[Amps] AL-80A troubleshooting

[2]

>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 (1820ºK) 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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