...
>I just got an dead TL 922 from a friend who took part for only 13 hours
>in the contest, than the 922 signed away....
>
>He reports that just from the start, 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!).
- The most likely cause of the tank tuning anomalies is
melted/evaporated bandswitch contacts.
[see "Parasitics Revisited" in the 9/90 and 10/90 issues of *QST*}
- The TL-922 companion article to "Circuit Improvements for the Heath
SB-220 Amplifier" (11/90 - 12/90): -- which *QST* decided not to
publish -- is available at:
http://www.vcnet.com/measures
- note: At the time when these two articles were written, Heath was not
advertising the SB-220 in *QST*. So what about the TL-922? . . If
you guessed that Kenwood was still advertising the TL-922 in *QST*,
congratulations.
>After 13 hours the amp would indicate HF out on the inbuild meter, but
>after a few seconds the needle would drop to zero. After retuning with
>the TUNE knob it worked for some seconds and than went to zero again.
>
- I do not know what could be causing such a thing.
>I haven't opened the amp yet, maybe you could give me a hint where to
>look first.
>
I would unplug the TL-922 and:
1. Remove the right side of the cabinet (5mm Allen screws), and inspect
the output bandswitch contacts.
2.. Grid/filament high-pot test the 3-500Zs to see if the filaments
helices are bent. Normal grid/filament breakdown potential is above 6kV
with a cold filament. {Ibid, photograph of 3-500Z grid and filament} If
the breakdown V is much less, a grid/filament short may occur when the
filament is hot.
3. Measure the R of the VHF suppressor resistors. If the R is
substantively higher than the marked value, an intermittent vhf parasitic
oscillation is indicated.
4. Check the zener bias diode for a short in both directions.
5. Measure R from tube socket pin 3 to ground. Normal R is rouhly 20
ohms.
Danger: In the factory-stock configuration, if a 3-500Z in a TL-922 or
SB-220 develops a grid to filament short, the amplifier will summarily
melt down its (unfused) filament transformer during Standby [Rx] mode --
unless the amplifier is switched OFF immediately. To avoid the problem,
remove the factory's +110V connection to the cathode bias contacts on the
T/R relay.
- good luck -
Rich---
R. L. Measures, 805-386-3734, AG6K
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