My non-technical buddy across town is having a problem with his
AL-1500 (8877 final). This problem shows up about 80% of the time
when he first turns on the amp.
When he first turns it on, after the 3 minute time delay relays
click in, the amp shows about 275 ma plate current. The grid
current meter is pinned negative. After about 30 more seconds,
the meter readings return to zero. When he goes to transmit, he
finds that the protective circuits have tripped, and he has to
cycle the standby/operate switch to reset the protective
circuits. After all these steps are done, the amplifier works
fine, and he used it all weekend in the CQWW SSB contest.
Ameritron suggested to him that the stud-mounted cathode zener
diodes may be shorting to ground or be defective, but, to me,
that doesn't really explain the transitory nature of the problem,
or the grid current anomaly.
I am thinking more along the lines of a cathode-grid short when
the tube is cold, that somehow clears up when it is hot.
I am going over to his place tomorrow. My plan is to check out
the zeners as Ameritron suggested, but I really don't expect to
find anything there. I plan to measure the grid-cathode isolation
of the tube with a DVM while the tube is cold. If that doesn't
show any problems, I plan to switch tubes with his other AL-1500
to see if the problem follows the tube.
I would appreciate any suggestions that you all might offer.
Many thanks.
Dave Hachadorian, K6LL
Yuma, AZ
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