[Amps] AL-1500 problem

[Carl]

Id also start with the zener as the mica insulating washer may be dirt 
encrusted or have a tiny puncture. Has this amp ever had a big bang failure 
or a lightning hit?

Just hope its not the tube and you have to try and arc the short clear with 
fingers and toes crossed!.

Carl
KM1H



----- Original Message ----- 
From: "Dave Hachadorian" <K6LL at ARRL.net>
To: <amps at contesting.com>
Cc: "W7WW, David" <w7ww at roadrunner.com>
Sent: Monday, November 02, 2009 3:52 PM
Subject: [Amps] AL-1500 problem


> 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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