I hope someone can help with the problem that just occurred with my Titan 425.
I moved the on/off switch to the on position and the amp started it's
warmup procedure. I had it tuned up on 15 meters, last used during CQ
WPX Phone on this band. I was tuning the transceiver, looking for the
N8S. The Titan completed it's warmup, I heard the relay click in,
then Pow, it sounded like a .38 going off. Lamps remained on, fan
remained on, and I tripped the VOX when I was startled. On the
monitor scope, I was able to see output when I grunted into the mic.
Everything looked normal after that. I did the "helllllllo' into the
mic and the scope and output seemed OK, about 1400W on the LED meter,
about 25 ma on the grid meter, and the scope looked normal. I assumed
it was a spider that built a web inside, and when HV was applied, she
vaporized. Then, I tripped the VOX again and POW, lights out. No
lamps, fan, anything.
Removed the RF deck and opened it up. It looks like L2, a 15uh choke
on the 81216 meter shunt board exploded.
I measured across L1, the 60uh plate choke, and got about 1.2 ohms.
R1, the series plate resistor however is really wierd. The 10 ohm 25
watt resistor now reads almost 8000 ohms. Also, there is a big black
carbon spot on the back of the TUNE capacitor where it arced on the
HV input side of R1.
I pulled the tubes and checked with an ohmmeter and there do not
appear to be any measurable shorts between elements.
After the last POW, the two 20 amp fuses in the power supply also blew.
I can easily understand the L2 choke vaporizing with a short to
ground on the output side but do not understand why R1 now reads so
high. I did not notice any high current indication on the plate meter
where current would have flowed through the triodes, plate choke then
R1 to ground. Since the carbon arc is on the power supply side of R1
and not the tube side, I assume the current flowed through the HV
line, L2 and to the input side of R1 to ground.
Examining R1, it looks like there is a tight fit for that resistor,
sandwiched in between the Tune capacitor and the cooling fan. If I
get this working again, it looks like a good place to mount some
dielectric material, maybe some silicon rubber used in the exhaust stack.
Has anyone ever seen this before? ( I did not find anything like this
in the archives).
Any help would be very much appreciated.
Thanks,
73,
Rick, W6RKC
|