[R-390] R390A VFO backlash problem

[Drew P. via R-390]

Roger Gates wrote:

[snipped] 

"I have a Collins type PTO manufactured by Progressitron Corp. for Capehart.
The PTO seems to have some backlash. If I set it for zero beat, it will jump
back about 300Hz when I let go of the knob or shaft (with PTO removed). I
have cleaned and lubricated the leadscrew and put a drop of oil in the front
bearing, but it still does this. Any suggestions??"

I have found that if the anti-backlash nut affixed to the leadscrew nut is set too tight, excessive torque will be applied to the spring loaded guide which rides on the guide bar. Turning the VFO shaft / leadscrew will then"wind up" the spring guide slightly, and cause the problem you described when the spring "unwinds". This also will contribute to "R-390A Sore Wrist-itis".

I have remedied this problem by disassembling the anti-backlash nut from the leadscrew nut, then lubricating and reassembling with less tension. The anti-backlash nut has spring "petals" protruding radially, clamped in place to the leadscrew nut by a ring secured by screws. The "petals" provide tension, and this may be reset by loosening/removing the small screws, rotating the anti-backlash nut, and then tightening the screws. If the anti-backlash nut has too much spring tension applied by the "petals", there will be excessive drag, with attendant "jump back" problem you described. If tension is insufficient, there will be simple mechanical hysteresis, the backlash we all know and love.

Aside: Some of the drag which can cause tuning the R-390A to be less than pleasant is attributable to the o-ring sealing the shaft where it passes through the front housing of the VFO. A new o-ring, lubricated, helps here. For the ultimate in low-drag tuning, it should also be possible (I haven't tried it) to operate with the o-ring removed entirely - we don't pump up our VFO's with dry nitrogen anymore, do we? Comments, anyone?

Drew