[Amps] A intermittent 87a

[Charles Harpole]

1.  Looking for ideas re an intermittent flakey Alpha 87a.  SN 91480337 with factory Alpha Max and set on 220VAC with line voltage consistently at 230VAC (not counting dips occasionally but not apparently relevant).  The problem was same with earlier internal xformer tap set at 240VAC.
 
2.  Amp has has all PIN diodes replaced seeking an answer to an earlier Fault 9, which has cleared up totally lately.
 
3.  Shack air temp at 27C to 30C  (86F), the amp goes to Fault 20 either immediately upon turn-on or up to measured 4 hours of continuous stand-by ON condition.  Position of the STBY/OPR switch does not matter in this kind of test.  Sometimes cycling the STBY/OPR switch resets the amp to normal and sometimes does not.  When the reset works, the amp does function normally with 40-50 watts of drive into 1.9:1 SWR antenna.  At times--with no discernable pattern--the amp goes to Fault 20 and will not respond to reset for hours.
 
4.  CHANGED Shack air temp after 24hrs., then test............. 25C  (77F).   At this lower temp. the amp works normally and puts out the full rated output.  The amp then usually waits 2 to 4 hours to go to Fault 20 during normal use (xmit 1 to 3 minutes, stand by 3 to 5 minutes or more).   A wait of 5-10 minutes, then reset via the STBY/OPR switch, is effective (works ok) about 70% of the time.  The 30% of the time, all other variables are the same, including ambient temp. in the shack, but the amp will not reset.
 
5.  In the latter case, turning the whole amp OFF, and back ON after a 5-10 minute wait gives full operation again in about 90% of cases, but does not produce consistent full operation because amp goes back to pattern noted in paragraph 4 above (that is, again at 25C).
 
6.  Note that the LOWER shack temp. seems to be the only variable that occurs along with the Fault 20 failures.  The amp has the factory added external fan, the internal blower puts out equal and good air volume, and none of the vents are obstructed.
 
7.  Any help and advice short of sending it to factory will be very much appreciated.   73,

Charles Harpole k4vud at hotmail.com