I think Ed's comments on the money. I have had an Omni 6 and a 811H for over
a year and I have had no problems. Looks like a solder joint or an open
choke.
73 Art
WA8VSJ
----- Original Message -----
From: "Rick Denney" <rick@rickdenney.com>
To: <tentec@contesting.com>
Sent: Monday, March 31, 2008 2:54 PM
Subject: [TenTec] Ameritron amp issues and looking for a Centaur
> I'm having a persistent problem with an AL-811 and now I'm wondering
> if the way my Omni V is keying it might be part of the problem.
>
> Synopsis: When working Ducie Island, I tried to tune on 10 meters and
> blew the finals in my AL-811. I replaced them with 572b's, which
> worked fine for about ten minutes of testing into a dummy load, at
> which time I heard arcing and smelled smoke. Inspection revealed blown
> parasitic suppressor resistors in the old-style suppressors. I
> replaced the suppressors with Ameritron's newer style, which is a PCB
> that sits atop the plate choke and provides a resistor protected from
> DC by caps, and an inductor to bypass low frequences around the
> resistor. It also provide new tube ceramic caps, etc.
>
> Everything worked fine with no smoke smell for about 30 minutes of 30
> or 40% duty cycle testing on all bands.
>
> Then, the amp stopped putting out any power at all. Nothing. The
> filaments are still lit, the high voltage reads correctly. When keyed,
> the amp shows that it's keyed, and the plate and grid meters both read
> about 10% of scale (100 and 20 ma or so, respectively). These readings
> remain completely unchanged for any setting of input drive power or
> plate and load. Of course, there is no ALC hooked up.
>
> I have checked the bias voltage filter cap on the output of the
> anodes, and it checks out okay (.001 uF with my Fluke DMM). I can find
> no shorts anywhere, and there is continuity in the entire RF chaing
> from input to output where there is supposed to be. The capacitors
> that pass RF from the input to modulate the filament biasing voltage
> check out okay, too. The input band switch is undamaged and seems to
> work fine.
>
> I have spent a lot of time with the T/R relay in the amp, and it's
> fine. It's working per spec.
>
> The Omni V reads high reflected power when attempting to excite the
> amp, but when I put the amp in bypass, the Omni happily puts out full
> drive power into my dummy load. It's almost as if the exciter's output
> is not connected to anything when the amp is not in bypass. All this
> tells me that it's gotta be on the input side of the tubes. The
> behavior is identical on all bands, telling me that it's not because
> of the tuned input components.
>
> The input band switch shows appropriate continuity. But if it is the
> input bandswitch, I'm wondering if it was damaged as a result of
> hot-switching. I'm keying the amp with the Relay output of the Omni V,
> and using the Tune button to put out keyed CW. Maybe I should have
> been testing with a whistle into the mike, heh, heh.
>
> I'm at a loss. Any ideas of things to check?
>
> And--I'm in the market for a Centaur if anyone has one they want to
> sell.
>
> Rick, KR9D
>
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