they have heard many complaints over the years from
>> TL-922 owners who were insulted by Trio-Kenwood factory-service's rotten
>> excuse.
>
>I had a Kenwood TS-922A that had a bandswitch arcing problem.
>Keying the amp while running VOX would result in an arc
>typically once per 20 transmissions.
>Upon investigating, I found that the problem was that the RF input
>and output DPDT relay was a large, SLOW, contactor style relay that
>was energized at the same time as another smaller, faster relay that
>controlled the bias and indicator lights. I replaced the RF
>input / output relay with a P&B 3PDT relay, and used the third
>pole to switch the bias.
>After this change, all bandswitch arcing ceased completely. I
>replaced the burned bandswitch, and never had an arc again over a
>period of about 6 years of use with VOX.
>IMHO, this problem had nothing to do with parisitic oscillations,
>just poor design of the RF and bias switching.
>
Agreed, Jack--- stock 922 switching produces transients. If one of the
922's problems is not parasitic-related, then how do so many 3-500Zs
sustain bent filament helices (which causes a filament-to-grid short)?
......... I can not visualize how a HF switching transient could possibly
cause enough grid current flow to bend a 3-500Z's filament---and
simultaneously damage a VHF parasitic-suppressor resistor, whilst causing
a mysterious 'big-bang'. .
Rich---
R. L. Measures, 805-386-3734, AG6K
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