Steve, this sounds like an output padder C that has lost ESR and heats up
with power applied. Heat will cause the capacitance to change. Many
amplifiers use large value capacitors which are known to develop this problem
over
time. If this is the problem, lowering the power output will take longer to
see the power drop.
73,
Gerald K5GW
In a message dated 6/22/2017 4:05:27 P.M. Central Daylight Time,
steve@nr4m.com writes:
Hello all,
Need some input from the brain trust.
I recently acquired an AL-1200 amp from a non-ham third party, that got
it from the estate of silent key. I think he was a relative, or at
least a family friend.
Because this amp was new to me, I was taking it slow. All is fine and
working as it should be up to about 300-400 watts.
At that point, the output suddenly and immediately dropped to about 12
watts, just like flipping a switch. The 'power output' function of the
amp's meter confirms what my outboard meter showed.
Plate current remains unchanged at about 250 ma. Input from the K3
remains unchanged in power level and standing wave.
I can repeat it, over and over, with both CW and SSB bias. It takes a
few seconds for this to happen, anywhere from 5 to 8 seconds, or so.
No vacuum relay in the output. No noises heard; no clicks, no snaps, no
'nothing'.
I recently had another amp given to me that had similar symptoms.
Turned out, in that case, that the vacuum output relay was stuck in the
N/C position and would not switch. Relay coil was fine and pulled
current trying to switch the contacts.
Anyone with experiences similar to this.
73 de Steve, NR4M
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