[AMPS] TL-922 Filament Transformer Protection

[Rich Measures]

>
>	I just aquired a Kenwood TL-922. Before placing it in service I will
>add some circuit refinements such as step start and other changes as
>described by others.
>	One item that I cannot find information for is detail about protecting
>the filament transformer against meltdown 

€  Briefly:  
The filament transformer is unfused.  It has an 11v winding and an 80v 
winding.   If either secondary winding is shorted, the transformer will 
literally melt down. 
The 80v winding powers a +110v power supply which runs the relays and 
supplied cut-off bias to the cathode / fil. winding CT.  .  
.  The Kenwood 922 has a tendency to intermittently oscillate at its 
anode-resonance freq. of c. 130MHz.  Such oscillations may cause 
intermittent spitting at the Tune-C and or bandswith, large changes in 
the R of the vhf suppressor resistors, and bursts of high grid current 
which place lateral electromagnetic force on the hot filament-helices.  
With repeated oscillations, the filament may eventually be bent far 
enough to touch the grid cage.  When the filament touches the 
grounded-grid, the +110v power supply shorts to ground.  If the amplifier 
is not switched off, the filament transformer will melt down.  One fix is 
to cut the wire on the bias relay that connects from the coil to the bias 
contacts.  For purists, the existing 100k resistor can be connected 
across the NO bias contacts on the relay.  .  
- later, Court



Rich...

R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures  


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