[AMPS] TL-922 went dead....

Rich Measures measures@vc.net
Mon, 27 Oct 97 09:01:20 -0800

>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:
-  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*, 

>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 

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  -

R. L. Measures, 805-386-3734, AG6K   

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