[AMPS] TL922 troubleshooting....

[Rich Measures]

>
>
>
>
>I am troubleshooting a TL922 for a friend. Here's the story:
>
>Amp was working fine on 20m. Took amp offline, removed covers to remove dust.
>Tubes were removed from sockets, then replaced after cleaning, in their
>original positions (L/R). 
>
>Upon firing up the amp after re-assembly, he heard what he described as a
>"soft sizzling sound, around a 800ma indication on the plate current meter,
>then the AC fuses blew. All with the amp in stby mode, CW position.
>
>That's when I got involved.
>
>Upon initial inspection, the 470uH choke for the left tube was melted/pulled
>together. 

>I performed a simple ohm-meter test on each tube, no apparent
>shorts. The 7.5v bias zener and associated components test good (as I would
>expect since this happened in stby mode, bias cutoff). 
>
>Question: Would a gassy tube break down and short only when HV is applied ? 
>
to find out, one needs to test the tube out of the amplifier.  .  

>               Would this cause the grid choke to fry ?
>
a tube with a bent filament helix can fry the choke.  A vhf parasite can 
fry the choke.  

>I am guessing that the handling of the tubes during removal/insertion may 
>have introduced a leak of the tube's vaccuum. Is this possible?  

Only if the tubes have no phenolic reinforcing disk on the pins and the 
tubes were rocked sideways during removal.  

>Both tubes look
>identical, no visible differences that I noticed.
>
>Since I think the amp will need new tubes, let me ask now. 

There is no way to tell which tubes are bad, and from what, without a 
high-pot. .  

>What is the best
>500Z to go with since Eimac 3-500's are no longer to be had (easily) ? I 
>would prefer a drop-in replacement, with no bias mods or other changes 
required. 
>
RF Parts reportedly selsl a Mu=130 3-500z.  

>What else can I check/test ?
>
>Thoughts/comnments/suggestions ?
>
There is an article on the TL922 on my Web site.  
-  later, Eric KK2ED


Rich...

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


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