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[AMPS] TL922 repaired (long)

To: <amps@contesting.com>
Subject: [AMPS] TL922 repaired (long)
From: measures@vc.net (Rich Measures)
Date: Sun, 23 Nov 97 07:51:32 -0800
>Good morning to the group.
>I experienced a grid-filament short in my TL922 while testing
>a pair of unused 3-500Z tubes which I had kept in storage for
>several years.  The reason for trying the unused tubes was that
>the amp was giving low output after service in the CQWW SSB.
>The grid short took out several small components.
...snip...
Grid shorts seems to result from an intermittent parasitic-oscillation at 
the TL-922's anode-resonance of roughly 130MHz.  To measure the anode 
resonance freq.  in your 922, couple a dipmeter to the 12 gauge bus-wire 
between the DC blocking capacitor and the HV-RFC.  IMO, the nature of 
this dip suggests that the anode resonant circuit has a high Q.
-  To determine whether a parasite actually occurred, measure the 
resistance of the parasitic-suppressor resistors.   If the resistance is 
within tolerance, a parasite is not indicated.  If the resistance is 
substantially above the marked value, a parasite is indicated.  However, 
if the parasite which bent the filament in the shorted 3-500Z took place 
in another amplifier, the suppressor resistors in your TL-922 may not be 
damaged.  

Note:  If a factory-stock 922 is operated on Standby with a grid/fil. 
shorted tube, the unfused filament transformer will melt down.  To avoid 
this problem, remove the +110v source from the cathode bias relay 
contacts that connect to the fil. CT.  
-  An article on circuit improvements for the TL-922 is available at:
http://www.vcnet.com/measures
good luck, Martin
Rich---

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


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