Hello Joe:
The Zener diode doesn't handle spikes very well, even a small current spike
can easily destroy your bias supply. Since you seem to return to normal
operation after replacing the diode, it is possible that your AL 82 is
occasionally hot switching, if you have a conventional open frame relay
handling the amp bi-passing and switching, and your amp is non QSK.
If you do have the open frame relay, check the contacts for contamination, a
possible sign of hot switching. Of course, check anything that might have
changed in your relay driving, such as any interface module you might be
using between the transceiver and the amp, it is always good practice to use
an interface in my opinion. Some manufacturers actually bend the open frame
relay contacts to attempt to make sure that the output closes before the
drive is applied to the input. This is not a great system, and I normally
change the switching in that case over to a QSK vacuum relay design, using
the less expensive Chinese relays which have worked very well also. I like
to use the GH 1 from Gigavac in California.
I also like to replace the Zener, with a string of diodes, using enough
diodes to idle the 3-500s at about 160 mils. Optionally and more expensive,
you can increase the wattage of the Zener, but I prefer the diode string
which not so spike sensitive as the Zener.
Those open frame relays can often lead to hot switching, and ultimately,
tube failure, not just Zener failure. Your Zener problems maybe a blessing
in disguise, saving you from much greater cost later. Check the contacts,
clean them, and if the problem clears up, you can consider the QSK upgrade.
73 and Happy DXing,
Mike
W2AJI
----- Original Message -----
From: "Joe Musto" <joe@jtme.com>-1
To: <amps@contesting.com>
Sent: Monday, May 28, 2007 2:43 PM
Subject: [Amps] AL-82 Zener problems
> I have an AL-82 that periodically blows the zener diodes in the biasing
> circuit. When it happens, I hear a pop, after which the plate current
> increases and the tubes run hot. I generally operate on the same
frequency
> (on 75m), so I don't have to retune the amp very often. It has happened
> when running the amp at anywhere from 500W to 1500W. I most often run at
> about 500W (SSB). I tune as directed with 100W drive then lower drive to
> obtain desired output. Some other trivial facts are I am using a resonant
> dipole and SWR is under 1.2:1. After replacing the diodes, the amp will
> run fine anywhere from a week to 3 weeks and then at some given point,
I'll
> hear a pop and know that another zener blew. I checked/tightened all the
> grounds on the power supply board and also the diodes on that board. I
> also blew out the amp with compressed air to be sure there was no dust
> causing anything to arc. Tubes do not appear to be gassy and the amp will
> put out full power. The amp has Amperex tubes. Any ideas?
>
> Joe, N1FCJ
>
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