>
>
>
>
>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
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|