[Amps] Alpha 77Dx

[Carl]

If you have access to a megger Id test the tube interelemrnt breakdown, 
keeping within the tube specs.

It could also be a leaky electrolytic in the bias circuit, monitoring 
the cut-off voltage may yield a hint.

Your symptom is rather common in other amps with a bad cap.

Carl
KM1H



----- Original Message ----- 
From: "Paul Christensen" <w9ac at arrl.net>
To: "Amps Amps" <amps at contesting.com>
Sent: Wednesday, January 21, 2009 3:21 PM
Subject: [Amps] Alpha 77Dx


>I had an unusual condition today that seems to be the prelude of my 
>Alpha
> 77Dx's 8877 going bad.
>
> Shortly after applying AC power (about 20 sec.), the HV was reduced 
> from
> what is normally 2600VDC during filament warm-up down to 2300 VDC and 
> plate
> current ran about 40 mA where it is normally biased off until the 
> 3-minute
> timer function completes.  After the 3-minute timer completed, the 
> amp's key
> line was working fine, but both LV and HV read much lower than normal 
> and
> plate current was still idling around 30-40 mA where it is otherwise
> normally biased off until PTT is applied.  Observing the amp's meters, 
> there
> was no indication of RF drive from the transceiver whatsoever.  In 
> other
> words, there was no change between standby and either LV or HV. except 
> that
> plate current idled at 40 mA and HV was lower in both positions.
>
> I re-started the amp a few times.  On  each successive attempt the 
> fault
> condition as described above lengthened, beginning at 20 seconds, then 
> about
> 60 seconds on the 2nd power-up, then 2 minutes on the third power up
> attempt, and finally everything was normal on the fourth and power-up
> attempt.  Full RF power now and everything is running 100%.  No snaps,
> crackles, or pops at any time.
>
> Thoughts?
>
> Paul, W9AC
>
>
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