[ARC5] BC-191/375 Tuning Units - Master Oscillator Dial Slippage

[Michael Hanz]

I think you're probably right, Chris - it's likely slippage between the 
bakelite thumbwheel and the shaft, which is solid.  The two arrows in 
the attached photo are pointing to tiny thrust washers. The nut in the 
second photo is used to preset the thrust pressure. The shaft is solid, 
so no problem there.  I would remove the thrust adjuster and see if the 
knob slips with respect to the shaft.  A little glue or loctite is 
probably all you need.

73,
Mike  KC4TOS

On 4/7/2023 11:13 AM, Christopher Bowne wrote:
> The master oscillator dial on the 3 Mc TU of my BC-191 has developed intermittent slippage which makes it difficult to move around between various frequencies I operate the set on 80 and 75 meters.  It appears that the slippage is occurring on or in the shaft of the front panel thumbwheel that drives the set of bevel gears of the MO variable capacitor.  I say on or in the shaft because as best I can tell the shaft is not a simple solid shaft between the thumbwheel and the gear.  It appears that the shaft is actually two pieces with some sort of slip
> slip clutch arrangement possibly involving a spring.
>
> I was able to get it working for a while
> by applying some silicone lube to the bevel gears and the bushings the variable cap shafts, but the problem has returned.
>
> Has anyone encountered this issue and can provide some enlightenment on what’s going on?  I was expecting to find something simple like a loose set screw on the thumbwheel collar, but it apparently is something else.
>
> Not clear to me why anything but a solid shaft between the thumbwheel and the input bevel gear driving the MO cap shaft would be used since the thumbwheel carries the 0- 100 digit vernier markings of the 0-2000 scale of the dial calibration.  Any slippage of the thumbwheel throws the dial settings of the MO frequency on the calibration charts out the window!
>
> 73 de Chris AJ1G Stonington CT
>
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