...... ....
>... ... the TL922 needed a new home. One problem, one of
>the tubes had developed an intermittent grid short (apparently there was
>a buzzing noise just before the amp died)
The buzzing noise is caused by a short on the DC output of the
halfwave-rectified 80V-rms. winding on the filament transformer. Since
the rectified / filtered output of the 80v winding is used to bias the
3-500Zs off during receive, a grid short also shorts the supply and that
destroys the filament transformer in short order.
What follows is an out-take from an article I wrote on the 922. The
entire article on circuit improvements for the TL-922 is available at:
http://www.vcnet.com/measures
-------------
...snip...
IMPROVING AMPLIFIER STABILITY
The stock TL-922 has a tendency to intermittently oscillate at roughly
130MHz. 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--indirectly--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 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.
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.
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
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 less than10uA of grid to filament leakage @ 8kV.
...snip...
-------
...snip...
>As a parting note I believe this gentleman to be just that and not trying
>to pull a fast one.
>
I agree, Denis. Few people understand the nature of this convoluted
problem. Trio-Kenwood was made aware of the problem in August of 1988
when their US rep. received a xeroxed manuscript of the forthcoming
article on parasitics in the October 1988 issue of *QST*. However, in
Japanese industry, the principle of 'ringesho' thwarts improvements that
do originate from top management. Kenwood's solution to the
extraordinarily high return rate of the 922 was to discontinue it instead
of straightening out several easily-fixed design problems.
Rich---
R. L. Measures, 805-386-3734, AG6K
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