>A ham friend of mine asked me to troubleshoot his newly acquired but
>inoperative AL-80A amp. yesterday. The first problem I found was the
>antenna relay had been hit by something, knocked out of position and
>bending the contacts.
>
>This was relatively easy to fix, but after that, a continued visual
>inspection revealed that 2 of the 3 wafers of the band switch had
>several vaporized contacts.
This is usually the result of an intermittent parasitic oscillation c.
155MHz. Does the toasted bandswitch look like the AL-80 bandswitches at:
http://WWW.VCNET.COM/measures/bandsw.html
?
One solution is to trade the new, high gain tube that apparently
oscillated for an old worn-out tube with lower gain that couldn't
oscillate if you wanted it to. Wanna swap tubes? An alternative would
be lower VHF-Q suppressors.
>My next step will be to replace the
>bandswitch, but now I am beginning to wonder if there may be a high
>probability of other problems present that caused the extreme arcing of
>the bandswitch. I have looked in the archives and see the info about vhf
>parasitics as a potential problem, but are there other known problems
>that I should look for ?
>
The burst of grid/cathode current that accompanies a vhf parasite
creates an EMF that can bend the hot (1820?K) filament perpindicular to
the flow of grid current. With repeated parasitic events, the tube may
short. With a high-pot tester, a healthy tube will exhibit a
withstanding potential of 5 to 8 kV. Also, metering damage is possible.
>Also, is the bandswitch in the AL-80B interchangeable with the AL-80A
>and are they still available from Ameritron ?
>
Yes, and for a reasonable price.
cheers, Kent
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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