[Boatanchors] What I did on the day we celebrate Memorial Day

[Brian Harris via Boatanchors]

Some of you know I am selling some rigs due to my need to downsize.  Today I decided to get a couple of Harvey Wells TBS-50D Bandmasters and their optional VFOs ready for sale.  You should know that I've had these around for at least ten years or more.

In my haste, I fired one up without removing it from its cabinet on the outside chance it might actually work because, after all, the one without a VFO that I worked on yesterday worked fine.  After a few attempts in vain to get grid or plate current I decided to take the thing out of the cabinet.  Much to my surprise (embarrassment) I discovered that whoever shipped it to me had stuffed some plastic packing material all around the 807 and 6L6s and that some plastic had melted to the 807 envelope.  After some alcohol and a paper towel the 807 looked like new.  Fortunately the 6L6s hadn't gotten hot enough to melt their packing.  In fact, one of the 6L6s was cold.  A check of the tube with an ohmmeter showed the filament was good. 

With a little closer inspection of the rear panel I discovered a loose jumper which, when tightened, fixed the cold 6L6 and yielded grid current, plate current and about 20 Watts of RF in CW.  It's simply amazing how much better the 6AQ5s in the oscillator and driver stages work when they too have filament voltage.  


When I tried it in the phone position is when the little transmitter decided to let some smoke out.  What was odd is that it came from the vicinity of the 6L6 tube sockets where there are no components.  The smoke quickly disappeared but returned again intermittently until I actually saw what looked like a miniature white dwarf burning between a couple of tube pin sockets.  Apparently repeated heat from the high resistance of the filament pin/socket connection caused enough carbon buildup to allow the 450V plate voltage on pin 3 to arc to the filament on pin 2.  No amount of cleaning will ever fix the socket so out it will come and a NOS ceramic will go in its place.  I'll probably replace both sockets just because I like things to match.  Too bad the manufacturer didn't spend the extra money on ceramic sockets in the first place.


What I did I learn?  Check the inside of a sealed cabinet before energizing.  Clean and Deoxit all tube pins and sockets <before> energizing.  Of course I already knew these things but when you've been away from the work bench for awhile and get in a hurry trouble will likely visit you.  Now where's my drill for those darned socket rivets?

Brian Harris, WA5UEK 
cell 214-763-5977 
email cosmophone at yahoo.com