[Amps] issue with AL-1200 output

[TexasRF at aol.com]

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 at 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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