>
>Hello all,
>
>Our local club station VE7SCC, has owned and operated a TL-922 for the
>last 13 years. We bought the unit new in the late 80's and it has been
>working quite FB until about 3 months ago.
>
>At that time, I began to notice that the output power was not arriving
>instantaneously, but rather creeping up to it's normal 1KW in about 3-4
>seconds. The steady climb is noticeable even when operating phone. The
>dummy load and watt meter confirmed my observation. Note: This can be
>duplicated over and over with various rigs, each time when the tubes are
>allowed to "cool" and sit idle for 30seconds between tests. The
>metering both on the external Bird and the internal TL-922 IP meter
>indicate the slow rise in output.???
>
>Over the last month, during normal SSB operation it has been noted that
>one tube seems to be working harder then the other, visually getting
>much more red when drive under normal conditions 60watts and with meager
>voice processing. The filament also seems *slightly* brighter on the
>tube that is drawing more current.
>
>The real reason for writing this message is because of a subsequent
>filament transformer failure. About a week ago, while the amp was
>idling, I and several others heard the amp emit a small "crack". At
>this point the filaments having been on for over 1 hour and the amp had
>gone without use for over 30 minutes. The smell that filled the room
>was not easily identified but later through an autopsy it was revealed
>that it was emanating from the filament/bias transformer. We removed
>the 3-500
>S and started the amp up again, all interlocks defeated, we stood back
>and watched, NO smoke, so we assumed that the problem has occurred on
>the secondary of filament winding. When the tubes were put back in, the
>problem came back and we could hear the windings cracking with heat.
>
>After all is said and done, I would like to know if anyone else has
>experienced similar problems and if they could provide any insight.
>
>In the end it would also be helpful to find someone who knows where a
>replacement filament/bias transformer can be found for the TL-922.
>
// The stock TL-922 has a tendency to intermittently oscillate at 120 -
130 MHz. This problem is exacerbated if above-average gain tubes are
used. The intermittent parasitic-oscillation can cause the bandswitch to
arc. The arcing can melt the contacts on the output sections of the
bandswitch.[4] If a full-blown parasitic-oscillation occurs, a loud bang
is usually heard. This noise is caused by a one-shot high-current pulse
that can damage: the 3-500Zs, the Zener cathode-bias diode, and the
bandswitch.
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 are burned by a (stupid) operator
(that's us) rapidly switching the bandswitch while transmitting."
If anyone 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. This photo, and an article on the TL-922, is available on my Web
site. 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.
Parasitic damage to 3-500Zs is indicated by a sudden change in
inter-electrode spacing. This may result in a grid to filament short.
Such a short in one of the 3-500Zs in turn places a short on the +110V
power-supply. This supply is powered by the 80Vrms winding on the unfused
filament-transformer. Unless the amplifier is switched off quickly after
a grid to filament short occurs, the filament-transformer will overheat
and melt-down. The SB-220 has the same problem, except that it
oscillates occasionally at 110MHz. .
Some parasitic-damaged 3-500Zs will not short until they are hot. Thus,
the best way to test a cold 3-500Z for the problem is with a high voltage
breakdown tester. A cold tube that will not withstand at least 5kV
between its grid and filament may short during actual use. New, cold,
upright, 3-500Zs typically exhibit <10µA of grid to filament leakage @
8kV.
Kenwood has replacement transformers. Peter W. Dahl is another source.
cheers, Ian
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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