Little Things

[Max Harelik]

I've been refurbishing an SB-200, doing the usual stuff (Harbach pwr 
sply bd, soft-start, cooling fan, new tubes & sw/relay/pot cleaning).  
Finally fired it up Friday, with my TS-530S and MFJ's interface. worked 
fine, with 2-3 S-unit increase over barefoot reported on 80, 40 and 20. 
Thought, "Done!" Put the amp back into its cabinet, cinched it down with 
the feet/screws,  and placed it at its appointed place at my station.

Saturday, sat down to get in some serious "See what I done?" time. 
Checked out the bands, settled on 80m, tuned up the driver, flipped on 
the amp, and checked idle plate current (~90 mA). OK. Began to crank up 
drive to get the 200mA to tune the amp. Nothing. Yes, I had drive, 
cabling OK, plate voltage good. I could hear the amp antenna relay click 
w/transmit, the filaments were lit, everything  as it should be. 
Thought, "Well, maybe I lost some switching function," and worked all 
the switch positions. No reaction. Sniffed the amp. No burnt smells, no 
smoke, nothing. Fooey.

Pulled the amp case back off, discharged the HV caps, and proceeded with 
yet another careful visual/wiring inspection. Nothing amiss. OK. Fired 
it up again (still out of the case, but with the cage cover on). It 
worked. Perfectly. Tried to make it fail again, without effect. Stumped.

Then serendipity strikes.

As I sat, pondering intermittents, my eyes fell upon the four rubber 
feet, which I had carefully placed on the bench, right side up, with 
screws inserted. Noticed one screw (of the four) about 1/4" longer than 
its brothers. Grasping at straws (screws) by this time, I checked for 
interference/contact of the screw with underchassis components. Eureka! 
One screw mounting position is immediately below the antenna relay 
actuator arm, and the screw was just long enough to hold one side of the 
actuator down without grounding out the contact set. The driver input 
contact set. Thinks I, "It can't be this easy," and fires it up again. 
Performs flawlessly, and nothing damaged.


It was as if the amp antenna relay had not been actuated, and the driver 
output bypassed the amp on its way to the antenna. I recased the amp, 
making very sure that the culprit screw couldn't offend, and "Voila, !" 
All is well.

A long story to say/see again what my first radio shop boss always 
said:" Always. ALWAYS check first what the monkeys (myself included) can 
fiddle with.

73,
Max, K5OVW

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