Some of those HV circuits can cause problems when the components come close
to ground. I had a disc ceramic capacitor on the meter shunt board get too
close the chassis and blew it up. It sounds like your problem may be
farther down the HV line than mine was, but the cause and fix are probably
similar.
Start by replacing components you know are bad and clean up any burn tracks
and make sure everything that carries HV is clear of the chassis or anything
it can arc to. Use insulating varnish (GC Electronics 10-9002) on potential
arc points. Look for any burn tracks you might have missed, like at the
base of the big horizontal plate RF choke. Check the DC blocking caps, or
simply replace them. Before replacing the 10 Ohm resistor, it might be a
good idea to power up the HV and make sure there is nothing wrong in the
power supply or the meter shunt board.
A few weeks ago, my 425 did a similar thing...big bang when the 'Wait' light
went off. I never did find anything wrong other than one of the fuses blew.
I took it all apart, including the power supply, cleaned and tested
everything, plugged it back in and it worked fine. I suspect it was because
it knew that I had ordered a new ACOM 2000A to replace it and it's feelings
were hurt ;-)
Ron N6IE
www.N6IE.com
----- Original Message -----
From: "Rick, W6RKC" <ab1u@volcano.net>
To: <tentec@contesting.com>
Sent: Wednesday, April 04, 2007 10:03 PM
Subject: [TenTec] Help with blown Titan 425
>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
>
>
>
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>
>
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