At 10:54 12/3/97 -0500, you wrote:
>
>On Wed, 03 Dec 1997 12:44:29 +1300 Martin Ellis <jmellis@ihug.co.nz>
>writes:
>>Good morning,
>>Here are details of repairs I found necessary when one 3-500Z
>>had a grid-filament short in my TL922:
>>
>>Destroyed components:
>>L7 470uH grid RF choke
>>R22 4.7K grid resistor
>>D1 diode 1N4007 type, 110v relay/bias power supply
>>C3 33uH 160v electrolytic (replaced, but may have been OK)
>>The good grid RF choke measured 3.0 ohms (with 4.7k across it).
>>The 7.5v 10w zener D2 tested OK.
>>With assistance from Charley W1TE I am planning to install
>>new RF chokes with higher current capacity.
>
>That would be a mistake Martin. In that amp the chokes make poor current
>limiters. In fact I would suggest even lower current ratings such as the
>160ma 1MH that Heath used in the SB-220. Those have an R of about 23
Ohms
>and I would rather lose an occasional choke as compared to a tube.
>
>>
>>I have one question:
>>Will modern type metal film 4.7K 1/2 watt resistors be suitable
>>for the grid resistors R22 and R23? Or should we still try
>>to use the old style composition resistors?
>>What is the best component to protect the filament/bias transformer
>>is the event of an unexpected grid/filament short in the TL922?
>
>Since many seconds of a short will not fry the xfmr a simple fuse may
>suffice. You would need to choose a Slow Blow type that could handle the
>initial inrush current but still be able to blow on overload....not a
>particularly easy task. If it was my amp I would also add a fuse in the
>bias line and drill 2 new holes on the rear apron for the fuse holders.
>
>GL Carl KM1H
>
>
>
>
>>Regards,
>>Martin ZL1ANJ
>>
>>--
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>>
>
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