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.
Jack, WB8BFS
--
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
|